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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY


    ENVIRONMENTAL HEALTH CRITERIA 69




    MAGNETIC FIELDS






    This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.

    Published under the joint sponsorship of
    the United Nations Environment Programme,
    the International Labour Organisation,
    and the World Health Organization

    World Health Orgnization
    Geneva, 1987


         The International Programme on Chemical Safety (IPCS) is a
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    International Labour Organisation, and the World Health
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    toxicology. Other activities carried out by the IPCS include the
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    promotion of research on the mechanisms of the biological action of
    chemicals.


        ISBN 92 4 154269 1 

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR MAGNETIC FIELDS

PREFACE

 1. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS FOR FURTHER STUDIES 

     1.1. Physical characteristics and dosimetric concepts
     1.2. Natural background and man-made magnetic fields
     1.3. Field measurement
     1.4. Biological interactions
          1.4.1. Interaction mechanisms
          1.4.2. Biological effects of magnetic fields
     1.5. Effects on man
          1.5.1. Static fields
          1.5.2. Time-varying fields
     1.6. Exposure guidelines and standards
     1.7. Protective measures
          1.7.1. Cardiac pacemakers
          1.7.2. Metallic implants
          1.7.3. Hazards from loose paramagnetic objects
     1.8. Recommendations for future research

 2. PHYSICAL CHARACTERISTICS, DOSIMETRIC CONCEPTS, AND MEASUREMENT 

     2.1. Quantities and units
     2.2. Dosimetric concepts
          2.2.1. Static magnetic fields
          2.2.2. Time-varying magnetic fields
     2.3. Measurement of magnetic fields
          2.3.1. Search coils
          2.3.2. The Hall probe
          2.3.3. Nuclear magnetic resonance probe
          2.3.4. Personal dosimeters

 3. NATURAL BACKGROUND AND MAN-MADE MAGNETIC FIELDS

     3.1. Natural magnetic fields
     3.2. Man-made sources
          3.2.1. Magnetic fields in the home and public premises
                 3.2.1.1  Household appliances
                 3.2.1.2  Transmission lines
                 3.2.1.3  Transportation
                 3.2.1.4  Security systems
          3.2.2. Magnetic fields in the work-place
                 3.2.2.1  Industrial processes
                 3.2.2.2  Energy technologies
                 3.2.2.3  Switching stations and power plants
                 3.2.2.4  Research facilities
                 3.2.2.5  Video display terminals
     3.3. Magnetic fields in medical practice
          3.3.1. Diagnosis, magnetic resonance imaging, and 
                 metabolic studies
          3.3.2. Therapy

 4. MECHANISMS OF INTERACTION

     4.1. Static magnetic fields
          4.1.1. Electrodynamic and magnetohydrodynamic 
                 interactions 
          4.1.2. Magnetomechanical effects
                 4.1.2.1  Orientation of diamagnetically
                          anisotropic macromolecules
                 4.1.2.2  Orientation of organisms with
                          permanent magnetic moments
                 4.1.2.3  Translation of substances in a
                          magnetic field gradient
          4.1.3. Effects on electronic spin states
     4.2. Time-varying magnetic fields
     4.3. Other magnetic field interactions under study
          4.3.1. Long-range cooperative phenomena in cell membranes
          4.3.2. Localized interactions of external ELF
                 fields with cell membrane structures

 5. EXPERIMENTAL DATA ON THE BIOLOGICAL EFFECTS OF STATIC MAGNETIC 
     FIELDS 

     5.1. Molecular interactions
     5.2. Effects at the cell level
     5.3. Effects on organs and tissues
     5.4. Effects on the circulatory system
          5.4.1. Linear relationship of induced flow potential and 
                 magnetic field strength
          5.4.2. Induced flow potentials and field orientation
          5.4.3. Dependence of induced blood flow potentials on 
                 animal size
          5.4.4. Magnetohydrodynamic effects
          5.4.5. Cardiac performance
     5.5. Nervous system and behaviour
          5.5.1. Excitation threshold of isolated neurons
          5.5.2. Action potential amplitude and conduction velocity 
                 in isolated neurons 
          5.5.3. Absolute and relative refractory periods of 
                 isolated neurons 
          5.5.4. Effects of static magnetic fields on the 
                 electroencephalogram 
          5.5.5. Behavioural effects
     5.6. Visual system
     5.7. Physiological regulation and circadian rhythms
     5.8. Genetics, reproduction, and development
     5.9. Conclusions

 6. BIOLOGICAL EFFECTS OF TIME-VARYING MAGNETIC FIELDS

     6.1. Visual system
     6.2. Studies on nerve and muscle tissue
     6.3. Animal behaviour
     6.4. Cellular, tissue, and whole organism responses
     6.5. Effects of pulsed magnetic fields on bone growth and 
          repair 
     6.6. Conclusions

 7. HUMAN STUDIES

     7.1. Studies on working populations
          7.1.1. Workers exposed to static magnetic fields
          7.1.2. Cancer epidemiological studies on workers exposed 
                 to ELF electromagnetic fields
          7.1.3. Conclusions
     7.2. Epidemiological studies on the general population
     7.3. Studies on human volunteers

 8. HEALTH EFFECTS ASSESSMENT

     8.1. Static magnetic fields
     8.2. Time-varying magnetic fields
     8.3. Conclusions

 9. STANDARDS AND THEIR RATIONALES

     9.1. Static magnetic fields
     9.2. Time-varying magnetic fields
     9.3. Magnetic resonance imaging guidelines
          9.3.1. United Kingdom
          9.3.2. USA
          9.3.3. Federal Republic of Germany
          9.3.4. Canada

10. PROTECTIVE MEASURES AND ANCILLARY HAZARDS

REFERENCES

WHO/IRPA TASK GROUP ON MAGNETIC FIELDS

 Members

Dr V. Akimenko, A.N. Marzeev Research Institute of General and
   Communal Hygiene, Kiev, USSR

Dr B. G. Bernardo, Philippine Atomic Energy Commission, Quezon
   City, Philippines

Professor J. Bernhardt, Institute for Radiation Hygiene of the
   Federal Health Office, Neuherberg, Federal Republic of
   Germanya

Dr B. Bosnjakovic, Ministry of Housing, Planning and Environment, 
   Directorate of Radiation Protection, Stralenbescherming 
   Leidschendam, Netherlandsa

Mrs A. Duchęne, Commissariat ŕ l'Energie Atomique, Département
   de Protection Sanitaire, Fontenay-aux-Roses, Francea

Professor J. Dumansky, A. N. Marzeev Research Institute of
   General and Communal Hygiene, Kiev, USSR

Professor M. Grandolfo, Radiation Laboratory, Higher Institute
   of Health, Rome, Italya

Dr H. Jammet, Commissariat ŕ l'Energie Atomique, Commissariat
   ŕ l'Energie Atomique, Institut de Protection et de Sűreté
   Nucléaire, Fontenay-aux-Roses, France  (Co-Chairman)a

Dr Y. A. Kholodov, Institute of Higher Nervous Activity and
   Neurophysiology, Moscow, USSR

Professor B. Knave, Research Department, National Board of
   Occupational Safety and Health, Solna, Swedena

Dr S. Mohanna, Radiation Protection Bureau, Environmental
   Health Directorate, Ottawa, Ontario, Canada

Dr M. H. Repacholi, Royal Adelaide Hospital, Adelaide, South
   Australia  (Rapporteur)a

Dr R. D. Saunders, National Radiological Protection Board,
   Chilton, Didcot, United Kingdom

Professor M. G. Shandala, A.N. Marzeev Research Institute of
   General and Communal Hygiene, Kiev, USSR  (Co-Chairman)

Mr J. Skvarca, National Direction of Environmental Quality,
   Ministry of Health and Social Action, Buenos Aires,
   Argentina

 Members (contd.)

Mr D. Sliney, Laser Microwave Division, US Army Environmental
   Hygiene Agency, Aberdeen Proving Ground, Maryland, USAa

Dr T.S. Tenforde, Lawrence Berkeley Laboratory, Biology and
   Medicine Division, Berkeley, California, USA

Secretariat

Dr M. Swicord, Division of Diagnostic, Therapeutic and
   Rehabilitative Technology, World Health Organization,
   Geneva, Switzerland  (WHO Consultant)

Dr P. J. Waight, Prevention of Environmental Pollution, World
   Health Organization, Geneva, Switzerland  (Secretary)

 Observers

Dr Zh.I. Chernaya, A. N. Marzeev Research Institute of
   General and Communal Hygiene, Kiev, USSR

Dr V. Voronin, Centre of International Projects, Moscow, USSR

Dr Z. Grigorevskaya, Centre of International Projects, Moscow,
   USSR

Dr T. Lukina, Centre of International Projects, Moscow, USSR

---------------------------------------------------------------------------
a  From the Committee on Non-Ionizing Radiation of the International 
   Radiation Protection Association.

NOTE TO READERS OF THE CRITERIA DOCUMENTS

    Every effort has been made to present information in the 
criteria documents as accurately as possible without unduly 
delaying their publication.  In the interest of all users of the 
environmental health criteria documents, readers are kindly 
requested to communicate any errors that may have occurred to the 
Manager of the International Programme on Chemical Safety, World 
Health Organization, Geneva, Switzerland, in order that they may be 
included in corrigenda, which will appear in subsequent volumes. 



                      *    *    *

PREFACE

    The International Radiation Protection Association (IRPA) 
initiated activities concerned with non-ionizing radiation by 
forming a Working Group on Non-Ionizing Radiation in 1974. This 
Working Group later became the International Non-Ionizing Radiation 
Committee (IRPA/INIRC), at the IRPA meeting held in Paris in 1977.  
The IRPA/INIRC reviews the scientific literature on non-ionizing 
radiation and makes assessments of the health risks of human 
exposure to such radiation.  Based on the Environmental Health 
Criteria Documents developed in conjunction with the International 
Programme on Chemical Safety (IPCS), World Health Organization, the 
IRPA/INIRC recommends guidelines on exposure limits, drafts codes 
of safe practice, and works in conjunction with other international 
organizations to promote safety and standardization in the non-
ionizing radiation field. 

    The first draft of this document was compiled by DR M. 
REPACHOLI.  An editorial group chaired by DR P. CZERSKI and 
including DR V. AKIMENKO, PROFESSOR J. BERNHARDT, DR B. 
BOSNJAKOVIC, MRS A. DUCHENE, PROFESSOR M. GRANDOLFO, DR M. 
REPACHOLI, MR D. SLINEY, and DR T. TENFORDE met in Neuherberg in 
May 1985 to develop the second draft.  A small editorial group 
consisting of DR P. CZERSKI, DR M. SWICORD, and DR P. WAIGHT met in 
Geneva in April 1986 to collate and incorporate the comments 
received from IPCS Focal Points and individual experts.  The final 
draft was then sent to WHO/IRPA Task Group members and formally 
reviewed in Kiev, USSR, 30 June - 4 July 1986.  Final scientific 
editing of the document was completed by DR M. REPACHOLI, with the 
assistance of DR M. SWICORD, in Geneva in July 1986.  The 
scientific assistance and helpful comments of DR T. TENFORDE, and 
the permission to use his extensive literature files, are 
gratefully acknowledged. 

    This document comprises a review of data of effects of magnetic 
field exposure on biological systems, pertinent to the evaluation 
of health risks for man.  The purpose of the document is to provide 
an overview of the known biological effects of magnetic fields, to 
identify gaps in this knowledge so that direction for further 
research can be given, and to provide information for health 
authorities and regulatory agencies on the possible effects of 
magnetic-field exposure on human health, so that guidance can be 
given on the assessment of risks from occupational and general 
population exposure. 

    Subjects reviewed include: the physical characteristics of 
magnetic fields; measurement techniques; applications of magnetic 
fields and sources of exposure; mechanisms of interaction; 
biological effects; and guidance on the development of protective 
measures, such as regulations or safe-use guidelines.  Health 
agencies and regulatory authorities are encouraged to set up and 
develop programmes that ensure that the maximum benefit occurs with 
the lowest exposure.  It is hoped that this criteria document will 
provide useful information for the development of national 
protection measures against magnetic fields. 

    The WHO Regional Office for Europe prepared a publication 
entitled Non-Ionizing Radiation Protection (WHO, 1982).  A revised 
and updated edition, completed in 1986, includes a section (5) on 
Electrical and Magnetic Fields at Extremely Low Frequencies. 

1.  SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS FOR FURTHER STUDIES

    This document includes a detailed review and evaluation of data 
on effects on human beings and other biological systems exposed to 
static magnetic fields or to time-varying fields at extremely low 
frequencies (ELF) of up to about 300 Hz.  Data from the biological 
effects of exposure to sinusoidally varying fields are mainly 
concerned with effects in the range up to 20 Hz or at 50 and 60 Hz, 
and only limited data are available on effects at higher 
frequencies.  Data on studies with higher frequencies and pulse 
repetition rates, and non-sinusoidal waveforms have also been 
considered, but radiofrequency magnetic fields in the frequency 
range 100 kHz - 300 GHz have been excluded because these have been 
treated in the Environmental Health Criteria 16: Radiofrequency and 
microwaves (WHO, 1981). 

    Information for health authorities on the biological effects 
and possible health effects of magnetic fields, is given to provide 
guidance for the assessment of the occupational and public health 
significance of exposure to magnetic fields and to indicate areas 
that may be hazardous.  Information on human exposure levels is 
provided, which, on the basis of present knowledge, is considered 
appropriate for the prevention of health hazards. 

1.1.  Physical Characteristics and Dosimetric Concepts

    A magnetic field always exists when there is an electric 
current flowing.  A static magnetic field is formed in the case of 
direct current, and a time-varying magnetic field is produced by 
alternating current sources. 

    The fundamental vector quantities describing a magnetic field 
are field strength, H (unit: A/m) and magnetic flux density, B 
(unit: T, tesla).  These quantities are related through B = µH, 
where µ is the magnetic permeability of the medium. 

    The term "dosimetry" is used to quantify exposure.  Present 
understanding of interaction mechanisms is insufficient to develop 
anything but preliminary dosimetric concepts for static or ELF 
magnetic fields. 

    In practical radiation protection, it is useful to consider 
static and time-varying magnetic fields separately.  In the case of 
static magnetic fields, protection limits tend to be stated 
primarily in terms of the external field strength or magnetic flux 
density and the duration of exposure.  Since time-varying magnetic 
fields induce eddy currents within the body, evaluation may be 
based on the electric eddy current density (electric field 
strength) in critical organs.  Derived protection limits can then 
be expressed as exposures to external magnetic fields, whereby 
field strength, pulse shape (rise and decay time) and frequency, 
orientation of the body, and duration of the exposure need to be 
specified. 

1.2.  Natural Background and Man-Made Magnetic Fields

    The natural magnetic field consists of a component originating 
in the earth, acting as a permanent magnet, and several small 
components with different spectral characteristics.  At the 
surface of the earth, the vertical component of the permanent field 
is maximal at the magnetic poles, amounting to about 6.7 x 10-5 T 
(67 µT), and is zero at the magnetic equator; the horizontal 
component is maximal at the magnetic equator, amounting to about 
3.3 x 10-5 T (33 µT), and is zero at the magnetic pole.  The flux 
density of the natural time-varying fields decreases from about 
10-7 to 10-14 T when the frequency of the atmospheric 
electromagnetic fields increases from about 0.1 Hz to 3 kHz. 

    The magnetic fields from man-made sources generally have higher 
intensities than the naturally occurring fields.  In the home and 
public places, magnetic flux densities ranging from 0.03 µT to 
30 µT are produced around household appliances, and up to 35 µT near 
transmission lines (50 and 60 Hz), depending on the current carried 
and the distance from the line.  For magnetically-levitated 
transportation systems, static magnetic fields of 6 - 60 mT are 
expected in the region of a passenger's head.  Security systems in 
libraries and storehouses operate at frequencies of between 0.1 and 
10 kHz and produce fields of up to about 1 mT. 

    Occupational exposure to magnetic fields is mainly encountered 
in industrial processes involving high electric current equipment, 
in certain new technologies for energy production and storage, and 
in specialized research facilities.  Around various types of 
welding machines, furnaces, and induction heaters, the magnetic 
flux densities at the operator location range from about 1 µT to 
more than 10 mT, depending on the magnetic field frequency and the 
distance from the coil.  Compared to devices operating at high 
frequencies, lower frequency induction heaters expose operators to 
higher magnetic flux densities.  At operator-accessible locations 
in industries using electrolytic processes, the mean static field 
level is about 5 - 10 mT. 

    In areas accessible to operations personnel in thermonuclear 
magnetic fusion and magnetohydrodynamic generating systems, the 
static magnetic field flux densities may reach 50 mT.  Similar 
field strengths occur near special research facilities, e.g., 
bubble chambers.  Typical values for the magnetic flux density at 
work-places near 50 or 60 Hz overhead transmission lines, 
substations, and in power stations are up to 0.05 mT. 

    In medical practice, exposure to magnetic fields results mainly 
from the use of magnetic resonance (MR) imaging or spectroscopy 
methods for diagnostic purposes or from devices generating magnetic 
fields for therapeutic purposes.  In the MR-devices in use at 
present, the patient is exposed to stationary magnetic fields with 
intensities of up to 2 T and, during examinations, to time-varying 
magnetic fields as high as 20 T/s.  However, most patients are not 
exposed to time-varying fields exceeding 1.5 T/s.  The peak 
exposure value for the patient caused by therapeutic magnetic 
devices is of the order of 0.1 - 2.5 mT. 

    The increasing use of magnetic field-producing equipment in 
industrial processes, research facilities, energy production and 
distribution, new transportation technologies, consumer products 
and medical practice, increases the possibility of human exposure 
to magnetic fields.  Although, up to now, both occupational and 
general-population exposures to magnetic fields have generally been 
at low levels, some new technologies, e.g., magnetically-levitated 
trains, might result in exposure of the general population to 
levels comparable with the highest ones in some working 
environments.  Thus, new technologies involving the production of 
magnetic fields should be carefully evaluated with respect to 
potential health risks. 

1.3.  Field Measurement

    In order to adequately characterize a magnetic field, the 
magnitude, frequency, and direction of the field must be 
determined.  The spatial properties of the field can become 
complicated by time-varying changes in the direction of the 
resultant magnetic field vector.  For example, for a circularly 
polarized field, the magnetic vector describes an ellipse during 
the course of a cycle and does not reach zero magnitude.  
Principles of calculation and measurement of these fields are 
outlined. 

    A human or animal body located in a magnetic field causes 
virtually no perturbation of the field.  A time-varying magnetic 
field induces electric currents in the exposed body.  The factors 
affecting the magnitude of the induced currents are discussed 
below. 

1.4.  Biological Interactions

    The following topics are summarized: the present state of 
knowledge on the mechanisms by which magnetic fields interact with 
living systems, and the biological effects of these fields.  On the 
basis of available information, the areas of future research that 
appear to hold the greatest potential for elucidating some poorly 
understood aspects of magnetic field interactions with biological 
systems are given at the end of this section. 

1.4.1.  Interaction mechanisms

    There are three established physical mechanisms through which 
static and ELF magnetic fields interact with living matter: 

A.   Magnetic induction

    This mechanism is relevant to both static and time-varying 
fields, and originates through the following types of interaction: 

    (a)   Electrodynamic interactions with moving electrolytes

    Both static and time-varying fields exert Lorentz forces on 
moving ionic charge carriers, and thereby give rise to induced 

electric fields and currents.  This interaction is the basis of 
magnetically-induced blood flow potentials that have been studied 
with both static and time-varying ELF fields.  It is also the 
physical basis of the weak induced potentials that provide sensory 
directional cues to elasmobranch fish as they swim through the 
static geomagnetic field. 

    (b)   Faraday currents

    Time-varying magnetic fields induce currents in living tissues 
in accordance with the Faraday law of induction.  Available evidence 
suggests that this mechanism may underlie the visuosensory 
stimulation that produces magnetophosphenes and other effects on 
electrically excitable tissues.  In addition, indirect evidence 
suggests that rapidly time-varying magnetic fields may exert 
effects on a variety of cellular and tissue systems by inducing 
local currents that exceed the naturally occurring levels.  This 
effect may be the basis for the wide spectrum of biological 
perturbations that have been observed with pulsed magnetic fields, 
such as those used clinically for bone fracture reunion. 

B.   Magnetomechanical effects

    The two types of mechanical effects that a static magnetic 
field exerts on biological objects are: 

    (a)   Magneto-orientation

    In a uniform static field, both diamagnetic and para-magnetic 
molecules experience a torque, which tends to orientate them in a 
configuration that minimizes their free energy within the field. 
This effect has been well studied for assemblies of diamagnetic 
macromolecules with differing magnetic susceptibilities along the 
principal axes of symmetry.  Included in this class of 
macromolecules are the arrays of photopigments in retinal rod disc 
membranes. 

    (b)   Magnetomechanical translation

    Spatial gradients of static magnetic fields produce a net force 
on paramagnetic and ferromagnetic materials that leads to 
translational motion.  Because of the limited amount of magnetic 
material in most living objects, the influence of this effect on 
biological functions is negligible. 

C.   Electronic interactions

    Certain classes of chemical reactions involve radical electron 
intermediate states in which interactions with a static magnetic 
field produce an effect on electronic spin states.  It is possible, 
that the usual lifetime of biologically relevant electron 
intermediate states is sufficiently short that magnetic field 
interactions exert only a small, and perhaps negligible, influence 
on the yield of chemical reaction products. 

    In addition to the mechanisms of magnetic field interactions 
for which there is direct experimental evidence, several other 
mechanisms have been proposed, on theoretical grounds, in an effort 
to explain various biological effects that have been reported to 
occur in static and ELF fields of very low intensity.  However, it 
must be emphasized, that many proposed mechanisms have not been 
subjected to direct experimental tests. 

1.4.2.  Biological effects of magnetic fields

    Some organisms possess sensitivity to static magnetic fields 
with low intensities comparable to that of the geomagnetic field 
(about 50 µT).  Phenomena for which there is substantial 
experimental evidence of sensitivity to the earth's field include: 

    (a)  direction finding by elasmobranch fish (shark, skate,
         and ray);

    (b)  orientation and swimming direction of magnetotactic
         bacteria;

    (c)  kinetic movements of molluscs;

    (d)  migratory patterns of birds; and

    (e)  waggle dance of bees.

In addition, a number of  in vitro studies have been made of 
magnetic orientation in assemblies of macromolecules, including 
retinal rod outer segments, muscle fibres, photosynthetic systems 
(chloroplast grana, photosynthetic bacteria, and  Chlorella cells), 
halobacteria purple membranes, and various synthetic liquid 
crystals and gels.  As discussed in the preceding summary of 
mechanisms of magnetic field interaction, certain classes of 
chemical reactions that involve a radical electron intermediate 
state may also be sensitive to static magnetic fields of moderate 
intensity (< 10 mT). 

    The available experimental information on the response of 
organisms, including land-dwelling mammalian species, to static and 
ELF magnetic fields indicates that three biological effects can be 
regarded as established phenomena: 

    (a)  the induction of electrical potentials within the
         circulatory system;

    (b)  magnetophosphene induction by pulsed and ELF magnetic
         fields with a time rate of change exceeding 1.3 T/s
         or sinusoidal fields of 15 - 60 Hz and field
         strengths ranging from 2 to 10 mT (frequency
         dependent); and

    (c)  the induction by time-varying fields of a wide
         variety of cellular and tissue alterations, when the
         induced current density exceeds 10 mA/m2; many of
         these effects appear to be the consequence of 
         interactions with cell membrane components.

    For static magnetic fields with flux densities of less than 2 
T, there exists a body of experimental data that indicates the 
absence of irreversible effects on many developmental, 
behavioural, and physiological parameters in higher organisms.  
Broadly summarized, available evidence suggests that the following 
9 classes of biological functions are not significantly affected by 
static magnetic fields at levels up to 2 T: 

    (a)  cell growth;

    (b)  reproduction;

    (c)  pre- and post-natal development;

    (d)  bioelectric activity of isolated neurons;

    (e)  behaviour;

    (f)  cardiovascular functions (acute exposures);

    (g)  the blood-forming system and blood;

    (h)  immune system functions;
    
    (i)  physiological regulation and circadian rhythms.

    For time-varying magnetic fields in the ELF frequency range, 
few systematic studies have been carried out to define the 
threshold field characteristics for producing significant 
perturbations of biological functions.  Nevertheless, available 
evidence suggests that ELF magnetic fields must induce current 
densities in tissues and extracellular fluids that exceed 
10 mA/m2, in order to produce significant alterations in the 
development, physiology, and behaviour of intact higher organisms.  
In  in vitro studies, various phenomena have been reported in the 
1 - 10 mA/m2 range, but their health significance has not been 
determined.  However, it should be noted that therapeutic 
applications of magnetic fields make use of this range. 

1.5.  Effects on Man

1.5.1.  Static fields

    Studies on workers involved in the manufacture of permanent 
magnets in the USSR indicated various subjective symptoms and 
functional disturbances including irritability, fatigue, headache, 
loss of appetite, bradycardia, tachycardia, decreased blood 
pressure, altered EEG, itching, burning, and numbness.  However, 
lack of any statistical analysis or assessment of the impact of 
physical or chemical hazards in the working environment 
significantly reduces the value of these reports.  Although the 
studies are inconclusive, they suggest that, if long-term effects 
occur, they are very subtle, since no cumulative gross effects are 
evident. 

    Recent epidemiological surveys in the USA have failed to reveal 
any significant health effects associated with long-term exposure 
to static magnetic fields.  A study of the health data on 320 
workers in plants using large electrolytic cells for chemical 
separation processes, where the average static field level in the 
work environment was 7.6 mT and the maximum field was 14.6 mT, 
indicated slight changes in white blood cell picture (still within 
the normal range) in the exposed group compared with the 186 
controls.  None of the observed changes in blood pressure or blood 
parameters was considered indicative of a significant adverse 
effect associated with magnetic field exposure. 

    The prevalence of disease among 792 workers at the US National 
Accelerator Laboratories, who were exposed occupationally to static 
magnetic fields, was compared with that in a control group 
consisting of 792 unexposed workers matched for age, race, and 
socioeconomic status.  The range of magnetic field exposures was 
from 0.5 mT for long durations to 2 T for periods of several hours.  
No significant increase or decrease in the prevalence of 19 
categories of disease was observed in the exposed group relative to 
the controls. 

    Workers exposed to large static magnetic fields in the 
aluminium industry were reported to have an elevated leukaemia 
mortality rate.  Although these studies suggest an increased cancer 
risk for persons directly involved in aluminium production, there 
is no clear evidence, at present, indicating the responsible 
carcinogenic factors within the work environment. 

    It can be concluded that available knowledge indicates the 
absence of any adverse effects on human health due to exposure to 
static magnetic fields up to 2 T.  It is not possible to make any 
definitive statements about safety or hazard associated with 
exposure to fields above 2 T.  From theoretical considerations and 
some experimental data, it could be inferred that short-term 
exposure to static fields above 5 T may produce significant 
detrimental effects on health. 

1.5.2.  Time-varying fields

    Time-varying magnetic fields generate internal electric 
currents.  For example, 3 T/s can induce current densities of about 
30 µA/m2 around the perimeter of the human head.  Induced electric 
current densities can be used as the decisive parameter in the 
assessment of the biological effects at the cellular level. 

    In terms of a health risk assessment, it is difficult to 
correlate the internal tissue current densities with the external 
magnetic field strength.  However, assuming worst-case conditions, 
it is possible to calculate, at least within one order of 
magnitude, the magnetic flux density that would produce potentially 
hazardous current densities in tissues.  The following statements 
can be made on induced current density ranges and correlated 

magnetic flux densities of a sinusoidal homogeneous field, which 
produce biological effects from whole-body exposure: 

    (a)  Between 1 and 10 mA/m2 (induced by magnetic fields
         above 0.5 - 5 mT at 50/60 Hz, or 10 - 100 mT at 3
         Hz), minor biological effects have been reported.

    (b)  Between 10 and 100 mA/m2 (above 5 - 50 mT at 50/60 Hz
         or 100 - 1000 mT at 3 Hz), there are well established
         effects, including visual and nervous system
         effects.  Facilitation of bone fracture reunion has
         been reported.

    (c)  Between 100 and 1000 mA/m2 (above 50 - 500 mT at
         50/60 Hz or 1 - 10 T at 3 Hz), stimulation of
         excitable tissue is observed and there are possible
         health hazards.

    (d)  above 1000 mA/m2 (greater than 500 mT at 50/60 Hz or
         10 T at 3 Hz), extra systoles and ventricular
         fibrillation, i.e., acute health hazards, have been
         established.

    For non-sinusoidal waveforms that have short duration pulses, 
the time rate of change of the magnetic flux density must be 
specified.  In analysing certain biological effects, especially the 
stimulation of excitable tissue, the peak current density values 
are more relevant than root mean square (rms) values.  In addition, 
non-homogeneous magnetic fields must be considered, since high 
field gradients exist near strong magnetic field sources.  The 
induction loops in extremities are usually smaller than those in 
the whole body, so higher magnetic field strengths are tolerable 
for extremities than for the whole body. 

    Several laboratory studies have been conducted with human 
subjects exposed to sinusoidally time-varying magnetic fields with 
frequencies in the ELF range.  None of these investigations has 
revealed adverse clinical or psychological changes in the exposed 
subjects.  The strongest field used in these studies with human 
volunteers was a 5-mT, 50-Hz field to which subjects were exposed 
for 4 h. 

    Several recent epidemiological reports present preliminary data 
indicative of an increase in the incidence of cancer among 
children, adults, and occupational groups.  In other 
epidemiological studies in the USA, no apparent increases in 
genetic defects or abnormal pregnancies were reported.  The studies 
that show an excess of cancers in children and adults suggest an 
association with exposure to very weak (10-7 - 10-6 T) 50 or 60 Hz 
magnetic fields that are of a magnitude commonly found in the 
environment.  These associations cannot be satisfactorily explained 
by the available theoretical basis for carcinogenesis by ELF 
electromagnetic fields.  The preliminary nature of the 
epidemiological evidence, and the relatively small increment in 

reported incidence, suggest that, although these epidemiological 
data cannot be dismissed, there must be considerable further study 
before they can be accepted. 

    From the available data on human exposure to time-varying 
magnetic fields, it can be concluded that induced current densities 
below 10 mA/m2 have not been shown to produce any significant 
biological effects.  In the range of 10 - 100 mA/m2 (from fields 
higher than 5 - 50 mT at 50/60 Hz), biological effects have been 
established, but these induced current densities from short-term 
exposure (few hours) may cause minor transient effects on health.  
The health consequences of exposure to these levels for many 
hours, days, or weeks are not known at present.  Above 100 mA/m2 
(greater than 50 mT at 50/60 Hz), various stimulation thresholds 
are exceeded and hazards to health may occur. 

1.6.  Exposure Guidelines and Standards

    Standards or guidelines limiting human exposure to static ELF 
magnetic fields have been developed in a few countries.  Of 
particular interest is the increasing tendency of countries to 
limit magnetic field exposure from particular devices (e.g., 
magnetic resonance diagnostic techniques).  Details of these 
standards and guidelines are given in section 9 of the document. 

1.7.  Protective Measures

    Two aspects of magnetic field safety that deserve special 
attention are the potential influence of these fields on the 
functioning of electronic devices, and the risk of injury due to 
the large forces exerted on ferromagnetic objects in strong static 
magnetic field gradients.  Of particular concern is the malfunction 
of cardiac pacemakers and the displacement of aneurysm clips and 
prosthetic devices. 

1.7.1.  Cardiac pacemakers

    Both static and time-varying magnetic fields can interfere with 
the proper functioning of modern demand pacemakers.  Some 
pacemakers may revert from a synchronous to an asynchronous mode of 
operation in time-varying fields with time rates of change above 
approximately 40 mT/s.  Certain pacemaker models also exhibit 
abnormal operation due to closure of a reed relay switch in static 
magnetic fields that exceed 1.7 - 4.7 mT.  Magnetic fields can also 
affect the functioning of other medical electronic monitoring 
devices, such as EEG and ECG equipment. 

1.7.2.  Metallic implants

    The sensitivity of implanted surgical devices to magnetic 
fields is dependent on their alloy composition.  A large number of 
metallic devices such as intrauterine devices, surgical clips, 
prostheses, infusion needles, and catheters may have a significant 
torque exerted on them by intense magnetic field gradients.  This 
may result in their displacement and produce serious consequences.  

All persons entering magnetic field environments should be screened 
carefully and, if necessary, prohibited from access. 

1.7.3.  Hazards from loose paramagnetic objects

    Depending on the weight and shape of a paramagnetic object 
subject to an intense magnetic field, it can become a missile with 
high momentum.  Care should be taken to exclude such objects as, 
for example, scissors, scalpels, and handtools from the vicinity of 
strong magnetic field sources. 

1.8.  Recommendations for Future Research

    On the basis of present knowledge of magnetic field bioeffects, 
several key areas of future research can be identified as being 
essential for achieving a comprehensive understanding of the 
biological consequences of exposure to these fields.  No attempt 
has been made to list all possible research areas.  Instead, 
emphasis has been placed on areas considered to have an impact on 
health hazard assessment. 

    For static magnetic fields, there is a clear need for 
additional studies in the following areas, in each of which the 
available information is either inadequate or contradictory: 

    (a)  studies on functional alterations in the cardiovascular 
         and central nervous system, where magnetic field 
         interactions have previously been observed; particular 
         emphasis should be placed on the effects of long-term 
         exposures; 

    (b)  sensitivity of enzyme reactions that involve radical 
         intermediate states, which may be an important issue in 
         long-term occupational exposures; 

    (c)  cellular, tissue, and animal responses to static fields 
         above 2 T, as proposed for use in clinical MR 
         spectroscopy. 

    For time-varying magnetic fields with repetition frequencies in 
the ELF range, key areas of future research can also be recommended 
on the basis of available information: 

   (a)  Comprehensive epidemiological studies should be
        carried out to resolve the issue of whether an
        elevated risk of leukaemia and other forms of cancer
        is associated with occupational and residential
        exposure to ELF fields.  These studies should include
        the use of appropriate techniques for the assessment
        of field exposure parameters (e.g., the  use of
        miniature personal dosimeters).  Relevant research
        with cellular and animal systems should also be
        conducted in an effort to elucidate interaction
        mechanisms of ELF fields that could lead to an
        elevated cancer risk.

   (b)  Studies on the response of developing embryonic and
        fetal systems, and other cell and tissue systems that
        have been identified as being responsive to ELF
        magnetic fields, should be continued with particular
        focus on effects mediated via interactions with cell
        membranes.

   (c)  Studies are needed on the effects of low levels of
        induced current density (< 100 mA/m2) on nerve tissue.

2.  PHYSICAL CHARACTERISTICS, DOSIMETRIC CONCEPTS, AND MEASUREMENT

    Just as an electric field is always linked with an electric 
charge, a magnetic field always appears when electric current 
flows.  A magnetic field can be illustrated by lines of force.  A 
static magnetic field is formed in the case of direct current, 
whereas a time-varying magnetic field is induced by alternating 
current sources. 

    The electric (E) and magnetic (H) fields that exist near 
sources of electromagnetic fields must be considered separately, 
because the very long wavelength (thousands of kilometres) 
characteristic of extremely low frequencies (ELF) means that 
measurements are made in the non-radiating near field.  The E and H 
fields do not have the same constant relationship that exists in 
the far field of a radiating source. 

    A description of the physical characteristics of static and ELF 
magnetic fields has been given by Grandolfo & Vecchia (1985a).  An 
animal or human body does not appreciably distort a magnetic field. 
Time-varying magnetic fields induce currents within the body.  The 
magnitude of these internal currents is determined by the radius of 
the current path, the frequency of the magnetic field and its 
intensity at the location within the body.  Unlike the electric 
field for which the internal field strength is many orders of 
magnitude less than that of the external field, the magnetic field 
strength is virtually the same outside the body as within.  The 
magnetically-induced electric field strengths and corresponding 
current density are greatest at the periphery of the body where the 
conducting paths are longest, whereas microscopic current loops 
anywhere within the body would have extremely small current 
densities.  The magnitude of the current density is also influenced 
by tissue conductivity where the exact paths of the current flow 
depend in a complicated way on the conducting properties of the 
various tissues. 

2.1.  Quantities and Units

    The quantities, units, and symbols used in describing magnetic 
fields are given in Table 1. 

    The fundamental vector quantities describing a magnetic field 
are the field strength (H) and the magnetic flux density (B) (or 
equivalently, the magnetic induction). 

    The magnetic field strength (H) is the force with which the 
field acts on an element of current situated at a particular 
point.  The value of H is measured in ampere per metre (A/m).  The 
trajectories of the motion of an element of current (or the 
orientations of an elementary magnet) in a magnetic field are 
called the magnetic lines of force. 

Table 1.  Magnetic field quantities and units in the SI 
System
-----------------------------------------------------------
Quantity                 Symbol   Unit
-----------------------------------------------------------
Frequency                f        hertz (Hz)

Current                  I        ampere (A)

Current density          J        ampere per square metre
                                  (A/m2)

Magnetic field strength  H        ampere per metre (A/m)

Magnetic flux            PHI      weber (Wb) = Vs

Magnetic flux density    B        tesla (T) = Wb/m2

Permeability             µ        henry per metre (H/m)

Permeability of vacuum   µo       µo = 1.257 x 10-6 H/m

Time                     t        seconds (s)
-----------------------------------------------------------

    As in the case of electric fields, single-phase and three-phase 
magnetic fields can be defined: the field at any point may be 
described in terms of its time-varying magnitude and invariant 
direction (single-phase), or by the field ellipse, i.e., the 
magnitude and direction of the major and minor semi-axes (three 
phase). 

    The magnetic flux density (B), rather than the magnetic field 
strength, (H = B/µ), is used to describe the magnetic field 
generated by currents in the conductors of transmission lines and 
substations.  Thus, the magnetic field is defined as a vector field 
of magnetic flux density B (B-field).  The value of µ (the magnetic 
permeability) is determined by the properties of the medium, and, 
for most biological material is equal to µo, the value of the 
permeability of free space (air).  Thus, for biological materials 
the values of B and H are related by a constant (µo). 

    Before the introduction of the International System of units 
(SI), the use of the CGS system (based on the three independent 
quantities: length (cm), mass (g) and time (s)) was customary.  SI 
is based on seven independent quantities: length (m), mass (g), 
time (s), electric current (A), thermodynamic temperature (K), 
luminous intensity (cd), and amount of substance (mol).  The 
equations describing the electromagnetic phenomena are equivalent 
but not identical in the SI and the CGS systems.  For an 
electromagnetic field, only the first four of the seven quantities 
mentioned above, are relevant.  The CGS unit of magnetic field 
strength is the oersted and that of the magnetic induction is the 
gauss. 

    In the CGS system, µo is a dimensionless quantity equal to 
unity, and as a result, for biological materials, B can be set 
equal to H, as a close approximation.  This convention has been 
used extensively in the biological literature, where many authors 
have used B and H as interchangeable quantities.  Thus, many 
publications contain equations that are appropriate for use only 
with the CGS system of units since the permeability of free space, 
µo, has been omitted. 

    The SI system has now been universally accepted.  The CGS 
system is obsolete and should not be used. 

    In addition, the term gamma is used and is equal to 1 nanotesla 
(10-9 tesla).  For convenience, the conversion factors relating the 
various quantities used in laboratory practice are given in Table 2. 
Table 2.  Conversion factors for units
---------------------------------------------------------------------------
To
obtain      T = Wb/m2      G              gamma  A/m           Oe
-----------
To convert
---------------------------------------------------------------------------
T = Wb/m2   1              104            109    7.96 x 105    104

G           10-4           1              105    79.6          1

gamma       10-9           10-5           1      7.96 x 10-4   10-5

A/m         1.256 x 10-6   1.256 x 10-2   1256   1             1.256 x 10-2

Oe          10-4           1              105    79.6          1
---------------------------------------------------------------------------
Symbols:  T  = tesla
          Wb = weber
          G  = gauss
          A  = ampere
          m  = metre
          Oe = oersted

    For a more complete inventory and discussion of quantities and 
units, the reader is referred to a report of the IRPA/International 
Non-Ionizing Radiation Committee entitled "Review of Concepts, 
quantities, units, and terminology for non-ionizing radiation 
protection" (IRPA, 1985). 

2.2.  Dosimetric Concepts

    In its broadest sense, the term "dosimetry" is used to quantify 
exposure to radiation.  Quantitative descriptions of exposure, for 
the purpose of formulating protection standards and exposure 
limits, require the use of appropriate quantities.  "Appropriate" 
means that the quantities should represent, as far as possible, the 
physical processes that are closely linked to the biological 

effects of the fields.  Since our knowledge of interaction 
mechanisms is incomplete, exposure conditions are often quantified 
in terms of the unperturbed external magnetic field strength and 
the duration of exposure. 

    The known physical mechanisms by which magnetic fields interact 
with living matter are described in section 4.  Some factors 
affecting the interaction of fields with organisms are summarized 
in Table 3.  To fully assess the data obtained in bioeffects 
research, exposure conditions must be well controlled and measured.  
In this case, the "dosimetry" in bioeffects research with magnetic 
fields is very complex, since all relevant factors must be taken 
into account.  The accuracy and sophistication of radiation 
protection dosimetry must be related to the conditions and actual 
or potential adverse consequences of exposure to magnetic fields. 

    In practical radiation protection, it is useful to consider 
static and time-varying magnetic fields separately. 

Table 3.  Factors affecting interaction of magnetic fields
---------------------------------------------------------------
 Parameters of the magnetic field source

1.   Frequency
2.   Modulation (Pulse, AM, FM), rise and decay times (dB/dt)
3.   Polarisation
4.   Field strength
5.   Field pattern (uniformity)
6.   Surrounding material properties

 Parameters related to exposure

1.   Tissue properties (conductivity, anisotropy, permeability)
2.   Size, geometry
3.   Orientation relative to polarization
4.   Mode of exposure (partial; whole body)
    
 Extraneous factors

1.   Metal implants (ferromagnetic)
2.   Metal objects in the field
3.   Drugs (medications)
4.   Chemical pollutants
---------------------------------------------------------------

2.2.1.  Static magnetic fields

    In the assessment of exposure to static magnetic fields for 
practical radiation protection purposes, the appropriate quantities 
are less well defined.  Protection limits tend to be stated in 
terms of the external field strength and the duration of exposure, 
where the integrated product of field and exposure time could be 
considered as a measure of exposure.  However, at present, there 
is no biological basis for choosing this dosimetric concept.  
Further development of dosimetric concepts and their theoretical 
and experimental basis is required. 

2.2.2.  Time-varying magnetic fields

    In evaluating human exposure to time-varying magnetic fields of 
frequencies between about 10 Hz and 100 kHz, the electric eddy 
current density can be employed as the decisive parameter in 
assessment of the biological effects at the cellular level 
(Bernhardt, 1979, 1985, 1986; Czerski, 1986; Tenforde, 1986a).  
Field strength and eddy current density are related by the specific 
conductivity of the medium. 

    By comparing the current densities, it may be possible to 
predict effects in human beings from those found in studies on 
animal and isolated cells.  In this context, it is irrelevant 
whether the current density surrounding a cell is introduced into 
the body through electrodes or induced in the body by external 
magnetic fields.  However, the current paths within the body may be 
different. 

    The evaluation of human exposure using current densities is 
based primarily on a concept of "dose" to the critical organs.  
Although this assumption is based on the most likely hypothesis, 
this mechanism of energy absorption in tissues should not be 
considered to the exclusion of all others.  The parameters of 
internal field strength and duration should also be taken into 
account.  Basic protection limits can be expressed in permissible 
current densities; derived protection limits can be expressed as 
exposures to external magnetic fields, where field strength, 
frequency, orientation of the body, and duration of exposure need 
to be specified.  Refinements may include field gradient values, 
partial body exposure, etc.  Induced eddy currents in organs cannot 
be measured, at present, under any practical conditions.  
Therefore, the only protection quantities that can be used to 
assess exposure to time-varying magnetic fields are the field 
strength distribution in time and space. 

2.3.  Measurement of Magnetic Fields

    During the last thirty years, the measurement of magnetic 
fields has undergone considerable development.  Progress in 
techniques has made it possible to develop new methods of 
measurement as well as to improve old ones.  Some of the incentive 
for considerable development in magnetic measurement techniques has 
arisen because of the necessity to accurately measure magnetic 
fields that often vary in both space and time in large particle 
accelerators.  The rapid development of plasma physics as well as 
that of astronautics has created new demands for magnetic field 
measurements. 

    A description of the most common measuring techniques follows, 
together with a comparison of their advantages and limitations.  
Further details can be found in Williamson & Kaufman (1981), 
Grandolfo & Vecchia (1985a), and Stuchly (1986). 

    The two most popular types of magnetic field probes are a 
shielded coil and a Hall-probe.  Most of the commercially available 
magnetic field meters use one of them.  Recently, in addition to 
Hall probes, other semiconductor devices, namely bipolar 
transistors and FET transistors, have been proposed as magnetic 
field sensors.  They offer some advantages over Hall probes, such 
as higher sensitivity, greater spatial resolution, and broader 
frequency response. 

    For measurements of very weak magnetic fields, such as those 
produced by endogeneous currents in biological systems, other 
sensors are used.  These include fluxgates, optically pumped 
magnetometers, magnetostrictive sensors with optical fibres, and 
superconducting quantum interference devices (SQUIDS).  These 
devices are rather specialized and expensive and are not normally 
used for the measurement of extraneous fields in biomedical 
applications (Stuchly, 1986). 

2.3.1.  Search coils

    The operating principle of a coil B-field probe can be 
explained by considering a closed loop of a conductor with area A 
immersed in a quasi-static, uniform magnetic field of flux density 
B, and angular frequency omega(= 2 pi f), as shown in Fig. 1 (Conti, 
1985). 

    An electromotive force (EMF) is induced in the loop (and a 
current (I) will flow) as a consequence of changes in the magnetic 
flux PHI(B) through the area A, in accordance with the following 
law: 

                    d
              EMF = -- PHI(B)                                (1)
                    dt

FIGURE 1

    If the vector B = Bo sin omega t is assumed to be uniform and 
to have its direction perpendicular to the plane of the loop, the 
EMF is given by the following relationship: 

        d
EMF = - -- (A Bosin omega t) = omega Bo A cos omega t       (2)
        dt

    Equation (2) shows that measurement of induced electromotive 
force provides a measure of the B-field strength. 

    For a loop of many turns, the EMF given by Equation (2) will 
develop over each turn and the voltage (V) will increase 
accordingly.  The induced current has been assumed to be so small 
that the opposing B-field generated by I can be ignored. 

    There is no theoretical limit on the frequency of operation of 
coils as sensors, except for the loop size.  In practice, factors 
such as the electric field perturbation and the pick up by the 
leads connecting the loop to the metering device require 
modifications of the sensor design. 

    A single coil has a directional spatial response 
characteristic, and has to be rotated to obtain a maximum reading 
to determine the actual magnitude and direction of the field. 
Alternatively, a probe consisting of three mutually perpendicular 
coils can be designed. 

2.3.2.  The Hall probe

    The most commonly used method in field mapping is the Hall 
probe.  When a strip of conducting material is placed along the Ox 
axis in a coordinate system Oxyz, with a current I running in the 
direction Ox while a magnetic field B is applied in the direction 
Oy at right angles to the surface of the strip, a potential 
difference appears in the direction Oz between the two sides of the 
strip. 

    The Hall effect can be explained as the result of the action 
exerted on the charge carriers by the magnetic field, which forces 
them sideways in the strip.  Thus, electric charges appear on the 
sides of the strip and, as a result, a transverse Hall electric 
field is created. 

    Several factors set limits on the accuracy obtainable, the most 
serious being the temperature coefficient of the Hall voltage.  
Another complication can be that of the planar Hall effect, which 
makes the measurement of a weak field component normal to the plane 
of the Hall plate problematical, when a strong field component is 
present parallel to this plane.  Many possible remedies have been 
proposed, but they are all relatively difficult to apply.  Last, 
but not least, is the problem of the representation of the 
calibration curve since the Hall coefficient varies with the 
magnetic field. 

    The measurement of the Hall voltage sets a limit of about 0.1 
mT on the sensitivity and resolution of the measurement, if 
conventional direct current excitation is applied to the probe.  
The sensitivity can be improved considerably by using alternating 
current excitation.  Higher accuracy at low field strengths can be 
achieved by using synchronous detection techniques for the 
measurement of the Hall voltage. 

    Hall plates are usually calibrated in a magnet in which the 
field is measured simultaneously using a nuclear magnetic resonance 
probe.  A well designed Hall-probe assembly can be calibrated to an 
accuracy of 0.01% (Germain, 1963). 

2.3.3.  Nuclear magnetic resonance probe

    Nuclear magnetic resonance (NMR) is the classical method of 
measuring the absolute value of a magnetic field. 

    If a charged particle possessing an angular momentum vector, J, 
is placed in a constant magnetic field B, the magnetic moment, u, 
of the particle becomes orientated with respect to B.  The vectors 
J and u are proportional, u = gamma J where gamma is the 
gyromagnetic ratio of the particle considered.  In a quantum 
mechanics description, this orientation can only be such that the 
component of J along B is equal to mh/2pi, where m = ±(I - k), 
I is the spin of the particle, and k is an integer smaller or 
equal to I.  Thus, m can take on several discrete values, each 
giving a different orientation for J and u.  Each of these 
orientations of u in the magnetic field corresponds with a 
different energy level, where these levels differ in energy by 
DELTA E = B gamma h/2pi. 

    If a sample containing a large number of particles, either 
electrons or protons, is irradiated with photons of the right 
frequency, upsilono, such that h upsilono = DELTA E, an exchange of 
energy occurs.  As a result of photon absorption, particles in the 
sample jump from the lower to the higher energy level.  The 
principle of the NMR measurement technique is to determine the 
resonant frequency of the test specimen in the magnetic field to be 
measured.  It is an absolute measurement that can be made with very 
great accuracy.  The measuring range of this method is from about 
10-2 to 10 T, without definite limits. 

    In field measurements using the proton magnetic resonance 
method, an accuracy of 10-4 is easily obtained with simple 
apparatus and an accuracy of 10-6 can be reached with extensive 
precautions and refined equipment. 

    The inherent shortcoming of the NMR method is its limitation 
to fields with a low gradient and the lack of information about 
the field direction. 

2.3.4.  Personal dosimeters

    A personal dosimeter suitable for monitoring exposures to 
static and time-varying magnetic fields has been developed by 
Fujita & Tenforde (1982).  Using thin-film Hall sensors that record 
magnetic induction (B) along three orthogonal axes, the time rate 
of change of the magnetic induction (dB/dt) is determined for 
values of B recorded during consecutive sampling intervals.  The 
parameters stored by the dosimeter include the average and peak 
values of B and dB/dt during a preset time interval, and the number 
of times that specified threshold levels of these parameters are 
exceeded.  An audible alarm sounds when B or dB/dt exceeds a preset 
threshold level.  This personal dosimeter is battery operated, and 
is capable of recording magnetic field exposure throughout an 8-h 
working day.  A microprocessor-controlled field dosimeter for 
monitoring personal exposures to power-frequency magnetic fields 
has been developed by Lo et al. (1986).  This dosimeter uses 
electrically-shielded, 500-turn copper coils and synchronous 
detector circuits for field measurements along three orthogonal 
axes.  For 60-Hz fields a measurement accuracy of 1 - 2% is 
achieved over the range of magnetic flux densities from 5 nT to 
60 µT (rms). 

3.  NATURAL BACKGROUND AND MAN-MADE MAGNETIC FIELDS

3.1.  Natural Magnetic Fields

    The natural magnetic field consists of one component due to the 
earth acting as a permanent magnet and several other small 
components, which differ in characteristics and are related to such 
influences as solar activity and atmospheric events (Aleksandrov et 
al., 1972; Polk, 1974; Benkova, 1975; Grandolfo & Vecchia, 1985b).  
The earth's magnetic field originates from electric current flow in 
the upper layer of the earth's core.  There are significant local 
differences in the strength of this field.  At the surface of the 
earth, the vertical component is maximal at the magnetic poles, 
amounting to about 6.7 x 10-5 T (67 µT) and is zero at the magnetic 
equator.  The horizontal component is maximal at the magnetic 
equator, about 3.3 x 10-5 T (33 µT), and is zero at the magnetic 
pole. 

    The naturally occurring time-varying fields in the atmosphere 
have several origins, including diurnally varying fields of the 
order of 3 x 10-8 T (0.03 µT) associated with solar and lunar 
influences on ionospheric currents.  The largest time-varying 
atmospheric magnetic fields arise intermittently from intense 
solar activity and thunderstorms, and reach intensities of the 
order of 5 x 10-7 T (0.5 µT) during large magnetic storms. 

    About 2000 thunderstorms are occurring simultaneously over the 
globe with lightning striking the earth's surface about 16 times 
per second; the currents involved may reach 2 x 105 A at the level 
of the earth (Kleimenova, 1963).  Electromagnetic fields having a 
very broad frequency range (from a few Hz up to a few MHz) 
originate the moment lightning strikes and propagate over long 
distances influencing the magnitude of magnetic fields.  
Superimposed on the magnetic fields associated with irregular 
atmospheric events is a weak time-varying field resulting from the 
Schumann resonance phenomenon.  These fields are generated by 
lightning discharges and propagate in the resonant atmospheric 
cavity formed by the earth's surface and the lower boundary of the 
ionosphere. 

    The characteristics of the time-varying components of the 
natural magnetic field can be summarized as follows: 

   (a)  The magnetic flux densities from 5 to 10 x 10-8 T are
        at pulsation frequencies from 0.002 to 0.1 Hz.

   (b)  The geomagnetic pulsations up to 5 Hz are of short
        duration, lasting from a few minutes to a few hours.

   (c)  The magnetic flux densities of the field decrease
        with increasing frequency from 10-11 T at 5 - 7 Hz to
        10-14 T at 3 kHz.

3.2.  Man-Made Sources

    The static and time-varying magnetic fields originating from 
man-made sources generally have much higher intensities than the 
naturally occurring fields.  This statement is particularly true 
for sources operating at the power frequencies of 50 or 60 Hz 
(e.g., home appliances), where fields occur that are many orders of 
magnitude greater than the natural fields at the same frequencies.  
Other man-made sources are to be found in research, industrial and 
medical procedures, and in several other technologies related to 
energy production and transportation that are in the developmental 
stage (Demetsky & Alekseev, 1981; Stuchly, 1986; Tenforde, 1986b).  
A list of applications of magnetic field technologies is given in 
Table 4. 

Table 4.  Magnetic field technologiesa
--------------------------------------------------------
 Energy technologies

    Thermonuclear fusion reactors
    Magnetohydrodynamic systems
    Superconducting magnet energy storage systems
    Superconducting generators and transmission lines

 Research facilities

    Bubble chambers
    Superconducting spectrometers
    Particle accelerators
    Isotope separation units

 Industry

    Aluminium production
    Electrolytic processes
    Production of magnets and magnetic materials

 Transportation

    Magnetically levitated vehicles

 Medicine

    Magnetic resonance
    Therapeutic applications
--------------------------------------------------------
a    From: Tenforde (1986b).

3.2.1.  Magnetic fields in the home and public premises

3.2.1.1  Household appliances

    Some common electrical appliances and the typical magnetic 
fields near them are listed in Table 5.  In a survey of magnetic 
fields around almost 100 different 60-Hz household appliances, 

levels from 0.03 µT to 30 µT were measured at a distance of 30 cm 
from the device (Gauger, 1984).  At approximately 150 cm from the 
appliance producing the highest magnetic field, the level had 
fallen to about 0.5 µT.  Background magnetic field flux densities in 
the homes where the fields from appliances were measured, ranged 
between 0.05 to 1 µT (Tell, 1983; Male et al., 1984; Stuchly, 
1986). 
Table 5.  Magnetic flux densities at 60 Hz near various appliances
in the USAa
---------------------------------------------------------------------------
Appliance                Magnetic flux density (µT) at distance z 
                         z  =  3 cm      z  =  30 cm     z  =  1 m
---------------------------------------------------------------------------
Can openers              1000 - 2000     3.5 - 30        0.07 - 1
Hair dryers              6 - 2000        < 0.01 - 7      < 0.01 - 0.3
Electric shavers         15 - 1500       0.08 - 9        < 0.01 - 0.3
Sabre and circular saws  250 - 1000      1 - 25          0.01 - 1
Drills                   400 - 800       2 - 3.5         0.08 - 0.2
Vacuum cleaners          200 - 800       2 - 20          0.13 - 2
Mixers                   60 - 700        0.6 - 10        0.02 - 0.25
Fluorescent desk lamps   40 - 400        0.5 - 2         0.02 - 0.25
Garbage disposals        80 - 250        1 - 2           0.03 - 0.1
Microwave ovens          75 - 200        4 - 8           0.25 - 0.6
Fluorescent fixtures     15 - 200        0.2 - 4         0.01 - 0.3
Electric ranges          6 - 200         0.35 - 4        0.01 - 0.1
Portable heaters         10 - 180        0.15 - 5        0.01 - 0.25
Blenders                 25 - 130        0.6 - 2         0.03 - 0.12
Television               2.5 - 50        0.04 - 2        < 0.01 - 0.15
Electric ovens           1 - 50          0.15 - 0.5      0.01 - 0.04
Clothes washers          0.8 - 50        0.15 - 3        0.01 - 0.15
Irons                    8 - 30          0.12 - 0.3      0.01 - 0.025
Fans and blowers         2 - 30          0.03 - 4        0.01 - 0.35
Coffee makers            1.8 - 25        0.08 - 0.15     < 0.01
Dishwashers              3.5 - 20        0.6 - 3         0.07 - 0.3
Toasters                 7 - 18          0.06 - 0.7      < 0.01
Crock pots               1.5 - 8         0.08 - 0.15     < 0.01
Clothes dryers           0.3 - 8         0.08 - 0.3      0.02 - 0.06
Refrigerators            0.5 - 1.7       0.01 - 0.25     < 0.01
---------------------------------------------------------------------------
a    From: Gauger (1984).
3.2.1.2  Transmission lines

    The magnetic field beneath high-voltage overhead transmission 
lines is mainly transversed to the line axis (Fig. 2).  The maximum 
flux density at ground level may be under the centre line or under 
the outer conductors, depending on the phase relationship between 
the conductors.  Apart from the geometry of the conductor, the 
maximum magnetic field strength is determined only by the magnitude 
of the current.  The maximum magnetic flux density at ground level 
for a double-circuit 500 kV overhead transmission line system is 
approximately 35 µT per kiloampere.  The field at ground level 
beneath a 765-kV, 60-Hz power line carrying 1 kA per phase is 

15 µT (Scott-Walton et al., 1979).  The magnetic flux density 
decreases with distance from the conductor to values of the order 
of 1 - 10 µT at a lateral distance of about 20 - 60 m from the 
line, as shown in Fig. 2 (Lambdin, 1978; Zaffanella & Deno, 1978). 

Figure 2

3.2.1.3  Transportation

    Several countries are currently designing and testing prototype 
vehicles that are suspended and guided by magnetic forces.  If 
successful, the magnetically-levitated vehicle could offer high-
speed public transportation (roughly 200 - 400 km/h), with greatly 
reduced levels of noise and pollution, compared with conventional 
modes of transportation. 

    The technical problems of magnetic levitation are consid-erable 
and include the presence of large fringe magnetic fields within the 
passenger compartment.  In some designs, the field level at the 
floor of the passenger compartment may be 50 - 100 mT, and 
estimates of the field at the location of a passenger's head range 
from 6 to 60 mT (Hassenzahl et al., 1978).  The magnetic flux 
density within the passenger compartment can be significantly 
reduced by several procedures and it may be possible to achieve a 
5- to 10-fold reduction in the magnetic field levels to which 
passengers are exposed. 

3.2.1.4  Security systems

    Different security systems have been developed for personnel 
identification or for electronic surveillance against theft in 
libraries and shops.  Such devices operate at frequencies ranging 
between 0.1 and 10 kHz.  Identification is achieved when a person 
passes through the coil carrying an identification tag or articles 

bearing a magnetic strip.  The maximum magnetic flux generated by 
the coil is about 1 mT at the ground. 

    The maximum magnetic flux for walk-through metal detectors used 
at airports is below 0.1 mT, and they have frequencies of operation 
below 1 MHz. 

3.2.2.  Magnetic fields in the work-place

3.2.2.1  Industrial processes

    Occupational exposure to magnetic fields comes predominantly 
from working near industrial equipment using high currents.  Such 
devices include various types of welding machine, electroslag 
refining, various furnaces, induction heaters, and stirrers.  
Details of surveys of magnetic field strengths in industrial 
settings are given in Table 6.  Surveys on induction heaters used in 
industry performed in Canada (Stuchly & Lecuyer, 1985), in Poland 
(Aniolczyk, 1981), and in Sweden (Lövsund et al., 1982), show 
magnetic flux densities at operator locations ranging from 0.7 µT 
to 6 mT, depending on the frequency used and the distance from the 
machine.  In their study of magnetic fields from industrial 
electro-steel and welding equipment, Lövsund et al. (1982) found 
that spot welding machines (50 Hz, 15 - 106 kA) and ladle furnaces 
(50 Hz, 13 - 15 kA) produced fields up to 10 mT, at distances up to 
1 m.  In the production of aluminium using a Soderberg cell, the 
final reduction process may lead to static field exposures of about 
40 mT. 
Table 6.  Occupational sources of exposure to magnetic fields
----------------------------------------------------------------------
Source             Magnetic flux     Distance    Reference
                   densities (mT)    (m)
----------------------------------------------------------------------
VDTs               up - 2.8 x 10-4   0.3         Stuchly et al. (1983)

Welding arcs       0.1 - 5.8         0 - 0.8     Lövsund et al. (1982)
(0 - 50 Hz)

Induction heaters  0.9 - 65          0.1 - 1     Lövsund et al. (1982)
(50 - 10 Hz)

50-Hz Ladle        0.2 - 8           0.5 - 1     Lövsund et al. (1982)
furnace

50-Hz Arc          up - 1            2           Lövsund et al. (1982)
furnace

10-Hz Induction    0.2 - 0.3         2           Lövsund et al. (1982)
stirrer

50-Hz Electroslag  0.5 - 1.7         0.2 - 0.9   Lövsund et al. (1982)
welding
----------------------------------------------------------------------

Table 6 (contd.)
----------------------------------------------------------------------
Source             Magnetic flux     Distance    Reference
                   densities (mT)    (m)
----------------------------------------------------------------------
Electrolyte        7.6 (mean)        operator    Marsh et al. (1982)
process                              position
(0 - 50 Hz)

Isotope            1 - 50            operator    Tenforde (1986c)
separation                           position
(static fields)
----------------------------------------------------------------------

    In the course of studies on the health of workers in industries 
using electrolytic processes, Marsh et al. (1982) found that the 
mean static magnetic field level at operator-accessible locations 
was 7.6 mT and the maximum was 14.6 mT.  Time-weighted-average field 
exposures were calculated to be about 4 and 11.8 mT for the mean 
and maximum field levels, respectively. 

    Vyalov (1974) characterized the average magnetic field levels 
to which Soviet workers in permanent magnet production plants were 
exposed.  He found that the static magnetic field at the level of a 
worker's hands was typically 2 - 5 mT.  At the level of the chest 
and head, the field was generally in the range of 0.3 - 0.5 mT. 

3.2.2.2  Energy technologies

    High static magnetic field strengths may be encountered around 
new and developing technologies used for energy production and 
storage, such as magnetohydrodynamic systems, superconducting 
magnetic energy storage systems, and thermonuclear fusion 
(Tenforde, 1986b). 

    The thermonuclear fusion process involves the combination of 
two light nuclei to form a heavier nucleus with a resultant release 
of energy.  Various methods can be used to confine an ignited 
plasma, including high-intensity magnetic fields.  It is now 
generally believed that fields as high as 9 - 12 T will be required 
for the sustained magnetic confinement of an ignited plasma.  
Fringe fields up to 50 mT will exist at locations within the main 
reactor building in areas accessible to operations personnel.  
Although only a limited number of scientists and maintenance 
personnel would normally be expected to enter fields of this 
intensity, it is expected that they will do so for brief periods 
during normal reactor operation. 

    Power generation by magnetohydrodynamic (MHD) separation of 
ionic charges has been studied as a potential means for increasing 
the net power output of a gas- or coal-fired electric power 
facility.  To a first approximation, a typical MHD generator can be 
represented as a magnetic dipole with a large net moment of 
approximately 8000 MA x m2 (Hassenzahl et al., 1978).  The field 

level at a distance of about 50 m from the device would then be 
approximately 10 mT and the field level would fall below 0.1 mT 
only at distances greater than 250 m. 

3.2.2.3  Switching stations and power plants

    Typical values for the magnetic flux density at work-places, 
near overhead lines, in substations, and in power stations (16, 
2/3, 50, 60Hz) range up to 0.05 mT (Krause, 1986). 

3.2.2.4  Research facilities

    Selected groups of workers in research laboratories may be 
exposed to high magnetic field strengths, particularly near bubble 
chambers and particle accelerators. 

    During the last three decades, bubble chambers have played a 
major role in the study of high-energy nuclear reactions.  The 
bubble chamber is contained within a solenoidal magnet operating at 
field levels up to approximately 3 T. 

    At the location where an operator changes the film cassettes, 
the field is estimated to be approximately 0.4 - 0.5 T at foot level 
and about 0.05 T at the level of the head.  The film changing 
procedure requires 5 min to complete, and is carried out 
approximately three times per day, i.e., once per 8-h work shift 
(Tenforde, 1986b). 

    Linear accelerators and synchrotrons have found applications 
in nearly every scientific field, including such areas as high-
energy physics, nuclear chemistry, cancer radiotherapy, and 
isotope production for research and medicine.  The scale of these 
devices ranges from a few metres to several kilometres.  Similarly, 
the focusing and beam extraction magnets used in various 
accelerator designs differ widely in field strengths and in the 
magnetic field profile.  Although high magnetic fields may be 
present near accelerator magnets, personnel are seldom exposed to 
these fields, because of exclusion from the high ionizing radiation 
zone surrounding the beam line. 

3.2.2.5  Video display terminals

    The use of computers with screen-based output units or video 
display terminals (VDT) grows at an ever increasing rate.  VDT 
operators have expressed concerns about possible effects from 
emissions of low-level radiations.  Magnetic fields (frequency 15 - 
125 kHz) as high as 0.69 A/m (0.9 µT) have been measured close to 
the surface of the screen (Bureau of Radiological Health, 1981) 
under worst-case conditions.  This result has been confirmed by many 
surveys (Roy et al., 1984; Repacholi, 1985a).  In a comprehensive 
review of measurements and surveys of VDTs by national agencies and 
individual experts, it was concluded that there are no radiation 
emissions from VDTs that would have any consequences for health 
(Repacholi, 1985a).  There is no need to perform routine radiation 
measurements since, even under worst-case conditions, the emissions 
are well below any international or national standards. 

3.3.  Magnetic Fields in Medical Practice

3.3.1.  Diagnosis, magnetic resonance imaging, and metabolic studies

    Magnetic resonance (MR) imaging used for diagnostic purposes 
involves both static and time-varying magnetic fields.  MR imaging 
applied to living tissues provides a promising new technique for 
medical imaging with high spatial resolutions (Budinger & 
Lauterbur, 1984).  In this technique, nuclear magnetic moments are 
aligned by the application of a static magnetic field (Bo), and 
undergo a precessional motion around the field direction with a 
Larmor frequency characteristic of each nucleus (section 2.3.3).  
When a radiofrequency (RF) field with a matching frequency is 
applied transverse to the direction of Bo, a resonant energy 
absorption occurs.  The return of the magnetic spin state to 
equilibrium following resonant energy absorption is characterized 
by two relaxation times, T1 and T2.  The T1 parameter is called the 
spin-lattice relaxation time, and reflects the local temperature 
and viscosity in the vicinity of the magnetic nuclei.  The T2 
parameter is called the "spin-spin" relaxation time, and reflects 
the local magnetic field resulting from the nuclear moments of 
neighbouring nuclei.  Both the T1 and T2 relaxation times provide 
information that can be converted into contrast differences in NMR 
images of tissue-proton density.  The intensity of the radiated 
signal reflects the tissue concentration of magnetic nuclei such as 
protons, 13C, 23Na, 31P, and 39K.  The selective detection of 
different magnetic nuclei is possible, because of their different 
characteristic resonant frequencies at a given magnetic field 
strength. 

    The decay of a MR signal occurs with a characteristic time 
variation that conveys detailed information about the local 
environment of the magnetic nuclei.  In proton MR images, large 
contrast differences can be observed between regions of tissue that 
have significantly different water or lipid contents.  Various MR 
imaging methods have been developed that are able to demonstrate 
differences between normal and pathological regions of the same 
tissue (Crooks & Kaufman, 1983).  Principles and applications of 
magnetic resonance techniques in medicine can be found in Mansfield 
& Morris (1982), Foster (1984), and Mathur (1984). 

    In addition to use as an imaging technique, MR spectroscopy 
based on 13C and 31P signals can provide unique information on 
tissue metabolism.  For example, 31P MR spectroscopy has been shown 
to give quantitative information on phosphate metabolism in the 
heart, liver, kidney, brain, and muscle tissue. 

    The present generation of MR imaging devices, used in clinical 
practice, employ stationary magnetic fields with intensities 
ranging from 0.3 T to about 2 T and RF fields with frequencies up 
to 100 MHz (the proton resonant frequency in a 2 T field is 85.15 
MHz).  In addition, weak spatial gradients of the stationary 
magnetic field (about 0.001 T/m) are used to define the tissue 
location of MR signals.  The gradient direction is rapidly switched 
from one projection axis to the next in order to reconstruct the 

entire image of the specimen.  These rapidly switched gradient 
fields produce a time-varying magnetic field within the tissue 
volume.  In the MR imaging devices that are currently in existence, 
the maximum time rate of change of the magnetic field is normally 
about 1.5 T/second, but may be considerably higher in a few 
specialized devices. 

    The feasibility of using static magnetic fields with strengths 
greater than 2T is being explored as a means of increasing the 
signal-to-noise ratio in MR images.  In addition, the use of higher 
fields could significantly reduce the time required to obtain 
chemical shift images, which provide high-resolution information on 
the spatial distribution of 31P nuclei and protons associated with 
tissue water and fat. 

3.3.2.  Therapy

    Patients suffering from bone fractures that do not heal well or 
unite have been treated with pulsed magnetic fields (Bassett et 
al., 1974, 1977, 1982; Mitbreit & Manyachin, 1984).  Studies are 
also being conducted on the use of pulsed magnetic fields to 
enhance wound healing and tissue regeneration. 

    Various devices generating magnetic field pulses are used for 
bone growth stimulation.  A typical example is the device that 
generates an average magnetic flux density of about 0.3 mT, a peak 
strength of about 2.5 mT, and induces peak electric field strengths 
in the bone in the range of 0.075 - 0.175 V/m (Bassett et al., 
1974).  Two different pulse patterns are used: a quasi-rectangular 
pulse of 250 - 400 µs duration with a secondary pulse of opposite 
polarity of 20 µs width, and a repetition rate of 40 - 77 Hz; and a 
train of pulses with a duration of 2.5 ms and a repetition rate of 
5 - 20 Hz (Fig. 3).  Near the surface of the exposed limb, the 
device produces a peak magnetic flux density of the order of 1.0 mT 
causing peak ionic current densities of about 10 to 100 mA/m2 (1 to 
10 µA/cm2) in tissue.  These ionic currents perturb cell function, 
even though most of the current flows around the cell  in the 
extracellular space (Pilla, 1979; Beltrame et al., 1980; Pilla et 
al., 1983).  Applications of magnetic field devices in medicine are 
rapidly expanding.  Further information can be obtained in the 
monograph edited by Bistolfi (1983).  Magnetic fields are being 
widely used in the USSR for various therapeutic applications 
(Bogolyubov, 1981). 

FIGURE 3

4.  MECHANISMS OF INTERACTION

    A broad spectrum of interaction mechanisms can occur between 
magnetic fields and living tissue.  At the level of macromolecules 
and larger structures, interactions of stationary magnetic fields 
with biological systems can be characterized as electrodynamic or 
magnetomechanical in nature.  Electrodynamic effects originate 
through the interaction of magnetic fields with electrolyte flows, 
leading to the induction of electrical potentials and currents.  
Magnetomechanical phenomena include orientational effects on 
macromolecular assemblies in homogeneous fields, and the 
translation of paramagnetic and ferromagnetic molecular species in 
strong gradient fields.  Magnetic fields that are time-varying also 
interact with living tissues at the macroscopic and microscopic 
levels to produce circulating currents via the mechanism of 
magnetic induction.  The theory behind each of these interaction 
mechanisms will be described in this section. 

    At the atomic and subatomic levels, several types of magnetic 
field interactions have been shown to occur in biological systems 
(Cope, 1971, 1973, 1978, 1981).  Two such interactions are the 
nuclear magnetic resonance in living tissues described earlier and 
the effects on electronic spin states and their relevance to 
certain classes of electron transfer reactions described in this 
section. 

    Other interaction mechanisms that are being studied at the 
present time are discussed at the end of this section.  Recent 
reviews of the theoretical bases for magnetic field interactions 
include those of Bernhardt (1979, 1986), Schulten (1982, 1986), 
Pirusyan & Kuznetsov (1983), Abashin & Yevtushenko (1984), Swicord 
(1985); Tenforde (1985a,c, 1986a,d), Kaune (1985), Frankel (1986), 
and Tenforde & Budinger (1986). 

4.1.  Static Magnetic Fields

4.1.1.  Electrodynamic and magnetohydrodynamic interactions

    Steady flows of ionic currents interact with applied stationary 
magnetic fields via the well known Lorentz force law (equation 3): 

                     F = q (v x B)                         (3)

where F is the net force exerted on a charge q moving with velocity 
v, and B is the magnetic flux density.  The term v x B represents a 
vector cross-product.  In the case of electrolytes flowing through 
channels (e.g., blood vessels), the interaction of an applied 
magnetic field with ionic charge carriers under steady-state 
conditions will result in a local force on the charge carriers of 
magnitude q v B sin THETA where THETA is the angle between the 
direction of charge motion and the magnetic field.  This force will 
be perpendicular to both the magnetic field and the direction of 
current flow, i.e., the induced field, Ei, is transverse to both 
v and B.  This phenomenon, which is the basis of the Hall effect in 
solid state materials, is also relevant to biological processes that
involve electrolyte flow.

    An interesting example of the role of magnetically-induced 
electrical potentials in a biological system is the geo-magnetic 
direction-finding mechanism used by elasmobranch fish, including 
the shark, skate, and ray (Kalmijn, 1974, 1978, 1981, 1984; Ilinsky 
& Brown, 1985).  The heads of these animals contain long jelly-
filled canals known as the ampullae of Lorenzini, which have a high 
electrical conductivity similar to that of seawater.  As the fish 
swims through the earth's magnetic field, a small voltage gradient 
is induced in the canals, which is detected by the sensory 
epithelia lining the terminal ampullary region.  The induced 
electric field, which can be detected at levels as low as 0.5 µV/m 
(Kalmijn, 1982), has a distinct polarity that is dependent on the 
relative orientation of the geomagnetic field direction of swimming. 
In this way, the marine elasmobranchs use the -(v x B) fields 
induced in their ampullary canals as a directional compass. 

    A second example of induced electric potentials is provided by 
blood flow in the presence of an applied static magnetic field.  
For the specific case of a cylindrical vessel with a diameter (d) 
and the local electric field strength (Ei), the magnitude of the 
induced potential PSI, is given by equation 4: 

          PSI =   |Ei|d  = |v||B|d sin THETA                 (4)

    The existence of magnetically-induced blood flow potentials in 
the central circulatory systems of several species of mammals has 
been demonstrated experimentally.  These induced potentials can be 
conveniently studied from electrocardiogram (ECG) records obtained 
with surface electrodes.  The ECG signal in the T-wave region shows 
a substantial augmentation in the presence of magnetic fields and 
this phenomenon is completely and immediately reversible on 
termination of the exposure.  Based on its temporal sequence in the 
ECG record, the increased amplitude of the T-wave in magnetic 
fields has been attributed to the superposition of an induced 
potential associated with pulsatile blood flow into the aortic 
vessel.  This effect is illustrated in Fig. 4 and discussed in more 
detail in section 5.  The occurrence of a change in the ECG is an 
excellent example of a physical effect of an applied magnetic field 
that does not result from a biological response to the field. 

    For a man with a peak blood flow rate of 0.63 m/s and an aortic 
diameter of 0.025 m, the predicted maximum value of the aortic flow 
potential is 16 mV per tesla (Mansfield & Morris, 1982).  The 
actual potential across the cardiac muscle fibres would be much 
smaller, so that the threshold change in cardiac potential required 
to initiate depolarization of cardiac muscle may not be reached, 
even in magnetic fields of a few tesla.  However, the induced 
potential differences can be significant in cases where the 
excitation stimulation or conduction of excitation is impaired.  
This is one of the reasons why, in recommendations for the safe 
medical use of magnetic resonance equipment where a field of more 
than 2 T is used, monitoring of cardiac and circulatory function of 
the patient is recommended (Bernhardt & Kossel, 1984, 1985). 

FIGURE 4

    Potential differences may also be induced by moving cross 
sections in a magnetic field, e.g. by cardiac contractions.  In 
theory (Bernhardt & Kossel, 1984), a field strength of 0.1 V/m or a 
current density of about 10 - 20 mA/m2 per tesla is induced in 
cardiac muscle.  A large current density in the vicinity of the 
heart may cause ventricular fibrillation.  However, the threshold 
magnetic field strengths for the induction of effects on cardiac 
function, including alteration of excitation or impulse conduction, 
are not known. 

    Another biological process involving ionic flows that are 
subject to electrodynamic interaction with an applied magnetic 
field is the conduction of electrical impulses in nerve tissue.  
Wikswo & Barach (1980) have calculated that a magnetic field 
strength of 24 T could produce a deflecting force on nerve ionic 
currents equal to one tenth of the force that they experience from 
interaction with the electric field of the nerve membrane.  A 
theoretical model suggests that magnetic fields with flux densities 
of 2 T or less should not produce any measurable change in the 
conduction velocity of nerve impulses.  This conclusion is supported 
by experimental data (section 5). 

    Theoretical analyses of magnetic field interactions with nerve 
ionic currents have also been made by Valentinuzzi (1965) and 
Liboff (1980).  Liboff has raised the interesting question of 
whether time variations in the magnetic flux linkage with ion 
current loops along the nerve membrane could lead to significant 
induced potentials.  Due to the rotational symmetry of the nerve 
axon, it is expected that these induced electrical fields would 
cancel.  However, for the unlikely condition of highly asymmetric 
current loops, Liboff (1980) suggests that applied fields of less 

than 1 T could theoretically introduce significant perturbations in 
the membrane current flows during impulse conduction.  At present, 
there are not sufficient data to test this hypothesis. 

    Theoretically, intracellular ionic fluxes are also susceptible 
to magnetic field interaction, but there is little experimental 
information relating to this possibility (Czerski, 1986). 

4.1.2.  Magnetomechanical effects

4.1.2.1  Orientation of diamagnetically anisotropic macromolecules

    A large number of diamagnetic biological macromolecules exhibit 
orientation in strong magnetic fields.  In general, these 
macromolecules have a rod-like shape, and magneto-orientation 
occurs as a result of an anisotropy in the magnetic susceptibility 
tensor (x) along the different axes of rotational symmetry.  The 
magnetic moment per unit volume (M) of these molecules in a field 
with intensity H is equal to xH.  The theoretical calculation of 
the interaction energy per unit volume has been discussed by 
Tenforde (1985a) and Frankel (1986).  The rod-like molecules will 
rotate to achieve a minimum energy in the applied magnetic field.  
For individual macromolecules, the magnetic interaction energy 
predicted theoretically will be small compared to the thermal 
interaction energy kT, unless enormous field strengths are used.  
This fact has been demonstrated for DNA solutions in which the 
extent of magneto-orientation has been studied from measurements of 
magnetically-induced bire-fringence (the Cotton-Mouton effect).  
Measurements on calf thymus DNA (Maret et al., 1975; Maret & 
Dransfeld, 1977), resulted in a degree of orientation of only 1% in 
an applied field of 13 T. 

    Despite the weak interaction of individual macromolecules with 
intense magnetic fields, there are several examples of 
macromolecular assemblies that exhibit orientation in fields of 1 T 
or less.  This phenomenon results from a summation of the 
diamagnetic anisotropies of the individual molecules within the 
assembly, thereby giving rise to a large effective anisotropy and 
magnetic interaction energy for the entire molecular aggregate.  
Examples of biological systems that exhibit orientation in fields 
of 1 T or less are retinal rod outer segments (Chalazonitis et al., 
1970; Hong et al., 1971; Vilenchik, 1982), photosynthetic systems 
such as chloroplast grana, photosynthetic bacteria, and  Chlorella  
cells (Geacintov et al., 1971, 1972; Becker et al., 1973, 1978a,b; 
Breton, 1974), purple membranes of Halobacteria (Neugebauer et al., 
1977), muscle fibres (Arnold et al., 1958), and "sickled" 
erythrocytes (Murayama, 1965).  A more detailed discussion can be 
found in Maret & Dransfield (1985). 

    Several of the physical principles underlying magneto-
orientation phenomena have been experimentally demonstrated for 
retinal rod outer segments.  The first observation that isolated 
rod outer segments, which consist of pigmented disc membranes 
stacked in a regular array, will orient in a 1 T stationary 

magnetic field was made in 1970 (Chalazonitis et al., 1970).  The 
oriented segments are aligned with the disc membranes perpendicular 
to the applied field direction, which indicates that magneto-
orientation results from the large summed diamagnetic anisotropy of 
the rhodopsin photopigments, as opposed to the lamellar membrane 
phospholipids (Hong, 1980; Becker et al., 1978b).  An estimate of 
the summed anisotropy for the rod outer segments can be obtained by 
observing the kinetics of the magneto-orientation process.  The time 
for rods to rotate by 90° is predicted to be approximately 4 
seconds in a 1.0 T field (Hong et al., 1971; Hong, 1977, 1980), and 
this value agrees well with experimental observations on the 
kinetics of rod orientation (Chagneux & Chalazonitis, 1972; 
Chagneux et al., 1977).  It should be noted that the slow 
orientational response of rod outer segments to an applied magnetic 
field makes this mechanism an unlikely basis for the 
magnetophosphene phenomenon that is observed in time-varying 
fields. 

4.1.2.2  Orientation of organisms with permanent magnetic moments

    An interesting specimen for the biophysical study of magnetic 
field interactions was provided by Blakemore's accidental discovery 
of magnetotactic bacteria (Blakemore, 1975).  Approximately 2% of 
the dry mass of these aquatic organisms is iron, which has been 
shown by Mossbauer spectroscopy to be predominantly in the form of 
magnetite (Fe3O4) (Frankel et al., 1979).  The magnetite inclusions 
are arranged as chains of approximately 20 - 30 single domain 
crystals.  The orientation of the net magnetic moment is such that 
magnetotactic bacteria in the northern hemisphere migrate towards 
the north pole of the geomagnetic field, whereas strains of these 
bacteria that grow in the southern hemisphere move towards the 
south magnetic pole (Blakemore et al., 1980; Rosenblatt et al., 
1982a,b).  Magnetotactic bacteria that have been found at the 
geomagnetic equator are nearly equal mixtures of south-seeking and 
north-seeking organisms (Frankel et al., 1981).  Because of the 
polarities of their magnetic moments, the magnetotactic bacteria in 
both the northern and southern hemispheres migrate downwards in 
response to the vertical component of the geomagnetic field.  It 
has been proposed that this downward directed motion, which carries 
the bacteria into the bottom sediments of their aquatic environment, 
is essential for the survival of these microaerophilic organisms 
(Blakemore, 1975; Frankel et al., 1979).  This phenomenon is an 
interesting example of an interaction between a physical response 
to a magnetic field and biological environmental adaptation and 
selection processes. 

4.1.2.3  Translation of substances in a magnetic field gradient

    A material with a net magnetic moment will experience a force 
in a magnetic field gradient (spatially non-uniform magnetic 
field).  As a result of this force, paramagnetic and ferromagnetic 
materials will migrate along the direction of the magnetic field 
gradient. 

    One of the interesting applications of the magnetomechanical 
force exerted by a magnetic field gradient is the differential 
separation of erythrocytes from whole blood (Melville et al., 1975; 
Paul et al., 1978).  In this procedure, deoxygenated erythrocytes, 
in which the haemoglobin is paramagnetic, are attracted to a wire 
mesh with a strong gradient field and thereby separated from other 
classes of blood cells.  Magneto-mechanical forces are also applied 
in: surgical traumatology to fix skeleton elements in specific 
position (Yarovitsky, 1986); in designs of various intestinal and 
oesophageal valves; and for the accumulation of drugs (in compounds 
with ferromagnetics) in specific parts of the human body.  In 
ophthalmology, strong constant magnets are applied to extract 
foreign (ferromagnetic) objects. 

    An important safety consideration is the displacement of 
metallic inclusions or implants in human beings exposed to strong 
magnetic field gradients, as this could pose a health risk (New et 
al., 1983). 

4.1.3.  Effects on electronic spin states

    A number of organic reaction processes that involve electron 
transfer via radical pair intermediates are highly sensitive to 
magnetic field interactions.  A well-studied example that is 
biologically relevant is the photo-induced charge transfer reaction 
that occurs in bacterial photosynthesis (Blankenship et al., 1977; 
Werner et al., 1978; Haberhorn & Michel-Beyerle, 1979; Michel-
Beyerle et al., 1979; Hoff, 1981; Ogrodnik et al., 1982).  Within 
10 picoseconds (ps) following excitation of bacteriochlorophyll 
(BChl) to its first excited singlet state, a radical pair 
intermediate state is formed that consists of a (BChl)+2 cationic 
dimer and a bacteriopheophytin (BPh)- anion.  Within 200 ps, 
electron transfer occurs to the ultimate acceptor, an ubiquinone-
iron complex.  However, if the acceptor molecule is chemically 
reduced, the lifetime of the radical pair intermediate state 
increases to approximately 10 nanoseconds (ns).  With an extended 
lifetime, hyperfine interactions between the nuclear and electron 
spin magnetic moments lead to an interconversion of the radical 
pairs between the singlet and triplet states.  Under this condition, 
the intermediate state decays directly back to the singlet ground 
state, or decays via a metastable triplet state.  Because of the 
weakness of the hyperfine interaction, the triplet states are 
nearly degenerate and the electron spins of the radical pair 
intermediate can move with nearly equal probabilities between the 
singlet So and the triplet To and T±1 states.  However, in the 
presence of an applied magnetic field that exceeds approximately 10 
mT, the resulting Zeeman interaction with the radical electron 
spins will lift the degeneracy of the triplet state and effectively 
block the T±1 triplet channels.  Theoretically, the yield of 
triplet product should be reduced by two thirds in the presence of 
the external field, and this has been confirmed experimentally by 
laser pulse excitation and optical absorption measurements (Michel-
Beyerle et al., 1979). 

    In considering the biological implication of these studies, it 
should be kept in mind that the ultimate electron acceptor 
molecules have been altered by chemical reduction and such 
conditions do not normally occur in nature.  However, the 
possibility cannot be excluded that similar phenomena may occur in 
other radical-mediated biological processes under normal 
conditions.  It has been proposed, for example, by Schulten et al. 
(1978), that an anisotropic Zeeman interaction with a radical 
mediated reaction system could provide a basis for geomagnetic 
direction finding. 

    Magnetic field effects on organic chemical reactions in which 
the splitting and subsequent recombination of a non-excited singlet 
molecule involves a radical pair as a short-lived intermediate 
stage have also been well documented (Molin et al., 1979; 
McLauchlan, 1981).  As described above, the effect of the magnetic 
field is on the singlet-triplet transition rate of the radical 
pair, thus affecting the relative proportion of recombinant and 
escape products, by up to 30% in some cases.  The magnitude of the 
response depends on the difference in the magnetic properties of 
the two radical intermediates, and will be particularly enhanced in 
reactions involving transition metals such as iron (Molin et al., 
1979).  These effects also increase with the lifetime of the 
radical pair.  This is typically 100 ps - 100 ns in solution, but 
is longer when the reacting molecule is held in a micellar "cage" 
or bound to an enzyme. 

    A number of enzyme reactions involving radical intermediates 
have been identified (Saunders & Cass, 1983), though the evidence 
is tenuous because they are generated and react at the enzyme 
active site and their presence can only be inferred by indirect 
methods.  Enzymes, the action of which may involve radical 
intermediates, are: 

   (a)   Cytochrome P-450:  A class of haem-containing enzyme
        involved in drug metabolism and steroid hydroxylation;

   (b)   Lipoxygenase:  A non-haem iron enzyme that is a key
        enzyme in prostaglandin and thromboxane synthesis; and

   (c)   Cyclo-oxygenase:  The enzyme involved in converting
        arachadonic acid to prostaglandins.

These enzymes all contain iron and use oxygen (O2) as one of the 
substrates.  They can be expected to be sensitive to magnetic 
fields, if the radical recombination is rate determining, i.e., if 
the radical is relatively long-lived. 

4.2.  Time-Varying Magnetic Fields

    In accordance with Faraday's law, magnetic fields that vary in 
time will induce potentials and circulating currents in biological 
systems. 

                   pi r2     dB           sigma r  dB
   J = E sigma =   ------- x -- x sigma = -------  --          (5)
                   2 pi r    dt           2        dt

    where J = current density (A/m2)
          E = induced potential (V/m)
          r =  radius of the inductive loop (m)
          sigma = tissue conductivity (S/m)
          dB = rate of change of magnetic flux density
          dt

For sinusoidal fields of frequency f, equation (5) reduces to:

          J = pi r f sigma Bo,

          where Bo is the magnetic field amplitude

Thus, the magnitude of the induced electric fields and current 
densities is proportional to the radius of the loop, the tissue 
conductivity, and the rate of change of magnetic flux density. 

    The dependence of the induced field and current on the radius 
of the loop through which magnetic flux linkage occurs is an 
important consideration for biological systems.  Time-varying 
fields of modest strength may induce significant circulating 
currents at the macroscopic level, but substantially smaller 
currents at the cellular level. 

    An important factor to be considered in the response of 
biological systems to a time-varying magnetic field is the 
waveform.  Many different types of magnetic field waveform are used 
in practice, including sinusoidal, square-wave, saw tooth, and 
pulsed fields.  For these fields, the two parameters of key 
importance are the rise and decay times of the signal, which 
determine the maximum time rates of change of the field, (dB/dt), 
and hence the maximum instantaneous current densities induced in 
tissues.  These also depend on tissue conductivity, which is 
frequency dependent and differs between tissues. 

    Luben et al. (1982) and Cain et al. (1984) demonstrated  in 
 vitro that pulsed magnetic fields, generated in pulse trains (72 
Hz) or recurrent bursts (15 Hz), blocked the response of mouse 
osteoblasts to parathyroid hormone.  The effects seemed to be 
mediated at the cell membrane by blocking receptor-adenylate 
cyclase coupling in the membrane (Cain et al., 1985).  The 
adenylate cyclase and cyclic AMP systems are part of the hormone 
response amplification system.  These effects were associated with 
current densities of 10 - 100 mA/m2 and electric field strengths of 
0.1 - 1 V/m in extracellular fluids.  Effects at the cell membrane 
receptor level seem to be involved in the effects of 450 MHz fields 
sinusoidally modulated at ELF frequencies on T-lymphocyte cytotoxic 
functions (Lyle et al., 1983). 

    Numerous effects resulting ELF electric fields in cells and 
tissues, induced by pulsed magnetic fields, have been described.  
These effects include stimulation of bone growth, nerve and limb 
regeneration, cell differentiation, effects on ionic fluxes and on 
DNA, RNA, and protein synthesis (Sheppard, 1985).  Experimental 
data seem to indicate that the site of the primary interaction is 
the cell membrane and proposed mechanisms presume a role of the 
induced electric field.  Effects on gene expression, such as the 
initiation and alteration of transcription (Goodman et al., 1983; 
Goodman & Henderson, 1986), or effects on  Escherichia coli lac 
operon (Aarholt et al., 1982) may be mediated through interaction 
with the genetic apparatus (chromosomes).  Effects on cell membrane 
and/or gene expression may be responsible for abnormalities in 
chick embryo development described by Delgado et al. (1982) and 
Ubeda et al. (1983).  However, the latter studies have not been 
confirmed by Maffeo et al. (1984). 

    A well-documented biological effect of time-varying magnetic 
fields is the occurrence of magnetophosphenes.  Various 
investigations leading to the elucidation of this phenomenon are 
summarized in Table 7.  First observed by d'Arsonval (1896), 
magnetophosphenes are detected as a sensation of flickering light 
induced in the eye, when it is exposed to magnetic fields with flux 
densities greater than about 10 mT and frequencies greater than 10 
Hz.  The minimum field strength required to produce visual 
phosphenes (Fig. 5) occurs at a frequency of 20 Hz (Barlow et al., 
1947b; Lövsund et al., 1979, 1980a,b; Tenforde & Budinger, 1986).  
There is evidence (Lövsund et al., 1981) to suggest that the time-
varying magnetic field effect occurs in the photoreceptors rather 
than in the post-synaptic neurons.  Furthermore, Lövsund et al. 
(1980a,b, 1981) concluded from their studies on volunteers that the 
mechanisms of underlying magnetically and electrically-induced 
phosphenes are possibly the same. 

    The idea that the magnetically-induced electric field strength 
(and hence current density) in tissues is the physical quantity 
determining the biological effects at the cellular level has been 
pursued by Bernhardt (1979, 1985).  He used electrophysiological 
data in order to find "safe" and "hazardous" current densities and 
to define corresponding magnetic field strengths.  The problem is 
the correlation of the internal current densities with the external 
magnetic field strengths.  Bernhardt (1985) concluded that, using 
his calculations, it was possible to estimate the current density 
levels within one order of magnitude.  Satisfactory agreement of 
theoretical predictions with data on the threshold for 
magnetophosphene perception in human volunteers was obtained 
(Bernhardt, 1985).  This was established to be between 2 and 10 mT 
(for frequencies greater than 10 Hz). 

Table 7.  Magnetophosphene studies
---------------------------------------------------------------------------
Reference            Principal findings
---------------------------------------------------------------------------
d'Arsonval (1896)    Initial report of magnetophosphenes produced by a
                     42-Hz field

Thompson (1909-10)   Described magnetophosphenes produced by a 50-Hz
                     field as a colourless, flickering illumination that
                     is most intense in the peripheral region of the eye

Dunlap (1911)        Demonstrated that magnetophosphenes produced by a
                     25-Hz field are more intense than those produced by
                     a 60-Hz field of comparable intensity

Magnusson &          Demonstrated the production of magnetophosphenes by
Stevens (1911-12)    pulsed DC fields as well as by time-varying fields
                     with frequencies from 7 to 66 Hz; observed strongest
                     magnetophosphenes with fields oscillating at 20 - 30
                     Hz
Barlow et al.        Demonstrated threshold field intensity of 20 mT
(1947a,b)            (rms) at 30 Hz, and showed that the threshold for
                     magnetophosphenes is relatively insensitive to back-
                     ground illumination compared with that for electro-
                     phosphenes; characterized "fatigue" phenomenon with
                     a 60 Hz magnetic field applied for 1 min, which was
                     followed by a refractory period of 40 s, during
                     which a second phosphene could not be elicited;
                     demonstrated that magnetic fields must be applied in
                     the region of the eye to produce phosphenes, and
                     that sensitivity is abolished by pressure applied to
                     the eyeball

Seidel et al.        Observed comparable light patterns associated with
(1968)               visual stimulation by ELF electric and magnetic
                     fields, but found different probabilities of occur-
                     rence of certain types of phosphene patterns

Lövsund et al.       Analysed threshold field intensity for production of
(1979-81)            magnetophosphenes over frequency range of 10 - 45
                     Hz; demonstrated maximum sensitivity to a 20-Hz
                     field; studied effects of dark adaptation, back-
                     ground illumination, and visual defects on sensi-
                     tivity to magnetophosphenes; compared threshold
                     stimuli required to produce electrophosphenes and
                     magnetophosphenes; characterized changes in electro-
                     physiological responses of isolated frog retinas
                     exposed to ELF magnetic fields
---------------------------------------------------------------------------

Table 7.  (contd.)
---------------------------------------------------------------------------
Reference           Principal findings
---------------------------------------------------------------------------
Silny (1981);        Found minimum time rate of change for magneto-
Bernhardt (1985)     phosphenes in sinusoidal fields at 17 Hz to be
                     0.3 T/s

Budinger et al.      Found minimum time rate of change of pulsed magnetic
(1984a)              field to be 1.3 - 1.9 T/s to produce magneto-
                     phosphenes
---------------------------------------------------------------------------

FIGURE 5

    Another potentially important target of ELF magnetic field 
interactions is the nervous system.  From a consideration of the 
naturally occurring fields in the central nervous system, Bernhardt 
(1979) concluded that magnetic fields in the 1 - 100 Hz frequency 
range, which can induce current densities in tissue of approximately 
1 mA/m2 or less, should not have a direct effect on the brain's 
electrical activity.  The strength of a 60-Hz magnetic field that 
would induce a peak current density of this magnitude in the 
cranium of a human subject was calculated to be about 0.5 mT 
(Tenforde, 1985a). 

    In a careful study on human perception to 60 Hz magnetic 
fields, Tucker & Schmitt (1978) did not find any significantly 
perceptive individuals among more than 200 subjects exposed to a 
field with an amplitude of 2.1 mT.  Several behavioural tests on 

mice exposed to a 60-Hz magnetic field that induced a peak current 
density approaching 1 mA/m2 in the peripheral cranial region also 
yielded negative findings (Davis et al., 1984).  The results of 
these studies suggest that ELF magnetic fields must have 
significantly greater amplitudes than the theoretically calculated 
threshold values in order to perturb animal behaviour.  However, it 
is important to recognize the inherent deficiencies of a simple 
theoretical model that treats the central nervous system as a 
region of uniform conductivity.  In addition, the induced current 
in a loop of maximum radius at the brain's surface may not be the 
relevant parameter to consider in predicting the response to ELF 
magnetic fields.  The regions of the central nervous system that 
might be responsive to these fields may have significantly smaller 
dimensions than the entire cranium.  Thus, a large increase in the 
ELF magnetic field strength would be necessary to evoke a 
measurable electrical and/or behavioural perturbation. 

    It is reasonable to suppose that these effects result from the 
interaction of the induced electric fields and currents with the 
membranes of nerve and muscle cells, thereby causing changes in the 
electrical excitability of these cells in the same way as naturally 
occurring or directly applied electric fields. 

    The permeability to ions of the nerve (and muscle) cell 
membranes depends on the membrane potential.  It is this voltage-
dependent permeability that gives the cells the property of being 
electrically excitable.  When an electric field is applied, various 
charged side-groups of certain proteins embedded in the membrane 
change their configuration, thereby causing a larger structural 
change in the protein as a whole.  In this new conformation, ions 
are able to pass through the membrane by binding temporarily with 
the protein molecule at various sites, thus "hopping" through the 
membrane.  In any area of a membrane, there are a large number of 
"gating" molecules, and the effect of an induced electric field may 
involve an alteration in the proportion of gates that are open.  
This type of interaction could significantly influence membrane 
permeability. 

    In addition, there is a specific type of protein molecule for 
each species of ion, permitting different ionic responses to the 
same electric field.  Thus, in response to a depolarizing electric 
field, there is a large increase in Na+ permeability in the 
membrane of a nerve cell tending to depolarize the cell further.  
This event is followed by a slower change in K+ permeability and an 
inactivation of the Na+ channel, resulting in a repolarization.  
Induced fields sufficient to exceed a threshold depolarization 
value can result in an action potential that is capable of 
stimulating other excitable cells.  These effects are well 
understood.  ELF magnetic fields inducing such large depolarizations 
may result in nerve stimulation or muscle contraction, or even in 
fibrillation.  ELF magnetic fields inducing weak electric fields 
may also interact with, or modulate, nervous system activity in a 
manner that is less well understood.  However, these interactions 
can produce changes in electrical excitability.  Such interactions 
may be involved in, for example, magneto- or electrophosphenes. 

    The ELF field interactions described above exhibit frequency-
dependent thresholds characteristic of nervous tissue, and have 
been well documented by Bernhardt (1979, 1985).  This frequency 
dependence is very important when relating experimental results 
obtained using high frequencies or very short pulses to effects 
anticipated at 50/60 Hz, at least as far as acute responses are 
concerned.  The main factors governing this dependence are 
accommodation and ionic mobility.  As a result, there is a 
characteristic U-shaped dependence of threshold current density on 
frequency, with the lowest values for most nervous tissues 
occurring between 10 Hz and 100 kHz.  At low frequencies, the 
effects of accommodation predominate, which is thought to be 
related to the slow inactivation of the Na+ channel.  At higher 
frequencies, the time available during each cycle for ions to 
migrate across the membrane, an all or nothing event, becomes 
limiting; direct electrical excitation gives way to heating 
somewhere between 100 Hz and 300 kHz. 

    It should be noted that the theoretically calculated field 
intensities at 20 Hz for stimulating the visual system are only 
slightly lower than the perception threshold for magnetophosphenes 
(Bernhardt, 1985).  With regard to "hazardous values" and the upper 
limit of the field strength that leads to injury, the ultimate 
criterion for the definition of injury may be the initiation of 
heart fibrillation.  The threshold for extra-systole induction at 
60 Hz is estimated to be above 300 mT for stimulation times of 1 
second or longer, and the threshold for ventricular fibrillation is 
higher by a factor of 3 - 5 (Bernhardt, 1985).  For shorter 
exposure times, higher field strengths are necessary to produce 
similar biological effects. 

    Silny (1986) measured the stimulation threshold of the heart in 
8 dogs exposed to time-varying magnetic fields.  He converted the 
thresholds found in dogs to the equivalent thresholds expected in 
human beings.  From his data, the fibrillation threshold for the 
human heart was estimated to be 1 T at 50 Hz, for magnetic fields 
acting perpendicular to the body axis. 

4.3.  Other Magnetic Field Interactions Under Study

    The transduction mechanism for ELF magnetic fields described in 
section 4.2 is supported by experimental data for electrically 
excitable tissues.  For other biological effects observed with ELF 
fields that induce smaller current densities (below the level that 
could significantly affect the cell membrane potential), other 
transduction mechanisms have been proposed.  For example, changes 
in cell-surface receptor molecules and in ion binding to membrane 
surfaces have been reported to occur, as a result of exposure to 
ELF magnetic fields.  It has been proposed that the pericellular 
currents induced by an ELF field may produce electrochemical 
alterations in components of the cell membrane surface.  These 
events, in turn, send signals across the cell membrane barrier that 
produce alterations in intracellular biochemical and physiological 
functions.  This hypothesized scheme of transductive coupling 

between induced electric currents in the extracellular medium and 
the intracellular events occurring in living cells is illustrated 
schematically in Fig. 6. 

FIGURE 6

    The key element in the sequence of events through which 
externally applied ELF fields influence cellular properties is the 
transductive signalling event within the cell membrane.  Numerous 
theoretical models have been proposed for the transmembrane 
signalling process (or processes) that are triggered by induced 
pericellular electric currents (Adey, 1980, 1981, 1983).  In the 
broadest sense, these hypothesized mechanisms can be grouped into 
two general classes: 

   (a)  long-range cooperative phenomena established within
        the matrix of glycoproteins and lipoproteins that
        constitute the cell membrane; and

   (b)  localized events occurring at specific ligand-binding
        sites (receptors) at the outer membrane surface, or
        events occurring within ion-selective channels that
        span the membrane and electrically couple the
        intracellular and extracellular fluids.

These classes of phenomena are depicted by the boxes at the left 
and right sides of Fig. 6 and will be discussed separately. 

4.3.1.  Long-range cooperative phenomena in cell membranes

    The electric fields induced in tissue by externally-applied 
low-amplitude ELF electromagnetic fields are several orders of 
magnitude less than the voltage gradient that exists across the 
living cell membrane.  It has therefore been proposed that the 
cellular response to external ELF fields may involve an 
amplification process in which a weak electric field induced in the 

extracellular fluid acts as a "trigger" for the initiation of long-
range cooperative events within the cell membrane (Adey, 1981).  
The basic premise underlying this theoretical concept is that the 
cell membrane exists in a metastable, non-equilibrium state that 
can be significantly perturbed by a weak electrical stimulus.  
Various physical models of such interactions have generally treated 
the cell membrane as a lattice in which nonlinear oscillations are 
established by weak electrical (or electrochemical) stimuli.  These 
oscillations are amplified by the collective excitation of patches 
of membrane molecules that extend over a significant portion of 
the cell surface.  The stored energy resulting from this collective 
mode of molecular excitation is then released as metabolic chemical 
energy through the activation of ion pumps or enzymatic reactions 
within the membrane (Frohlich, 1968, 1977; Grodsky, 1976, 1977; 
Kaczmarek, 1977; Lawrence & Adey, 1982, 1983; Adey, 1983). 

4.3.2.  Localized interactions of external ELF fields with
cell membrane structures

    Recent experimental evidence and theoretical models have given 
support to the concept that the interactions of ELF electromagnetic 
fields with living cells occur at specific loci on the cell 
membrane.  In many ways, this concept is more attractive than the 
hypothesized long-range membrane interactions described above.  
Apart from the abstract nature of such theories, the concept of 
long-range interactions that involve a large fraction of the cell 
membrane surface is generally feasible only for electromagnetic 
fields with frequencies well above the ELF range.  Recent 
theoretical efforts have therefore focused on the possibility that 
weak ELF field interactions could significantly alter either 
ligand-receptor interactions at the membrane surface, or the 
transmembrane movement of electrolytes.  Theoretical and 
experimental developments in this area include the following: 

    (a)   Ligand-receptor interactions

    Chiabrera et al. (1984) proposed a model of membrane 
    interactions in which a microelectrophoretic motion induced in 
    the cell membrane by weak ELF electric fields influences the 
    average distance between charged ligands and the cell-surface 
    receptors to which they are bound.  In this theoretical model, 
    the effect of the imposed electric field is to decrease the 
    mean lifetime of the ligand-receptor complexes on the membrane 
    surface.  The authors propose that this effect could influence 
    various biological phenomena such as the activation of 
    lymphocytes by antigens and various lectins.  An ELF field 
    interaction of this type could also influence the gating 
    mechanisms that control the membrane transport of various types 
    of cations such as calcium. 

    (b)   Combined static and ELF field interactions

    Some experimental evidence suggests that ion cyclotron 
    resonance effects could occur between ELF fields and static 
    magnetic fields with intensities comparable to that of the 

    geomagnetic field.  Briefly summarized, it has been reported 
    that magnetic resonance conditions influence the dielectric 
    properties and growth rate of yeast cells (Jafary-Asl et al., 
    1982), the rate of lysozyme reaction with a cell membrane 
    substrate (Jafary-Asl et al., 1982), the behaviour of rats in a 
    timing discrimination task (Liboff et al., 1985), and the rate 
    of calcium ion release from the surfaces of brain cells exposed 
     in vitro to low-intensity electromagnetic fields (Blackman et 
    al., 1985a,b).  The first two of these biological effects were 
    claimed to occur in response to conventional nuclear magnetic 
    resonance conditions under which the static field intensity and 
    the frequency of the electromagnetic field were related by the 
    Larmor relationship for various nuclei, including 1H, 23Na, 
    31P, 35Cl, and 39K (Jafary-Asl et al., 1982).  In a third study 
    (Liboff et al., 1985), reversible changes in rodent timing 
    behaviour were observed when rats were simultaneously exposed 
    to a horizontal 60-Hz magnetic field and a vertical 
    magnetostatic field with a flux density of 26 µT.  This 
    combination of static field intensity and oscillating field 
    frequency satisfies the cyclotron resonance conditions for 
    lithium ions, which are thought to exert neuropharmacological 
    effects.  In the fourth study (Blackman et al., 1985b), a 
    generalized relationship was derived between the biologically 
    effective electromagnetic field frequency and the static 
    magnetic field flux density.  This relationship established a 
    proportionality between the frequency of the oscillating field 
    and the static magnetic field flux density multiplied by an 
    index, (2n + 1), where n = 0 or 1. 

    Liboff (1985) proposed that these weak interactions, which 
involve energy transfer from the external field that is 8 orders of 
magnitude less than the Boltzmann thermal energy (kT), could 
nevertheless impart kinetic energy to ions, such as calcium, moving 
through transmembrane channels.  The theoretical argument was made 
by McLeod & Liboff (1986) that ion channels provide an environment 
in which damping effects on ion motion due to collision may be 
reduced relative to the high collision frequencies that exist in 
bulk aqueous media.  Nevertheless, a simple calculation indicates 
that, under the various experimental conditions described above, 
the induced electric field within ion transport channels is of the 
order of 10-10 V/m.  This field level is 2 orders of magnitude less 
than the Nyquist thermoelectric noise present in membrane channels 
(Bawin & Adey, 1976).  Overall, the experimental data that suggest 
a possible role of cyclotron resonance effects on ion binding to 
membrane surfaces and on cation transport through cell membrane 
pores are intriguing, but there is a clear need for refinements in 
the theoretical description of this phenomenon and to substantiate 
the experimental results. 

5.  EXPERIMENTAL DATA ON THE BIOLOGICAL EFFECTS OF STATIC MAGNETIC FIELDS

    In this section, the aim is to present a review of experimental 
observations on the biological effects of exposure to magnetic 
fields, and to relate them to data presented in the preceding 
section, in which the mechanisms of interaction were discussed.  
Empirical observations for which no theoretical explanations are 
available, at present, will be pointed out, and an attempt will be 
made to identify gaps in knowledge.  The data discussed here were 
selected on the basis of their relevance for the assessment of 
health risks.  Thus, many papers have been omitted from the 
discussion. 

    Several comprehensive sources of experimental data on the 
biological effects of magnetic fields are available.  Older results 
have been collected in two volumes edited by Barnothy, M.F., ed. 
(1964, 1969) and the monograph by Kholodov (1966, 1974); more 
recent results can be found in the report of the American Institute 
of Biological Sciences (1985).  Some recent reviews include those 
prepared by Bogolyubov (1981), Kholodov (1982), Schulten (1982), 
Galaktionova et al. (1985), Sidjakin (in press), Tenforde (1979, 
1985a,b,c, 1986a), and Tenforde et al. (1985).  Valuable information 
and extensive bibliographies can be found in review papers by 
Budinger (1979, 1981), Budinger & Collander (1983), Persson & 
Stahlberg (1984), and Tenforde & Budinger (1986), which address the 
biological effects of magnetic fields in the context of the safety 
of magnetic resonance imaging and  in vivo spectroscopy. 

    All the above reviews are concerned with potential risks for 
human health from exposure to magnetic fields of a strength greater 
than that of the geomagnetic field.  This document does not deal 
with magnetic fields of a strength below that of the geomagnetic 
field.  However, readers interested in this aspect are referred to 
reviews by Nakhil'nitskaya et al. (1978) and Kopanov & Shakula 
(1985). 

    The organization of this section will follow the order of 
increasing biological complexity of the system studied. 

5.1.  Molecular Interactions

    Research on magnetic field interactions with biological 
molecules has led to a diversity of findings as exemplified by the 
results of studies on various enzymes summarized in Table 8.  A 
total of 15 reports has appeared in which the reaction rates of 17 
different enzymes were studied during exposure to stationary 
magnetic fields over a broad range of field strengths, and with 
widely varying exposure times, reaction temperatures and pH levels, 
and conditions of field uniformity.  Overall, 58% of the 
experimental studies showed no effects of the field exposure, while 
33% and 8% of the tests showed increases and decreases, 
respectively, in the rate of enzyme reactions in the exposed 
samples relative to controls.  As discussed earlier in section 4, 
in certain systems, such as enzymes that involve radical 

intermediate stages as part of their reaction pathways, it might be 
anticipated that the reaction would be sensitive to the presence of 
a magnetic field.  However, for several other enzyme systems there 
is no obvious physical mechanism that could explain the observed 
magnetic sensitivity at the field intensities that were used.  It 
is interesting to note, for example, that Cook & Smith (1964) found 
that the activity of trypsin increased by up to 23% during a 2-h 
exposure to a 0.8-T field, whereas Vajda (1980) and Nazarova et al. 
(1982) did not observe any change in enzyme activity during 
exposures of 2-8 h duration in a 1.4-T field.  Furthermore, 
Nazarova et al. (1982) found that trypsin activity was not affected 
by a 2.5 h exposure to a 10-T field, and Rabinovitch et al. 
(1967a,b) did not observe any change in trypsin activity either 
during a 9 min exposure to a 22-T field, or following a 3.7 h 
pretreatment of the enzyme in a 20.8-T field. 

    Another aspect of the data presented in Table 8 that merits 
comment is the finding in two different laboratories of an increase 
in the reaction rate of the metalloenzyme catalase in response to 
exposure to a magnetic field (Haberditzl, 1967; Vainer et al., 
1978).  Vainer et al. (1978) reported that the reaction rate of 
catalase varied linearly with field between 0 and 0.8 T, increasing 
by 20% at 0.8 T.  The action of this enzyme may involve a radical 
intermediate state which, as discussed in the preceeding section, 
might be anticipated to exhibit magnetic sensitivity.  Several 
other biologically important enzymes that may have radical 
intermediate steps in their pathways include the cytochrome P-450 
enzymes, which are involved in steroid hormone metabolism, and 
lipoxygenase and cyclo-oxygenase, both of which are involved in the 
synthesis of prostaglandins (Saunders & Cass, 1983).  Further 
studies on these enzyme systems would provide useful insight into 
whether enzymatic pathways that involve radical intermediate states 
exhibit sensitivity to a stationary magnetic field, with possible 
consequences for cellular and tissue functions (Schulten, 1986). 

    A well-studied mechanism by which static magnetic fields can 
influence macromolecules is through a magneto-orientational effect.  
As discussed in section 4, this phenomenon produces measurable 
effects on single molecules, only at field strengths greater than 
10 T.  Various macromolecular assemblies, such as retinal 
photopigments, can be oriented in fields of less than 1 T.  
However, at present, there are no data suggesting that magneto-
orientation of these various macromolecules exerts profound effects 
on vital membrane, cellular, or tissue function.  For example, 
mammalian visual functions have been found to be unaffected by 
static magnetic fields up to 1.5 T (Gaffey & Tenforde, 1984). 

Table 8.  Magnetic field effects on enzyme systems
-------------------------------------------------------------------
Enzyme                  Applied    Effect on   Reference
                        field      enzyme         
                        strength   activity
                        (tesla) 
-------------------------------------------------------------------
Acetylcholinesterase    1.7        increase    Young (1969)
Alcohol dehydrogenase   1.4        none        Muller et al. (1971)
Aldolase                17         none        Rabinovitch et al.
                                               (1967a,b)
Asparaginase            1.7        increase    Shishlo (1974)
beta-galactosidase      1          none        Thomas & Morris
                                               (1981)
Carboxydismutase        2          increase    Akoyunoglou (1964)
Catalase                6          increase    Haberditzl (1967)
Catalase                0.8        increase    Vainer et al. (1978)
Cytochrome oxidase      1.3        increase    Gorczynska et al.
                                               (1982)
DNase                   0.3        increase    Komolova et al.
                                               (1972)
Glumatic dehydrogenase  7.8        decrease    Haberditzl (1967)
Histidase               1.7        decrease    Shishlo (1974)
Lactic dehydrogenase    1.4        none        Muller et al. (1971)
Peroxidase              17         none        Rabinovitch et al.
                                               (1967a,b)
RNase                   17         none        Rabinovitch et al.
                                               (1967a,b)
RNase                   4.8        none        Maling et al. (1965)
RNase                   1.4        none        Muller et al. (1971)
RNase                   0.3        none        Komolova et al.
                                               (1972)
Succinate-cytochrome-C
 reductase              4.8        none        Maling et al. (1965)
Trypsin                 0.8        increase    Cook & Smith (1964)
Trypsin                 20.8       none        Rabinovitch et al.
                                               (1967a,b)
Trypsin                 10         none        Nazarova et al.
                                               (1982)
Trypsin                 1.4        none        Vajda (1980)
Tryosinase              17         none        Rabinovitch et al.
                                               (1967a,b)
-------------------------------------------------------------------

5.2.  Effects at the Cell Level

    The results of a number of studies conducted in the 1960s and 
earlier suggested that exposure to stationary fields might lead to 
physiological, morphological, and growth abnormalities at the 
cellular level (Barnothy, M.F., 1964, 1969).  Degenerative changes 
such as pycnosis (Mulay & Mulay, 1961; Pereira et al., 1967), 
decreased DNA synthesis (D'Souza et al., 1969) and growth 
inhibition (Gerencer et al., 1962; Butler & Dean, 1964) were noted 
for various types of normal and tumour cells.  In contrast to these 
observations, a large number of more recent studies using magnetic 

field intensities and exposure times that were equal to or greater 
than those used in the earlier work have failed to produce effects 
on cell growth (Montgomery & Smith, 1963; Halpern & Greene, 1964; 
Hall et al., 1964; Rockwell, 1977; Iwasaki et al., 1978; Frazier et 
al., 1979, Nath et al., 1980).  It is also interesting to note that 
early reports (Barnothy, J.M., 1964; Gross, 1964) of  in vivo  
tumour growth inhibition by stationary magnetic fields have not 
been replicated in other studies (Eiselein et al., 1961; Chandra & 
Stefani, 1979).  All of the studies mentioned above were performed 
under different exposure conditions and thus are difficult to 
compare. 

    Malinin et al. (1976) reported that exposure of human WI-38 
fibroblasts and murine L-929 cells to a 0.5-T field for 4-8 h at 
4 °K led to subsequent growth inhibition compared with controls, when 
the cells were thawed and cultured at 30 °C.  The exposed cultures 
also appeared to undergo morphological transformation and to lose 
sensitivity to contact inhibition of cell division in long-term 
cultures.  These observations were later shown to be the result of 
using unconventional culture techniques in which control cells were 
subcultured at 5- to 6-day intervals, while cultures grown from 
exposed cells were only passaged at 28- to 45-day intervals.  When 
Frazier et al. (1979) used similar culture techniques, they were 
able to replicate, in unexposed cultures of WI-38 and L-929 cells, 
the morphological transformation that had been reported by Malinin 
et al. (1976) to result from magnetic field exposure.  Thus, the 
results of Malinin et al. (1976) should be discounted in an 
evaluation of magnetic field effects on cell cultures. 

    Although the preponderance of available experimental evidence 
indicates that stationary magnetic fields with intensities up to 2 
T exert little influence on cell growth properties, there are 
potential mechanisms, discussed by Tenforde (1985b), by means of 
which effects might occur.  These include: 

   (a)  Enzymatic pathways that contain radical intermediate stages 
        and may be sensitive to the presence of strong magnetic 
        fields; 

   (b)  The redistribution of paramagnetic oxygen molecules in the 
        presence of a strong magnetic field gradient (Aceto et al., 
        1970).  The magnetomechanical movement of dissolved oxygen 
        in an aqueous medium has been demonstrated experimentally 
        (Lyu et al., 1978; Ueno & Harada, 1982), but, as yet, there 
        are no clear tests of the potential biological consequences 
        of this effect. 

   (c)  As the lamellar phospholipids of cell membranes are 
        diamagnetically anisotropic, the orientational effect of an 
        applied magnetic field exceeding approximately 0.1 - 1 T 
        could significantly perturb membrane transport properties 
        (Labes, 1966).  In support of this proposal, direct 
        evidence has been obtained for magnetic field effects on 
        the diffusional properties of liquid crystals (Teucher et 
        al., 1971; Hakemi & Labes, 1974, 1975).  Using measured 

        values for the anisotropic diamagnetic susceptibility of 
        model phospholipid membranes (Boroske & Helfrich, 1978), 
        it can be estimated from theoretical considerations that 
        the magnetic interaction energy within a typical cell 
        membrane will exceed the Boltzmann thermal energy, kT, in 
        stationary fields greater than approximately 0.5 T 
        (Tenforde, 1985b).  At sufficiently high magnetic field 
        intensities, a perturbation of membrane properties might 
        therefore be expected to occur, with possible consequences 
        for other cellular functions. 

    (d) The sensitivity of cell membranes to magnetic field 
        interactions may be greatest at phase transition 
        temperatures (Amer, 1965; Aceto et al., 1970).  This 
        hypothesis is based on the concept that perturbations 
        introduced by relatively weak magnetic interactions should 
        be amplified near a phase transition temperature at which 
        membrane properties undergo abrupt changes.  Some indirect 
        support for this hypothesis was obtained from studies on 
        thermally-induced developmental failure in flour beetles 
        (Amer, 1965), in which higher temperatures were required to 
        elicit developmental wing abnormalities in the presence of 
        a strong magnetic field.  More direct evidence for membrane 
        sensitivity to static magnetic fields at phase transition 
        temperatures has recently been obtained (Liburdy et al., 
        1986; Liburdy & Tenforde, 1986).  These investigators 
        observed changes in the permeability of liposome bilayer 
        membranes composed of saturated phospholipids, when the 
        liposomes were exposed to static magnetic fields at 
        temperatures in the prephase transition region from 40 to 
        40.7 °C.  At temperatures of lower than 40 °C or higher than 
        40.7 °C, no effects on liposome membrane transport were 
        observed in fields as high as 7.5 T. 

5.3.  Effects on Organs and Tissues

    Examples of mammalian tissue and organ alterations that have 
been observed following magnetic field exposure include changes in: 

   (a)  blood and bone marrow cellular composition (Barnothy
        et al., 1956; Barnothy & Sumegi, 1969a,b; Nakagawa et
        al. 1980; Gorchonskaya, 1984);

   (b)  serum chemistry (Nakagawa et al. 1980; Tvildiani et
        al. 1983);

   (c)  microcirculation (Demetsky et al., 1979; Grohmann et
        al., 1986)

   (d)  thrombocyte coagulation (Rusyayev, 1979);

   (e)  electrolyte balance in blood, urine, and various
        tissues (Hanneman, 1969; Markuze et al., 1973;
        Tvildiani et al., 1981);

   (f)  functional and structural properties of various
        organs and tissues (Reno & Nutini, 1963, 1964;
        Toroptsev, 1968; Galaktionova & Strzhizhovsky, 1973;
        Bucking et al., 1974; Wordsworth, 1974; Kholodov &
        Shishlo, 1980; Strzhizhovsky et al., 1980; Rabino-
        vitch et al., 1983; Strzhizhovsky & Mastryukova,
        1983);

   (g)  immune response (Pautrizel et al., 1969; Kandil &
        Elashmawy, 1981); and

   (h)  endocrine regulation (Klimovskaya & Maslova, 1981,
        1983; Friedman & Carey, 1972).

    With the exception of one study on endocrine changes 
(Klimovskaya & Maslova, 1983), all of the reported alterations in 
tissue and organ properties were observed at static magnetic field 
levels below 1 T.  These observations are therefore difficult to 
reconcile with the growing body of evidence that the development, 
growth, and homeostatic regulation of mammals is not significantly 
affected by prolonged exposure to fields of this magnitude. 

    Many of the experimental reports have been based on studies 
with small numbers of exposed and control subjects, and often no 
attempt has been made by the investigators to replicate their 
experimental results.  Furthermore, the magnetic field exposure 
conditions frequently have not been well documented.  In some 
studies, inadequate controls were used, such as the use of cage-
control animals, instead of sham-exposed controls.  In this case, 
effects attributed to magnetic fields may have occurred in response 
to stresses imposed by other factors, such as adaptation to new 
caging conditions, differences in ambient temperature, sound 
levels, lighting conditions, and so forth.  There have been few 
attempts to verify the findings of tissue and organ effects through 
independent replication in other laboratories.  In the few cases 
where such attempts have been made, the original results have not 
been successfully replicated.  For example, the early reports 
(Barnothy et al., 1956; Barnothy & Sumegi, 1969a,b; Nakagawa et 
al., 1980) of haematopoietic alterations have not been confirmed in 
other studies (Eiselein et al., 1961; Nahas et al., 1975; Viktova 
et al., 1976; Battocletti et al., 1981).  Similarly, earlier 
reports (Pautrizel et al., 1969; Kandil & Elashmawy, 1981) that 
magnetic fields alter the immune status of exposed subjects have 
not been confirmed by Bellossi (1983) or in more recent studies 
designed to test humoral and cell-mediated immunity in mice exposed 
for 6 days to a 1.5-T stationary magnetic field (Tenforde & 
Shifrine, 1984). 

    It should be added that, where effects have been identified, 
efforts have seldom been made to explore the consequences for other 
related end-points, as a means of verifying the previous findings.  
Furthermore, only a few studies have addressed magnetic field 
effects on tissues and organs using classical anatomical and 
physiological methods.  In view of these considerations, the 

existence of deleterious effects of static magnetic fields on 
tissue and organ functions must, at present, be considered as 
questionable. 

5.4.  Effects on the Circulatory System

    The occurrence of magnetically induced potentials associated 
with blood flow in the aorta (Fig. 4, section 4.1) has been 
demonstrated on electrocardiogram (ECG) recordings from rats 
(Gaffey & Tenforde, 1981), rabbits (Togawa et al., 1967), dogs and 
baboons (Gaffey & Tenforde 1979; Gaffey et al., 1980), and monkeys 
(Beischer & Knepton, 1964; Beischer, 1969; Tenforde et al., 1983) 
exposed to static magnetic fields (Fig. 7).  The primary change in 
the ECG in the field is an alteration of the signal amplitude at 
the locus of the T-wave.  The repolarization of ventricular heart 
muscle, which gives rise to the T-wave, occurs in the normal ECG at 
approximately the same time in the cardiac cycle as the ejection of 
blood into the aorta.  It is therefore reasonable to expect that 
the flow potential induced by the magnetic field is superimposed on 
the T-wave. 

FIGURE 7

    From the theoretical discussion in section 4, four predictions 
can be made regarding the induced blood flow potential and the 
associated magnetohydrodynamic effects: 

   (a)  an induced flow potential should have a linear
        dependence on the applied magnetic field strength;

   (b)  the magnitude of the potential should be a function
        of the orientation of the animal relative to the
        field direction;

   (c)  the induced potentials observed in the ECG should
        increase with the size of the animal;

   (d)  the resultant magnetohydrodynamic effects should be
        small.

    In the following section, experimental data will be described 
that relate to these predictions. 

5.4.1.  Linear relationship of induced flow potential and magnetic 
field strength

    Experimental tests of the linear relationship between the 
magnetically-induced aortic blood flow potential and the applied 
magnetic field strength have been carried out by recording the ECG 
of several species of mammals during exposure to graded field 
intensities.  From the ECG records of rats exposed to static fields 
ranging from 0.1 to 2.1 T, a field-strength-dependent increase in 
T-wave amplitude was observed at field levels greater than 0.3 T 
(Gaffey & Tenforde, 1981).  The T-wave signal increase was a linear 
function of the applied field up to 1.4 T.  In dogs (Gaffey & 
Tenforde, 1979), baboons (Gaffey et al., 1980), and monkeys 
(Tenforde et al., 1983), the threshold for detection of the T-wave 
amplitude change was 0.1 T, and the increase in signal strength was 
a linear function of the magnetic field up to 1 T (Fig. 7). 

    These data support the concept that the T-wave alteration is a 
consequence of the superposition of an induced aortic blood flow 
potential, which is theoretically predicted to have a strictly 
linear dependence on the magnetic field strength. 

5.4.2.  Induced flow potentials and field orientation

    From theoretical considerations, it is predicted that the 
magnitude and the sign of the induced flow potential should be a 
function of the angle between the direction of blood flow and the 
direction of the applied magnetic field.  Consistent with this 
prediction, it has been shown for rabbits (Togawa et al., 1967) and 
for rats (Gaffey & Tenforde, 1981) that the amplitude of the T-wave 
signal can be increased, decreased, or remain unchanged by the 
superimposed aortic blood flow potential, depending on the 
orientation of the animal relative to the field.  It has also been 
demonstrated that the maximum change in the T-wave amplitude occurs 
when the long axis of a rat, and hence its ascending aortic vessel, 

is oriented perpendicular to the field (Gaffey & Tenforde, 1981).  
This observation is completely consistent with the theoretical 
prediction that the magnitude of the magnetically-induced aortic 
blood flow potential should achieve its maximum value when the flow 
vector and the magnetic field vector are orthogonal. 

5.4.3.  Dependence of induced blood flow potentials on animal size

    The theoretical considerations in section 4 suggest that the 
magnitude of induced aortic blood flow potentials should be 
significantly greater for large animal species than for the rodent.  
When ECG measurements were made on animals exposed to a 1-T field, 
with an orientation perpendicular to the body axis, the maximum 
aortic flow potentials recorded at the body surface were 75 µV for 
rats (average weight 0.25 kg) (Gaffey & Tenforde, 1981), 175 µV for 
baboons (5 kg) (Gaffey et al., 1980), 200 µV for monkeys (5 kg) 
(Tenforde et al., 1983), and 390 µV for dogs (9 kg) (Gaffey & 
Tenforde, 1979).  Thus, greater magnetically-induced blood flow 
potentials were observed with larger animal species, conforming to 
theoretical expectations. 

5.4.4.  Magnetohydrodynamic effects

    A test of potential alterations in haemodynamic parameters as a 
consequence of magnetohydrodynamic interactions was made by 
recording the arterial blood pressure of monkeys during exposure to 
homogeneous, static magnetic fields ranging from 0.1 to 1.5 T.  The 
study was conducted with an accuracy of ± 2 mmHg in the recording 
of systolic and diastolic blood pressures.  No measurable 
alteration in blood pressure was observed in fields up to 1.5 T 
(Fig. 7).  This observation is consistent with the theoretical 
prediction that negligible haemodynamic alterations result from 
magnetohydrodynamic interactions with blood flow in fields of less 
than 2 T (Tenforde et al., 1983). 

5.4.5.  Cardiac performance

    Several indices of cardiac function have been studied in order 
to assess the possible physiological effects of the electrical 
potentials induced by an applied magnetic field.  These indices 
include blood pressure, heart rate, and the bioelectric activity of 
heart muscle.  As described above, there is no measurable 
alteration in the blood pressure of monkeys exposed to a 1.5-T 
stationary field.  The heart rate and electrical properties of 
heart muscles have been determined from ECG measurements on rats 
exposed to stationary fields up to 2.1 T (Gaffey & Tenforde, 1981), 
rabbits in a 1-T field (Togawa et al., 1967), dogs (Gaffey & 
Tenforde, 1979) and baboons (Gaffey et al., 1980) in fields up to 
1.5 T, and monkeys exposed to fields of up to 1.5 T (Tenforde et 
al., 1983) and to a 10-T field (Beischer, 1969).  Significant 
changes in heart rate were not observed during acute magnetic field 
exposures in any of these studies.  Similarly, the amplitudes of 
the P, Q, R, and S waves of the ECG were not altered, indicating 
that the applied magnetic field had no effect on the depolarization 
characteristics of the auricular and ventricular heart muscle.  The 

data from these studies on various species of animals also 
indicated that no cardiac arrhythmias occurred during acute 
exposures to the field levels studied. 

    These experimental observations provide evidence that little or 
no cardiovascular stress should result from exposure to the highest 
static magnetic field levels routinely encountered by man.  
However, this conclusion must be tempered by the recognition that 
no data are available in the literature relating to cardiovascular 
performance during protracted exposure to large stationary magnetic 
fields.  Also, from the theoretical considerations discussed in 
section 4, it would be anticipated that measurable haemodynamic 
pertubations could occur during exposure to static fields that 
significantly exceed 2 T.  For example, it has been predicted 
theoretically (Tenforde, 1985a) that a 5-T field would produce a 
reduction in aortic blood flow velocity of up to 7% in a human 
adult. 

5.5.  Nervous System and Behaviour

    On the basis of the theoretical models described in section 4, 
it is not anticipated that stationary magnetic fields with 
intensities up to 2 T would produce measurable alterations in nerve 
bioelectric properties.  The theoretical expectations agree with 
the existing experimental information on the behaviour of isolated 
neurons in large static magnetic fields. 

5.5.1.  Excitation threshold of isolated neurons

    From the theoretical considerations of Wikswo & Barach (1980), 
it can be estimated that a static magnetic field of at least 24 T 
would be needed to reduce the velocity of action potential 
conduction in isolated neurons by 10%.  The threshold for neural 
excitation has been examined for both intact frog sciatic nerves 
and single myelinated sciatic nerve fibres during exposure to a 
homogeneous, static magnetic field (Liberman et al., 1959; Gaffey & 
Tenforde, 1983).  In both studies, the field orientation was 
transverse to the nerve axis.  No evidence was obtained in these 
studies of an effect of a 1-T magnetic field on the minimum 
electrical stimulus required to evoke action potentials in either 
single fibres or intact sciatic nerves. 

    An important observation that has a direct bearing on such 
studies was made by Gaffey & Tenforde (1983), who determined the 
temperature coefficient of the frog sciatic nerve excitation 
threshold, and found it to rise with increasing temperature.  To 
obtain reliable results, it was found that the temperature must be 
controlled to within 0.1 °C. 

5.5.2.  Action potential amplitude and conduction velocity in
isolated neurons

    Several groups of investigators have studied the properties of 
evoked action potentials in isolated nerve preparations during 
exposure to static magnetic fields oriented either parallel or 

perpendicular to the nerve axis.  Schwartz (1978) exposed the 
circumoesophageal nerve of the lobster to static fields with a 
maximum strength of 1.2 T.  The nerve preparation was maintained in 
an L-shaped chamber, and the field gradient along the sections of 
nerve oriented parallel and perpendicular to the field lines were 2 
and 15 T/m, respectively.  No effects of either the parallel or 
perpendicular fields, applied for periods of up to 30 min, were 
observed on the nerve conduction velocity.  Gaffey & Tenforde 
(1983) conducted similar measurements on intact sciatic nerves 
exposed to either parallel or perpendicular 2-T static fields that 
were homogeneous to within 0.1% over the entire length of the 
nerve.  With both field configurations, no effects were observed 
during a continuous 4-h exposure on either the amplitude or the 
conduction velocity of maximal evoked action potentials.  Extending 
the duration of exposure to 17 h was also found not to influence on 
the impulse conduction velocity. 

    Schwartz (1979) measured the membrane potentials and 
transmembrane currents in lobster circumoesophageal nerves exposed 
to a 1.2-T stationary field.  Both parallel and perpendicular field 
orientations relative to the nerve axis were used in these 
experiments, and the field gradients were identical to those 
described above in the discussion of Schwartz's studies on nerve 
conduction velocity (Schwartz, 1978).  No effects of the parallel 
or perpendicular magnetic fields were observed on either the action 
potentials or the transmembrane currents during nerve excitation. 

    In contrast to the negative results of the studies described 
above, the results of two other studies have shown effects of 
static magnetic fields on nerve bioelectric activity (Reno, 1969; 
Edelman et al., 1979).  However, Tenforde (1985b) suggested that 
the apparent magnetic field effects observed in these studies are 
probably attributable to a lack of precise temperature control, the 
importance of which has already been discussed above. 

5.5.3.  Absolute and relative refractory periods of isolated neurons

    Following the passage of a maximal action potential, an 
isolated peripheral nerve enters an absolute refractory period of 1 
- 2 ms duration, during which a second impulse cannot be evoked.  
Following the absolute refractory period, the nerve enters a 
relative refractory period during which action potentials of 
progressively increasing amplitude can be evoked by electrical 
stimulation.  After a period of approximately 4 - 6 ms, the second 
action potential reaches the same maximal amplitude as the impulse 
elicited by the initial stimulus, thus marking the end of the 
relative refractory period.  The characteristics of both the 
absolute and relative refractory periods have been examined during 
the exposure of frog sciatic nerves to a homogeneous 2-T field 
(Gaffey & Tenforde, 1983; Tenforde et al., 1985).  Using both 
parallel and perpendicular configurations of the magnetic field 
relative to the nerve axis, no influence of the field was observed 
on the duration of either the absolute or the relative refractory 
periods.  In addition, the amplitudes of impulses evoked during the 
relative refractory period were unaffected by the magnetic field 
exposure. 

    In summary, the majority of the experimental studies that have 
been conducted to date indicate that static magnetic fields up to 2 
T have little or no influence on the bioelectric properties of 
isolated neurons. 

5.5.4.  Effects of static magnetic fields on the electroencephalogram

    Several reports have been made of changes in brain electrical 
activity during the exposure of experimental animals to static 
fields ranging from approximately 0.1 to 9.1 T.  The information is 
inconsistent, at times contradictory, and requires additional 
investigations before a definite judgement can be made. 

    In a series of electroencephalogram (EEG) studies on squirrel 
monkeys, Beischer & Knepton (1966) observed that exposure to static 
magnetic fields produced a significant increase in the amplitude 
and frequency of brain electrical signals recorded below the scalp 
in the frontal, parietal, temporal, occipital, and median cranial 
regions.  Recordings of the EEG were made in homogeneous fields 
with field strengths ranging from 1.47 to 9.13 T.  EEG recordings 
were also made in strong gradient fields.  During exposures ranging 
from 3 to 45 min, it was found that the predominant EEG frequencies 
shifted from their pre-exposure range of 8 - 12 Hz to 14 - 50 Hz, 
independently of the field intensity or homogeneity.  The 
amplitude of the signals also increased from the control level of 
25 - 50 µV to 50 - 400 µV.  These changes were uniformly observed 
in the different cranial regions, which were simultaneously 
monitored; there was no latency in the response on application of 
the field.  When the field was removed, both the amplitude and 
frequency spectrum of the EEG signals returned to their pre-
exposure levels, indicating the transient nature of this effect. 

    In analysing the results of their studies, Beischer & Knepton 
(1966) considered several potential sources of artifacts, 
including ripple currents from the magnet power supply, animal 
movements associated with heart contractions and breathing, pick-up 
of stray 60-Hz fields by the EEG electrodes and leads, and skeletal 
muscle tremors.  All of these factors, except for muscle tremors, 
could be excluded because their characteristic frequencies were 
outside the frequency range observed for the predominant EEG 
signals in the presence of a static magnetic field.  However, the 
characteristics of the EEG tracings obtained from monkeys in the 
magnetic field suggest that "myographic noise" from skeletal 
muscles may have been superimposed on the brain electrical signals.  
It is also possible that other uncontrolled factors, present only 
during excitation of the magnet coils, including mechanical 
vibrations, audible noise, and an increased ambient temperature, 
could have led to an altered pattern of brain electrical activity. 

    In contrast to the above findings with monkeys, Kholodov 
reported that the exposure of rabbits to relatively weak static 
fields (0.08 - 0.10 T) produced an EEG signature characteristic of 
a general inhibitory state in the central nervous system (Kholodov, 
1964, 1966; Kholodov et al., 1969).  The major changes in the EEG 
during magnetic field exposure were the occurrence of slow waves 

and high-amplitude spindles observed in the electrical activity 
recorded from different regions of the brain.  The phenomenon was 
not uniformly exhibited in all of the tests conducted by Kholodov; 
in a series of 100 field exposures on 12 rabbits, the author 
observed the occurrence of spindles in 30% of the tests, and an 
increase in the number of slow waves with frequencies of less than 
4 Hz in 19% of the tests (Kholodov, 1966).  The percentage of 
animals exhibiting EEG responses to the field was not stated.  Both 
spindles and slow waves in the EEG occurred with a latency of 
approximately 15 s after the field was turned on, and reached 
maximum levels after 45 s of exposure.  The increased number of 
spindles and slow waves persisted during exposure to a 0.1-T field 
for 3 min, and decreased immediately after the field was turned 
off.  However, 15 to 25 s after the exposure was terminated, a 
transient increase in the number of spindles and slow waves 
occurred with a duration of approximately 20 - 30 s. 

    Kholodov (1966) presented evidence that the EEG alterations 
observed in his studies on rabbits were not artifacts resulting 
from the induced potentials that occur during the switching on and 
off of an electromagnet.  This possibility was excluded on the 
basis of trials in which the magnet was energized and de-energized 
at various rates, with no resulting change in the character of the 
observed EEG alterations. 

    Kholodov (1966, 1981, 1982) also described histological changes 
in the brains of mammals exposed to static magnetic fields for 
brief periods.  The significance of these anatomical changes was not 
clearly established.  The differences in the results obtained by 
Beischer & Knepton (1966) and by Kholodov (1966) may be related to 
the one order of magnitude difference between the field strengths 
used in their studies.  Furthermore, Battocletti et al. (1981) did 
not find changes in potentials evoked by stimulation of extremities 
in rhesus monkeys exposed to 2 T for 48 h. 

    The positive observations may be explained by additive effects 
on elements of the central nervous system (Valentinuzzi, 1965).  
It should be noted that no recent studies on the effects of static 
magnetic fields on the bioelectric activity of the brain were found 
in the published literature.  It seems that this area deserves 
further study using modern electroencephalographic methods.  The 
application of recordings from single locations in the brain to 
elucidate the neural basis for sensitivity to magnetic cues in 
pigeons (Semm et al., 1984; Semm, 1986) may serve as an example. 

5.5.5.  Behavioural effects

    An inherent sensitivity to the weak geomagnetic field and 
correlated behavioural responses has been demonstrated for a number 
of different organisms and animal species.  It has been well 
documented experimentally that weak magnetic fields influence the 
migratory patterns of birds (Keeton, 1971; Emlen et al., 1976; 
Bookman, 1977), the kinetic movement of molluscs (Ratner, 1976), 
the waggle dance of bees (Martin & Lindauer, 1977), the direction-
finding of elasmobranch fish (Kalmijn, 1978, 1982), and the 

orientation and swimming direction of magnetic bacteria (Blakemore, 
1975; Blakemore et al., 1980).  The mechanisms underlying the 
magnetic sensitivity of elasmo-branchs and magnetotactic bacteria 
have been discussed in sections 4.1.1 and 4.1.2.2. 

    A precise mechanism underlying the magnetic sensitivity of 
other organisms has not been elucidated, although small deposits of 
magnetite crystals have been discovered in the cranium of pigeons 
(Walcott et al., 1979), in the tooth denticles of molluscs 
(Lowenstam, 1962; Kirschvink & Lowenstam, 1979), and in the 
abdominal region of bees (Gould et al., 1978).  Magnetite has been 
also reported in various anatomical sites in dolphins (Zoeger et 
al., 1981), tuna (Walker et al., 1984), butterflies (Jones & 
MacFadden 1982), turtles (Perry et al., 1981), mice (Mather & 
Baker, 1981), and human beings (Kirschvink, 1981; Baker et al., 
1983).  The possible role of magnetite in the geomagnetic 
direction-finding mechanism possessed by some of these species has 
not been established, nor is it clear for all of the mammalian 
species in which magnetite deposits have been reported to occur 
(Baker, 1980; Gould & Able, 1981). 

    Although the directional cues derived from the weak geomagnetic 
field by certain species of animals have been demonstrated by 
careful study, the possible effects of magnetic fields on the 
behaviour of higher organisms are by no means established.  Several 
studies with rodents have reported effects of static magnetic 
fields of less than 1 T on locomotor activity and patterns of food 
and water consumption (Aminev et al., 1967; Russell & Hendrick, 
1969; Pelyhe et al., 1973; Nakagawa, 1979; Nakagawa et al., 1980; 
Shust et al., 1980).  In contrast to these earlier reports, Davis 
et al. (1984) did not observe any behavioural abnormalities in mice 
exposed for prolonged periods to a 1.5-T field.  The behavioural 
end-points examined in this extensive study included memory 
retention of an electroshock-motivated passive avoidance task, 
general locomotor activity, and sensitivity of the subjects to a 
neuropharmacological agent (pentylenetetrazole).  Smirnova (1982) 
also did not find any behavioural effects in rats exposed to 0.3 T 
or 1.6 T for 5 min/day for 3 successive days. 

    The effects on primate behaviour of exposure to intense 
magnetic fields was studied by Thach (1968).  In one study, 3 
squirrel monkeys  (Saimiri sciureus) were conditioned to respond to 
a visual vigilance task and subsequently exposed to static magnetic 
fields in the core of a water-cooled Bitter magnet.  Response was 
greatly suppressed by fields of 7 T or more.  A threshold seemed to 
exist between 4.6 and 7 T.  In a second study (deLorge, 1979), 8 
squirrel monkeys were trained in several operant tasks and a 
similar suppression response was observed in fields up to 9.7 T.  
In addition, 2 of the monkeys regurgitated when exposed to these 
higher fields.  All of these effects were reported to be 
reproducible. 

5.6.  Visual System

    As discussed in section 4.2, one of the most clearly 
established magnetic field effects in biological systems is the 
phenomenon of magnetophosphenes, in which a flickering light is 
produced in the visual field during exposure to time-varying 
magnetic fields. 

    Although the phenomenon of phosphenes has not been reported by 
human observers during exposure to large static magnetic fields, 
there are two potential interaction mechanisms between these fields 
and elements of the retina that are involved in the visual response 
to photic stimulation.  First, the photoreceptor outer segments 
are subject to orientation in a static magnetic field as the result 
of their large diamagnetic anisotropy (Chalazonitis et al., 1970; 
Hong et al., 1971; Becker et al., 1978b; Hong, 1980).  Second, the 
initial photoisomerization event elicited by photon absorption in 
retinal photopigments is followed by a series of ionic fluxes that 
lead to excitation of the retinal neurons, and ultimately the 
visual cortex via a complex neural pathway.  This component of the 
phototransduction process could be influenced by static magnetic 
fields as the result of ionic current distortion and/or inductive 
effects, as discussed in section 4.  However, electrophysiological 
studies on the retinal response to photic stimuli in cats and 
monkeys have not shown any effects of exposure to a 1.5-T static 
magnetic field (Gaffey & Tenforde, 1984; Tenforde et al., 1985). 

    That photoreceptors may play a crucial role in magneto-
reception is suggested by the fact that inhibitory effects of low 
magnetic fields on pineal melatonin synthesis were not found in 
albino rats (Olcese et al., 1985) or in rats exposed to magnetic 
fields in total darkness (Reuss & Olcese, 1986), thus supporting 
the theory of Leask (1978) that incident radiation is an important 
factor in magnetic field sensitivity, i.e., light might be 
essential to the process of magnetoreception.  In contrast to rats 
(both albino and pigmented strains), hamsters did not respond to 
magnetic stimuli, as measured by the inhibition of pineal gland 
metabolism (Olcese & Reuss, 1986), and species-specific differences 
as well as reciprocal effects between photoreceptors and retinal 
pigments should be taken into consideration. 

    Since birds possess a direction-finding sense that appears to 
be based on simultaneous detection of the earth's magnetic and 
gravitational fields, Semm et al. (1984) undertook a study to 
explore possible neural mechanisms for the integration of magnetic 
and gravitational cues.  Leask (1977) proposed that the magnetic 
field compass was located in the retina of the bird.  Thus, Semm et 
al. (1984) recorded single unit electrical activity in the lateral 
and superior vestibular nuclei, the vestibulo-cerebellum, and the 
nucleus of the basal optic root, which has a projection to the 
vestibular system, in pigeons, under magnetic stimulation by fields 
of about 42 µT.  The responses of these cell systems were 
direction-selective, i.e., different cells responded to different 
directional changes in the magnetic field.  The interpretation of 
this was that magnetic cues may be conveyed from the visual to the 

vestibular system via a projection from the basal optical root, and 
then related to the movement of the bird. 

    The effects of static magnetic fields on turtle retinas  in 
 vitro were studied by Raybourn (1983) (see also Tenforde et al., 
1985).  No changes were seen in electroretinograms (ERG) from dark- 
or light-adapted eyes during exposure to 1-T fields.  However, 2- 
to 3-mT fields suppressed the B-wave of the ERG in eyes prepared 
during the light-to-dark adaptation phase, which lasts for about 2 
h.  No effects of 1.5-T fields on the ERG in cats and monkeys were 
observed (Gaffey & Tenforde, 1984; Tenforde et al., 1985), but 
circadian variations were not studied.  These findings have not 
been interpreted.  The static magnetic field strength at which 
effects were noted in turtle retinas was too low to influence ionic 
fluxes that occurred in the retina following stimulation by light. 

5.7.  Physiological Regulation and Circadian Rhythms

    In assessing the responses of living organisms to static 
magnetic fields, an important aspect is the maintenance of normal 
homeostatic regulation.  The literature on this subject is often 
contradictory.  For example, the finding by Sperber et al. (1984) 
that thermoregulation in rodents is affected by strong magnetic 
field spatial gradients, could not be replicated by Tenforde 
(1986c).  It should be noted that Gremmel et al. (1984) described 
changes in thermoregulation in human beings exposed to magnetic 
fields.  One of the central issues in this assessment is whether 
exposure to magnetic fields produces an alteration in the normal 
circadian rhythm of major physiological and behavioural variables.  
Several of the investigations discussed in this section indicate 
that exposure of mammals to static magnetic fields may lead to 
hormonal alterations and to other metabolic effects that could 
potentially affect physiological regulation, and thereby lead to an 
alteration in the normal circadian rhythm.  Although there is 
relatively little information available on this subject, several 
reports in the literature suggest that weak magnetic fields may 
influence circadian regulation. 

    Brown & Scow (1978) observed a modulation of the normal 24-h 
circadian activity period in hamsters, when a weak magnetic field 
with a maximum intensity of 26 µT was applied in 26-h cycles.  The 
nocturnal sensitivity of mice to morphine was found by Kavaliers et 
al. (1984) to be diminished, when the animals were exposed to a 
rotating magnetic field with an intensity ranging from 105 µT to 
9 mT.  A cancellation of the earth's magnetic field by Helmholtz 
coils was found to alter the circadian activity of birds (Bliss & 
Hepner, 1976).  It has recently been reported that artificial 
changes in the strength and direction of the local geomagnetic 
field are sufficient to alter the electrical activity of pineal 
cells in the guinea-pig (Semm et al., 1980; Semm, 1983), rat (Reuss 
et al., 1983), and pigeon (Semm et al., 1982, 1984; Semm, 1983, 
1986).  In related studies, it was demonstrated in albino rats that 
artificial changes in the ambient magnetic field reduced the 
nocturnal rise in pineal melatonin contents and the activity of the 

involved enzymes,  N -acetyltransferase (Welker et al., 1983; Olcese 
et al., 1985; Olcese & Reuss, 1986) and hydroxyindole- O -
methyltransferase (Reuss & Olcese, 1986).  Interestingly, this 
effect was not found using NMR-strength fields of 0.14 T (Reuss et 
al., 1985). 

    In other recent studies (Tenforde, 1985c; Tenforde et al., 
1986b,), prolonged exposures of mice to a 1.5-T static magnetic 
field did not produce any alterations in the circadian rhythm of 
several physiological and behavioural variables.  Noninvasive 
transducer techniques were used to provide continuous measurements 
of core body temperature, respiration, body mass, food intake and 
excreta, and two independent indices of locomotor activity.  The 
rodents were subjected to a homogeneous 1.5-T field under 3 
different exposure regimens: (a) continuous exposure for 5 days; 
(b) intermittent exposure in an 8 h-on/16 h-off cycle for 10 
consecutive days; and (c) serial exposures to the field under the 
5-day continuous and 10-day intermittent schedules.  In addition, 
the sensitivity of circadian oscillations to a 1.5-T field was 
tested both in mice that were maintained on a diurnal light/dark 
cycle, and in mice that were placed in a free-running circadian 
state by the maintenance of continuous dim illumination.  Under all 
of these conditions, no influence of a 1.5-T field was observed on 
the circadian variations in any of the physiological or behavioural 
parameters studied. 

    In an effort to elucidate whether static magnetic fields 
perturb the light-elicited electrical activity of the retina, 
Raybourn (1983) recorded the external ERG of isolated turtle 
retinas during light stimulation in the presence of magnetic fields 
of graded strength.  When the retinal preparations from light-
adapted or dark-adapted eyes were studied, no changes in the ERG 
occurred in fields up to 1 T.  However, the amplitude of the ERG 
b-wave, which results from the electrical activity of nerve cells 
in the retina, was consistently suppressed in retinas prepared 
during the light-to-dark transition phase of the diurnal 12 h-
light/12 h-dark cycle.  During this transition phase, which extended 
for approximately 2 h after the onset of darkness, the 
photoreceptor cells underwent rapid changes in both physiological 
and metabolic activities (Bubenik et al., 1978; Young, 1978). 

    The magnetic field effect was observed with intensities as low 
as 2 - 3 mT, and was rapidly reversible following termination of 
exposure.  This effect was observed in both the cone-dominant 
retinas of  Pseudemys scripts turtles, and the mixed rod-cone 
retinas of  Chelydra serpentina turtles, suggesting that it is 
independent of the photoreceptor cell type.  The circadian 
dependence of the magnetic field sensitivity was clearly 
demonstrated by studies in which the light/dark cycle was phase 
shifted by several hours (Tenforde et al., 1985). 

    An alteration in human twilight visual acuity has been reported 
to occur in response to changes in the strength of the ambient 
geomagnetic field (Krause et al., 1984).  It has been suggested 
that this visual alteration may have its origin in a quantum 

mechanical effect on biochemical reactions in the retina, similar 
to that discussed by Schulten et al. (1978). 

5.8.  Genetics, Reproduction, and Development

    Developing organisms frequently exhibit a strong response to 
noxious environmental factors.  This observation has stimulated a 
relatively large number of studies on the potential effects of 
static magnetic fields on the genetics, reproduction, and 
development of various organisms.  Investigations on a variety of 
non-mammalian test systems have led to several reports of mutagenic 
and developmental effects resulting from exposure to both gradient 
and homogeneous magnetic fields.  Effects observed with strong 
magnetic field gradients have included alterations in the sex ratio 
and development of  Drosophila pupae (Mulay & Mulay, 1964; Markuze 
et al., 1973; Tvildiani et al., 1981), and abnormal development of 
sea urchin, frog, and salamander eggs (Perakis, 1947; Neurath, 
1968; Levengood, 1969; Ueno et al., 1984).  Inhibition of limb 
regeneration in crabs (Lee & Weis, 1980) has also been observed.  
Homogeneous magnetic fields have been reported to alter the 
development of chicken embryos (Joshi et al., 1978), and guppies 
(Brewer, 1979), and the rate of fertilization of trout eggs (Strand 
et al., 1983).  It is interesting to note that Perakis (1947) did 
not find any effects of a homogeneous 3.3-T field on the 
development of sea urchin eggs, and Ueno et al. (1984) did not 
observe any effects of a 1-T homogeneous field on the development 
of frog embryos.  The absence of effects of homogeneous magnetic 
fields on frog egg development is also supported by the 
experimental observations of Iwasaki et al. (1978) and Mild et al. 
(1981).  In contrast, developmental abnormalities were observed in 
both sea urchin eggs and frog embryos exposed to large magnetic 
field gradients (Perakis, 1947; Ueno et al., 1984).  Ueno et al. 
(1984) suggested that the developmental effects of gradient fields 
may result from a redistribution of dissolved oxygen or from the 
orientation of mitochondrial cytochromes in large magnetic fields 
with gradients exceeding 104 T/m. 

    In studies on mammals, it has been reported that homogeneous 
and gradient fields up to 0.94 T inhibit weight gain in young mice 
and produce weight loss in older animals (Barnothy, J.M., 1964).  
The rate and number of live births and the average birth weight 
have also been reported to decrease following prenatal and 
postnatal exposure of mice to a homogeneous field (Nakagawa, 1979).  
In contrast to these reports, studies on young mice exposed for up 
to 15 days to a nearly homogeneous field with a maximum strength of 
1.44 T did not reveal any effects on growth rate (Eiselein et al., 
1961).  Bellossi et al. (1984) did not observe any variations in 
growth of either mice or rats exposed to static magnetic fields of 
up to 800 mT for up to 250 days.  The intrauterine exposure of mice 
and rats to either a 1-T homogeneous field or a 2.5-T/m gradient 
field was also found not to influence fetal or postnatal 
development (Sikov et al., 1979). 

    Exposure of mice to static magnetic fields of 1.6 T, during a 
30-day period, resulted in reversible changes in spermatogenic 
epithelium and in a considerable decrease in the number of mature 
germ cells (Galaktionova et al., 1985).  These and other authors 
(Toroptsev et al., 1974; Udintsev & Khlynin, 1979) considered the 
testes a vulnerable organ when exposed to static or time-varying 
(20 mT, 50 Hz) magnetic fields.  Morphological changes in the 
testes and other organs, which occur after a 6-h exposure to 
magnetic fields, revert to normal after approximately one month. 

    Several studies have been carried out to determine whether 
genetic defects can be detected following magnetic field exposure.  
No increase in mutation frequency was observed by Kale & Baum 
(1979) among the progeny of  Drosophila males exposed as eggs, 
larvae, pupae, and adults to 1.3 - 3.7-T homogeneous magnetic 
fields.  Similar results were obtained by Mittler (1971) and 
Diebolt (1978), who exposed  Drosophila males to fields of 1 - 
1.1 T.  Baum et al. (1979) also found that exposure of the plant 
 Tradescantia to homogeneous fields up to 3.7 T did not lead to any 
increase compared with controls in three mutagenic indices, namely, 
pollen abortion, micronuclei formation, and pink stamen hair 
production.  Dominant lethal assays have been conducted by Mahlum et 
al. (1979) with male mice exposed to either a uniform 1-T or a 2.5-
T/m gradient field for 28 days prior to mating.  No effects of 
exposure to either the homogeneous or the gradient field were 
observed.  This result and the study of Strzhizhovsky et al. 
(1980) indicate that such exposure does not induce chromosomal 
aberrations in male germ cells. 

    Recent studies have also demonstrated that the exposure of 
cultured Chinese hamster ovary cells to a 0.35-T homogeneous field 
does not lead to alteration in DNA synthesis or chromosome 
structure (Wolff et al., 1980).  The structure and biological 
activity of bacteriophage DNA have also been found to be unaffected 
by exposure to a 2-T homogeneous field (Roots et al., 1982). 

5.9.  Conclusions

    Studies on the effects of static magnetic fields on enzyme 
reactions and cellular and tissue functions have provided diverse, 
and often contradictory, findings.  Nevertheless, available 
evidence indicates that there are few irreversible effects on such 
systems, with the possible exception of: 

   (a)  enzymes and photosynthetic systems that involve
        radical-mediated reaction intermediates; and

   (b)  cellular systems in which the membrane is undergoing
        a structural phase transition during magnetic field
        exposure.

    The occurrence of significant genetic or developmental 
alterations in cellular tissues and animal systems exposed to high-
intensity static magnetic fields appears unlikely from available 

evidence.  One possible exception relates to unconfirmed reports of 
alterations in the embryonic development of amphibian species 
exposed to strong magnetic field gradients. 

    The magnetic induction of electrical potentials and currents in 
the central circulatory system does not produce measurable 
cardiovascular stress during short-term exposure to static fields 
of up to 2 T.  This conclusion must be tempered by the recognition 
that data do not exist on cardiovascular performance during 
protracted magnetic field exposure. 

    The majority of experimental studies conducted with isolated 
neurons indicate that static magnetic fields of up to 2 T have no 
irreversible influence on neuronal bioelectric properties.  Several 
reports have referred to changes in brain electrical activity and 
behaviour in animals exposed to fields ranging from 0.1 to 9 T, but 
the data are inconsistent and at times contradictory. 

    An inherent sensitivity to the weak geomagnetic field and 
correlated behavioural responses have been demonstrated for a 
number of different organisms and animal species.  However, 
behavioural effects in higher organisms have not been established 
at field strengths of less than 2 T.  Although the data are 
inconsistent, effects on physiological regulation and circadian 
rhythms have been reported in animals, due to alterations in the 
local geomagnetic field.  Negative findings in higher organisms 
have been reported in studies involving field levels as high as 
1.5 T. 

    Thus, reversible or transient effects have been reported in 
lower animals due to exposure to low-intensity static fields or due 
to alterations in the ambient geomagnetic field.  However, no 
irreversible effects have been established due to static magnetic 
field exposures of up to 2 T. 

6.  BIOLOGICAL EFFECTS OF TIME-VARYING MAGNETIC FIELDS

    An extensive literature exists on the response of animals and 
isolated cellular and tissue systems to ELF magnetic fields.  At 
present, this body of research is difficult to interpret in a 
systematic manner because of two factors: 

   (a)  A wide range of intensities, frequencies, waveforms, and 
        exposure durations have been used.  Many of the earlier 
        studies involved sinusoidal fields oscillating at 
        frequencies below 100 Hz, but research during the last 
        several years has focused increasingly on the biological 
        effects of square-wave or pulsed fields with complex 
        waveforms. 

   (b)  Very few of the reported effects of ELF magnetic fields 
        have been independently replicated in different 
        laboratories. 

    In spite of these difficulties, there is a growing body of 
evidence that suggests that living systems exhibit a response to 
ELF magnetic fields under conditions in which the field intensity 
and rate of change in time (dB/dt) are sufficient to induce 
currents greater than the naturally occurring levels in tissues and 
extracellular fluids.  This effect is best illustrated by the 
phenomenon of magnetophosphenes, which is the one well established 
biological effect of ELF magnetic fields.  Although less well 
established, there is also evidence suggesting that pulsed magnetic 
fields, such as those used clinically to facilitate bone fracture 
reunion, may exert direct biological effects through the induction 
of tissue currents that exceed the endogenous levels. 

    The following topics are summarized in this section:

    (a)  magnetophosphene research;

    (b)  studies on the nervous system and animal behaviour;

    (c)  cellular, tissue, and animal responses to magnetic
         fields with various waveforms and repetition
         frequencies in the ELF range;

    (d)  studies on the effects of pulsed magnetic fields on
         bone growth and repair; and

    (e)  thresholds for biological effects as a function of
         field frequency and induced current densities.

6.1.  Visual System

    High-intensity magnetic fields oscillating in the ELF range 
produce visual sensations in human subjects that are known as 
magnetophosphenes.  This phenomenon has already been discussed in 
section 4.2. 

    Studies (Silny, 1981, 1984, 1986) have been performed to 
characterize ELF magnetic-field effects on visually evoked 
potentials (VEP).  Fields greater than 50 mT in the frequency range 
below 100 Hz were demonstrated to reverse the polarity and reduce 
the amplitude of VEP recorded from human volunteers.  This effect 
persisted after the termination of the magnetic field exposure.  
Approximately 40 min after the magnetic field exposure, the VEP was 
found to return to a normal form.  This effect of magnetic fields 
was shown to be frequency-dependent, the field strength required to 
elicit an alteration in the VEP decreasing as the field frequency 
increased from 5 to 100 Hz.  It should be noted that the field 
strength required to alter the VEP is approximately one order of 
magnitude greater than that required to elicit clear 
magnetophosphene patterns. 

6.2.  Studies on Nerve and Muscle Tissue

    Several studies have been made on the electrical response of 
neurons to stimulation with time-varying magnetic fields.  As 
discussed by Bernhardt (1979, 1985), the current densities induced 
by the field must exceed 1 - 10 mA/m2 in order to have an 
appreciable effect on the nerve bioelectric activity, and a 
threshold extracellular current density of about 20 mA/m2 has been 
found experimentally with  Aplysia pacemaker neurons stimulated 
by an ELF electric field (Wachtel, 1979).  In a subsequent study 
with  Aplysia (Sheppard, 1983), an induced current density of 
approximately 5 mA/m2 produced by a 10-mT, 60-Hz sinusoidal field 
was ineffective in altering the spontaneous neuronal electrical 
activity.  Ueno et al. (1981) were also unable to alter the 
amplitude, conduction velocity, or refractory period of evoked 
action potentials in lobster giant axons, by applying sinusoidal 
magnetic fields with intensities of 1.2 T at 5 - 20 Hz, 0.8 T at 50 
Hz, and 0.5 T at 100 Hz. 

    Using magnetic flux densities in the range of 0.2 - 0.8 T, 
Kolin et al. (1959) were able to stimulate frog nerve-muscle 
preparations at field frequencies of 60 and 100 Hz.  Oberg (1973) 
and Ueno et al. (1978) were also able to stimulate contractions in 
frog nerve-muscle preparations by using pulsed magnetic fields with 
pulse durations of less than 1 ms.  In addition, the excitation of 
frog sartorius and cardiac muscles (Irwin et al., 1970) and of the 
sciatic nerves of dogs and rabbits (Maass & Asa, 1970) has been 
reported to occur in response to pulsed magnetic fields.  On the 
basis of electromyographic recordings from the human arm, Polson 
et al. (1982) were able to characterize the pulsed magnetic field 
parameters that elicited a neural response.  They indicated that 
the threshold rate of change of the magnetic field (dB/dt), which 
was necessary to stimulate the major nerve trunks of the arm, was 
approximately 104 T/s.  These fields were discrete pulses, 180 µs 
long, which will result in a high threshold compared with that for 
a continuously applied sinusoidal stimulus of between 10 and 100 
Hz. 

    A threshold of perception of about 2 x 103 T/s was reported 
(McRobbie & Foster, 1984) in human volunteers whose forearms were 
exposed to a damped sinusoidal magnetic field (2 - 3 cycles of a 
period equal to 0.3 ms).  The currents induced in the peripheral 
tissues of the forearm were calculated to be approximately 5 A/m2. 

    Other effects of time-varying magnetic fields on electrically 
excitable tissue have been summarized by Bernhardt (1985) (section 
4.2).  The frequency dependence of these effects has been described 
by Bernhardt (1985, 1986) (section 8.2). 

    From these studies, it appears that sinusoidal magnetic fields 
with intensities in the range generally used in the laboratory or 
well above the levels encountered by human beings in occupational 
settings or in the home environment, are insufficient to alter the 
bioelectric properties of isolated neurons.  However, direct 
magnetic stimulation of nerve and muscle tissues can be achieved by 
using pulsed fields with a rapid time rate of change of the 
magnetic flux density.  It should also be borne in mind that the 
effects of sinusoidal fields on complex, integrated neuronal 
networks, such as those within the central nervous system, may be 
considerably greater than the effects that occur in single neurons 
or nerve bundles.  This amplification of a field effect could occur 
through a summation of the small responses evoked in individual 
neuronal elements (Valentinuzzi, 1965).  An additive response 
mechanism may also underlie the production of magnetophosphenes 
through the stimulation of multiple neuronal elements of the retina 
(Valentinuzzi, 1962). 

6.3.  Animal Behaviour

    During the past two decades, a large number of studies on 
animal behavioural responses to ELF magnetic fields have been 
reported.  A chronological listing of these reports and a summary 
of the principal findings are given in Table 9. 

    Several studies in which the behaviour of honeybees and birds 
was observed to be altered in the presence of combined ELF electric 
and magnetic fields (Southern, 1975; Larkin & Sutherland, 1977; 
Greenberg et al., 1981a,b) have not been included, because of 
difficulty in attributing these effects to either the electric or 
magnetic field component.  In the case of bees, it appears that ELF 
electric fields may induce step-potential currents in the hive that 
have harmful effects when the field intensity exceeds approximately 
2 kV/m (Greenberg et al., 1981b).  However, altered behavioural 
patterns in honeybees have also been reported to occur in strong 
60-Hz magnetic fields in the absence of an external electric field 
(Caldwell & Russo, 1968).  The mechanism underlying the observed 
disruption of avian migration by the 72- to 80-Hz electric and 
magnetic fields from an ELF communication test system is not known 
(Southern, 1975; Larkin & Sutherland, 1977).  However, there are 
numerous reports that weak static magnetic fields, comparable in 
strength to the earth's field, may influence the migration patterns 
of birds (Keeton, 1971; Emlen et al., 1976; Bookman, 1977) and very 
weak time-varying magnetic fields have also been claimed to affect 
avian orientation (Papi et al., 1983). 


Table 9.  Behavioural effects of exposure to time-varying magnetic fields
---------------------------------------------------------------------------------------------------------
Reference             Subject   Exposure conditionsa              Results
---------------------------------------------------------------------------------------------------------
Friedman et al.       human     0.1 and 0.2 Hz, 0.5 - 1.1 mT;     Increased reaction time in 0.2-Hz field
(1967)                being     acute exposures

Caldwell & Russo      honey     60 Hz, 2.2 - 30 mT;               Altered exploratory behaviour
(1968)                bee       10-min exposures

Persinger             rat       0.5 Hz, 0.3 - 3 mT; rotating      Decreased open-field activity and
(1969)                          field; exposure during entire     increased defecation when tested
                                gestational period                postnatally at 21 - 25 days

Persinger & Foster    rat       0.5 Hz, 0.3 - 3 mT; rotating      Decreased avoidance of aversive
(1970)                          field; exposure during entire     electrical shock when tested
                                gestational period                postnatally at 30 days

Grissett & deLorge    monkey    45 and 75 Hz, 0.3 mT; fields      No effect on reaction time
(1971)                          applied in 10 daily sessions
                                of 1 h duration

Grissett              monkey    45 Hz, 1 mT; continuous           No effect on reaction time
(1971)                          exposure for 42 days

Persinger & Pear      rat       0.5 Hz, 0.3 - 3 mT, rotating      Suppressed rate of response to a
(1972)                          field; exposure during entire     conditioned stimulus preceding an
                                gestational period                aversive shock when tested postnatally 
                                                                  at 70 days

Persinger et al.      rat       0.5 Hz, 0.3 - 3 mT, rotating      Increased ambulatory activity after
(1972)                          field; exposure of adult          removal from field
                                animals for 21 - 30 days

Ossenkopp & Shapiro   duck      0.5 Hz, 2 - 10 and 10 - 30 mT,    Increased ambulation and defecation
(1972)                egg       rotating field; exposure for      rate when tested postnatally
                                entire prenatal period
---------------------------------------------------------------------------------------------------------

Table 9.  (contd.)
---------------------------------------------------------------------------------------------------------
Reference             Subject   Exposure conditionsa              Results
---------------------------------------------------------------------------------------------------------
deLorge               monkey    10, 15, 45, 60, and 75 Hz,        No consistent influence on motor
(1972, 1973a,b,                 0.8 - 1 mT; fields applied        activity, reaction time, inter-response
1974, 1979, 1985)               in 4 - 13 daily sessions of       time, overall level responding, or
                                2- to 8-h duration                match-to-sample performance

Beischer et al.       human     45 Hz, 0.1 mT; 22.5-h exposure    No effect on reaction time
(1973)                being

Gibson & Moroney      human     45 Hz, 0.1 mT; 24-h exposure      No consistent effect on cognitive or
(1974)                being                                       psychomotor functions

Mantell               human     50 Hz, 0.3 mT; 3-h exposure       No effect on reaction time
(1975)                being

Medvedev et al.       human     50 Hz, 10 - 13 µT; acute          Increased latency of sensorimotor
(1976)                being     exposures                         reactions

Smith & Justesen      mouse     60 Hz, 1.4 - 2 mT; 2-min          Increased locomotor activity and
(1977)                          aperiodic exposures over          aggression-related vocalization
                                2 days

Andrianova            mouse     100 Hz, 10 mT; acute exposures    Heightened motor activity
& Smirnova
(1977)

Brown & Scow          hamster   10-5 Hz, 0.8 - 26 µT; 26-h        Modified circadian rhythm in locomotor
(1978)                          schedule of high (14 h) to        activity
                                low (12 h) field switching
                                over period of 4 - 5 months

Tucker & Schmitt      human     60 Hz, 1.06 mT over whole body,   No perception of field
(1978)                being     or 2.12 mT over head region;
                                repetitive acute exposures

Becker, von           termite   50 Hz, 0.05 µT in shielded room;  Stimulation of gallery building 
(1979)                          exposures up to several weeks     activity
---------------------------------------------------------------------------------------------------------

Table 9.  (contd.)
---------------------------------------------------------------------------------------------------------
Reference             Subject   Exposure conditionsa              Results
---------------------------------------------------------------------------------------------------------
Clarke & Justesen     chicken   60 Hz, 2.4 mT; aperiodic          Increased variability of response to
(1979)                          exposures during 1-h interval     electric shock stimulus when 60-Hz
                                for 10 days                       magnetic field used as conditional 
                                                                  stimulus

Udintsev & Moroz      Rat       50 Hz, 20 mT, 15 min/day for      Transitory stimulation of adrenal
(1982)                          7 days                            system

Udintsev & Moroz      rat       50 Hz, 20 mT, 6.5 h/day for       Significant changes in hormone levels
(1982)                          7 days

Delgado et al.        monkey    9 - 500 Hz, 0.1 mT (applied       Modification of threshold for
(1983)                          to cerebellum); 9-h daily         exictation of motor neurons
                                exposures for maximum of 
                                19 days

Papi et al.           pigeon    0.034, 0.043, and 0.067 Hz,       Initial disturbance of orientation, but
(1983)                          60 µT peak intensity;             no effect on homing performance
                                exposures up to 4 h

Graham et al.         human     60 Hz, 40 µT; acute exposures     No perception of field
(1984)                being

Creim et al.          rat       60 Hz; 3.03 mT; 1-h exposure      No field-associated avoidance behaviour
(1984)

Davis et al.          mouse     60 Hz; 2.33 mT; 3-day             No change in memory retention, 
(1984)                          continuous exposure               locomotoractivity, or sensitivity to a 
                                                                  neuropharmacological agent

Liboff et al.         rat       60 Hz, 56 µT (with a trans-       Changes in timing discrimination
(1985)                          verse 26-µT static field);
                                30-min exposures

Creim et al.          rat       60 Hz, 3 mT, 1- to 23-h           No avoidance of applied field
(1985)                          exposures                         in a shuttle box test
---------------------------------------------------------------------------------------------------------
a  The magnetic fields were sinusoidal unless otherwise indicated.
    In assessing the effects of time-varying magnetic fields on the 
behaviour of mammalian species, the publications on this subject, 
listed in Table 9, are nearly equally divided between positive 
findings and observations of no behavioural effects in mammals.  
However, a careful examination of this list leads to the 
interesting conclusion that most investigations in which 
behavioural effects were not observed, the time rate of change of 
the applied magnetic field was sufficient to induce peak 
intracranial current densities at, or above, the endogenous level 
of approximately 1 mA/m2.  In contrast, only one of the positive 
findings of behavioural alterations in mammals (Andrianova & 
Smirnova, 1977) involved the use of a time-varying magnetic field 
capable of inducing intracranial currents at this level. 

    In examining the possible reasons for this apparent disparity, 
it is important to assess the potential influence on animal 
behaviour of extraneous factors, such as mechanical vibration and 
audible noise, that may accompany the activation of magnet coils.  
The importance of these factors has been well demonstrated by 
Tucker & Schmitt (1978), who found that perceptive individuals 
could sense the presence of a 60-Hz magnetic field through 
auxiliary clues.  When these investigators developed an exposure 
chamber that provided extreme isolation from vibration and audible 
noise, none of the more than 200 individuals tested could detect 
60-Hz fields with intensities of 1.1 mT over the whole body or 2.1 
mT over the head region.  The sensitivity of behavioural indices to 
adventitious factors, such as changes in barometric pressure, was 
also discussed by deLorge (1973b), who emphasized that the 
correlation of such variables to positive findings of apparent 
time-varying magnetic field effects must be examined. 

6.4.  Cellular, Tissue, and Whole Organism Responses

    Magnetic fields with a broad range of intensities, ELF 
frequencies, waveforms, and exposure durations have been evaluated 
for their ability to induce effects at the cellular, tissue, and 
animal levels.  These studies have recently been reviewed 
(Tenforde, 1985c, 1986a,d), and only a brief summary will be given 
here of the cellular and tissue responses to ELF magnetic fields 
that have been reported on the basis of both  in vitro and  in vivo  
studies. 

    Reports of alterations produced in cellular, tissue, and animal 
systems as a result of exposure to low frequency magnetic fields 
are summarized in Table 10, where a brief summary is given of the 
principal findings in each study.  The following types of 
investigations have not been included in Table 10 for the reasons 
stated below: 

   (a)  Studies on time-varying magnetic field effects on the
        visual system (magnetophosphene induction), nervous
        tissues, and animal behaviour, and epidemiological studies
        on carcinogenic risk, because these subjects are discussed
        elsewhere in this section and in section 8;

   (b)  Reports lacking adequate documentation of field
        exposure conditions (e.g., frequency, waveform, intensity,

        and duration of exposure).  Similarly, studies in which
        the biological measurements were qualitative rather than
        quantitative, as in certain medical reports on bone
        fracture reunion following therapy with pulsed magnetic
        fields;

    (c) Reports of research that involved combined exposures
        to ELF electric and magnetic fields, because of the
        obvious difficulty in delineating the relative effects of
        the two types of fields.

    The reported changes resulting from ELF magnetic field exposure 
include the following: 

    (a) Altered cell growth rate (Batkin & Tabrah, 1977;
        Tabrah et al., 1978; Goodman et al., 1979; Greenebaum
        et al., 1979, 1982; Aarholt et al., 1981; Ramon et
        al., 1981; Phillips et al., 1986a);

    (b) Decreased rate of cellular respiration (Cook et al.
        1969; Goodman et al., 1979; Greenebaum et al., 1979,
        1982; Kolodub & Chernysheva, 1980);

    (c) Altered metabolism of carbohydrates, proteins, and
        nucleic acids (Udintsev et al., 1976, 1978; Kartashev
        et al., 1978; Udintsev & Khlynin, 1979; Kolodub &
        Chernysheva, 1980; Kolodub et al., 1981; Norton,
        1982; Archer & Ratcliffe, 1983; Buyavikh, 1984;
        Liboff et al., 1984);

    (d) Effects on gene expression and genetic regulation of
        cell function (Chiabrera et al., 1978, 1979; Beltrame
        et al., 1980; Aarholt et al., 1982; Goodman et al.,
        1983; Goodman & Henderson, 1986);

    (e) Endocrine alterations (Riesen et al., 1971; Udintsev
        & Moroz, 1974; Sakharova et al., 1977, 1981; Kolesova
        et al., 1978; Udintsev et al., 1978);

    (f) Altered hormonal responses of cells and tissues,
        including effects on cell surface receptors (Dixey &
        Rein, 1982; Luben et al., 1982; Marsakova, 1983;
        Jolley et al., 1983; Cain et al., 1984; Chan &
        Nicholson, 1986);

    (g) Altered immune response to antigens and mitogens
        (Odintsov, 1965; Mizushima et al., 1975; Conti et
        al., 1983; Budd & Czerski, 1985);

    (h) Morphological and other nonspecific tissue changes in
        adult animals, frequently reversible with time after
        exposure (Druz & Madiyevskii, 1966; Toroptsev et al.,
        1974; Sakharova et al., 1981; Toroptsev & Soldatova,
        1981; Soldatova, 1982; Shober et al., 1982);

    (i) Teratological and developmental effects (Ossenkopp et
        al., 1972; Delgado et al., 1981, 1982; Kreuger et
        al., 1972; Ramirez et al., 1983; Ubeda et al., 1983).

                                                                                                  
Table 10.  Effects of exposure to time-varying magnetic fields on cells, tissues, and whole animals     
--------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results                                 
                   specimen                                                                             
--------------------------------------------------------------------------------------------------------
Odintsov (1965)    mouse         50 Hz, 20 mT; 6.5-h single     Increased resistance to Listeria        
                                 exposure or 6.5 h daily for    infection                               
                                 15 days                                                                
                                                                                                        
Druz & Madiyevskii rat           3 Hz, 0.1 - 0.8 T, and 50 Hz,  Change in hydration capacity            
 (1966)                          0.05 - 0.2 T; 1-min exposures  of brain, kidney, and liver tissues     
                                                                                                        
Riesen et al.      guinea-pig    60 Hz, 10 mT; 10 - 110-min     No effect on respiration                
 (1971)            brain         exposures                      (oxidative phosphorylation)             
                   mitochondria                                                                         
                    in vitro                                                                             
                                                                                                        
Riesen et al.      rat brain     60 Hz, 5 - 10 mT; 30-min       Decreased uptake of norepinephrine      
 (1971)            synaptosomes  exposure                       at 0 °C, but not at 10 °C, 25 °C,      
                    in vitro                                     or 37 °C                                
                                                                                                        
Tarakhovsky et al. rat           50 Hz, 13 - 14 mT; exposure    Changes in serum chemistry,             
 (1971)                          for 1 month                    haematocrit, and tissue morphology      
                                                                                                        
Kreuger et al.     chicken       45 Hz, 0.14 mT, and 60 Hz,     Reduced growth rate in young animals    
 (1972)                          0.12-mT exposure for 1 month                                           
                                                                                                        
Ossenkopp et al.   rat           0.5 Hz, 0.05 - 0.30 or         Increased thyroid and testicle weights  
 (1972)                          0.3 - 1.5 mT, rotating field;  at 105 - 130 days of age; no change in  
                                 exposure during entire         thymus or adrenal weights compared      
                                 gestational period             with controls                           
                                                                                                        
Beischer et al.    human being   45 Hz, 0.1 mT; 22.5-h          Elevated serum-triglycerides; no        
 (1973)                          exposure                       effects on blood cell counts or other   
                                                                serum chemistry                         
                                                                                                        
DeLorge (1974)     monkey        15 and 45 Hz, 0.82 - 0.93 mT;  No alteration in blood cell counts or   
                                 fields applied in 5 - 8 daily  serum chemistry (including trigycer-    
                                 sessions of 2-h duration       ides)                                   
--------------------------------------------------------------------------------------------------------
                                                                                                        

                                                                                                        
Table 10.  (contd.)                                                                                     
--------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results                                 
                   specimen                                                                             
--------------------------------------------------------------------------------------------------------
Toroptsev et al.   guinea-pig    50 Hz, 20 mT; 6.5-h single     Pathomorphological changes in testes,   
 (1974)                          exposure or 6.5 h daily for    kidneys, liver, lungs, nervous tissues, 
                                 24 days                        eyes, capillaries, and lymphatic system 
                                                                                                        
Udinstev & Moroz   rat           50 Hz, 20 mT; 1 - 7 days       Increase in adrenal 11-hydroxy          
 (1974)                          exposure                       corticosteroids                         
                                                                                                        
Mizushima et al.   rat           50 Hz, 0.12 T; 3-h exposure    Anti-inflammatory effects of field on   
 (1975)                                                         carrageenan-induced oedema and          
                                                                adjuvant-induced arthritis              
                                                                                                        
Beischer & Brehl   mouse         45 Hz, 0.1 mT; 24-h exposure   No change in liver-triglycerides        
 (1975)                                                                                                 
                                                                                                        
Mantell (1975)     human being   50 Hz, 0.3 mT; 3-h exposure    No haematological changes               
                                                                                                        
Udintsev et al.    rat           50 Hz, 20 mT; 1-day            Increased lactate dehydrogenase         
 (1976)                          exposure                       activity and change in distribution in  
                                                                heart and skeletal muscles              
                                                                                                        
Batkin & Tabrah    mouse         60 Hz, 1.2 mT; 13-day          Decreased tumour growth rate            
 (1977)            neuroblastoma exposure                                                               
                                                                                                        
Sakharova et al.   rat           50 Hz, 20 mT; 1-day            Increased catecholamines in tissue      
 (1977)                          exposure                                                               
                                                                                                        
Kartashev et al.   yeast         0.1 - 100 Hz, 0.025 - 0.40     Changes in rate of anaerobic glycolysis 
 (1978)                          mT; 20 - 30-min exposure                                               
                                                                                                        
Kolesova et al.    rat           50 Hz, 20 mT; single 24-h      Development of insulin deficiency       
 (1978)                          exposure and 6.5 h daily                                               
                                 for 5 days                                                             
                                                                                                        
Tabrah et al.       Tetrahymena   60 Hz, 5 - 10 mT;              Cell division delay, reduced growth     
 (1978)             pyriformis    exposures up to 72 h           rate, increased oxygen uptake           
--------------------------------------------------------------------------------------------------------
                                                                                                        

                                                                                                        
Table 10.  (contd.)                                                                                     
--------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results                                 
                   specimen                                                                             
--------------------------------------------------------------------------------------------------------
Persinger et al.   rat           0.5 Hz, 0.1 T - 1 mT,          No significant changes in thyroid       
 (1978)                          rotating field;                follicle numbers, mast cells, adrenal   
                                 10-day exposure                and pituitary weights, body weight, or  
                                                                water consumption                       
                                                                                                        
Persinger &        rat           0.5 Hz, 0.01 T - 1 mT,         No significant change in thymus mast    
 Coderre                         rotating field; 5-day          cell numbers in animals exposed         
 (1978)                          exposure                       prenatally and postnatally or exposed   
                                                                as adults                               
                                                                                                        
Udintsev et al.    rat           50 Hz, 20 mT; 0.25- to         Changes in iodine uptake by the thyroid 
 (1978)                          6.5-h and 24 h exposures       and thyroxine uptake by tissues         
                                                                                                        
Udintsev &         rat           50 Hz, 20 mT; 1-day            Metabolic changes in testicle tissue    
 Khlynin (1979)                  exposure                                                               
                                                                                                        
Kronenberg &       cultured      60 Hz, 2.33 mT; 4-day          No effect on cell growth rate           
 Tenforde          mouse         exposure                                                               
 (1979)            tumour cells                                                                         
                                                                                                        
Chandra & Stefani  mouse         60 Hz, 0.16 T; 1-h daily       No effect on tumour growth rate         
 (1979)            mammary       exposures for 1 - 4 days                                               
                   carcinoma                                                                            
                                                                                                        
Goodman et al.     slime mould   75 Hz, 0.2 mT; 400-day         Lengthened nuclear division cycle and   
 (1979);                         exposure                       altered respiration rate (decreased O2  
 Greenebaum et al.                                              uptake)                                 
 (1979, 1982)                                                                                           
                                                                                                        
Chiabrera et al.   frog          Single bidirectional pulses    Changes in chromatin structure in the   
 (1979)            erythrocytes  at 40 - 70 Hz, or 4-kHz bursts cell nucleus, suggestive of             
                                 of bidirectional pulses with   dedifferentiation                       
                                 10 - 20 Hz repetition rate;                                            
                                 2-mT peak intensity; 12- to                                            
                                 24-h exposures                                                         
--------------------------------------------------------------------------------------------------------
                                                                                                        

                                                                                                        
Table 10.  (contd.)                                                                                     
--------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results                                 
                   specimen                                                                             
--------------------------------------------------------------------------------------------------------
Kolodub &          rat           50 Hz, 9.4, and 40 mT;         Altered brain metabolism at higher      
 Chernysheva                     5 h daily for 15 days          field intensity, including decreased    
 (1980)                                                         rate of respiration, decreased levels   
                                                                of glycogen, creatine phosphate and     
                                                                glutamine, and increased DNA content    
                                                                                                        
Fam (1981)         mouse         60 Hz, 0.11 T; 23 h daily for  Decreased body weight and increased     
                                 7 days                         water consumption; haematology, organ   
                                                                histology and reproduction not affected 
                                                                                                        
Aarholt et al.     bacteria      16.66 and 50 Hz, 0 - 2 mT;     Decreased growth rate                   
 (1981)                          10- to 12-h exposure                                                   
                                                                                                        
Ramon et al.       bacteria      60 and 600 Hz, 2 mT; 17- to    Decreased growth rate and cytolysis     
 (1981)                          64-h exposure                                                          
                                                                                                        
Toroptsev &        rat           50 Hz, 20 mT; 1- to 24-h       Pathomorphological changes in brain     
 Soldatova                       exposure                                                               
 (1981)                                                                                                 
                                                                                                        
Kolodub et al.     rat           50 Hz, 9.4 - 40 mT, daily      Changes in carbohydrate metabolism      
 (1981)                          3-h exposures for up to 6      in the myocardium                       
                                 months                                                                 
                                                                                                        
Sakharova et al.   rat           50 Hz, 20 mT, 1-day exposure   Changes in catecholamine content and    
 (1981)                                                         morphology in brain, heart, liver,      
                                                                spleen, and circulatory system          
                                                                                                        
Delgado et al.     chicken       10, 100, and 1000 Hz; 0.12,    Morphological abnormalities in nervous  
 (1981, 1982)      embryo        1.2, and 12 µT; 0.5-ms         tissue, heart, blood vessels, and       
                                 rectangular pulses; 2-day      somites                                 
                                 exposure                                                               
                                                                                                        
Soldatova          rat           50 Hz; 20, 40, and 70 mT;      Pathomorphological changes in brain     
 (1982)                          6.5 h daily for 5 days, or     tissue                                  
                                 24-h continuous exposure                                               
--------------------------------------------------------------------------------------------------------
                                                                                                        
Table 10.  (contd.)                                                                                     
--------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results                                 
                   specimen                                                                             
--------------------------------------------------------------------------------------------------------
Sander et al.      human being   50 Hz, 5 mT; 4-h exposure      No changes in ECG, EEG, hormones, blood 
 (1982)                                                         cell counts, or blood chemistry         
                                                                                                        
Luben et al.       mouse         Single bidirectional pulses    Reduced cAMP production in response     
 (1982)            osteoblast    at 72 Hz, or 4-kHz bursts of   to parathyroid hormone                  
                   culture       bidirectional pulses with                                              
                                 15 Hz repetition rate; 2 mT                                            
                                 peak intensity; 3-day exposure                                         
                                                                                                        
Shober et al.      mouse         10 Hz, 1 mT; 1-day exposure    Decreased sodium ion content of liver   
 (1982)                                                                                                 
                                                                                                        
Norton (1982)      cultured      4 kHz bursts of bidirectional  Increased hydroxyproline,               
                   chicken       pulses with 15 Hz repetition   hyaluronate, and DNA synthesis;         
                   embryo        rate; 2 mT peak intensity;     decreased glycosoaminoglycans;          
                   sternum       four 6-h exposures during 2    increased lysozyme activity             
                                 days                                                                   
                                                                                                        
Aarholt et al.     bacteria      Square wave pulses at 50 Hz;   Changes in rate of      
 (1982)             (E. coli)     0.20 - 0.66 mT, 2- to 3-h      beta-galactosidase synthesis                               
                                 exposures                                                              
                                                                                                        
Dixey & Rein       rat pheo-     500 Hz; bidirectional pulses;  Stimulation of noradrenaline release    
 (1982)            chromocytoma  160 - 850 µT; 3-h exposure                                             
                   cell  in                                                                              
                    vitro                                                                                
                                                                                                        
Conti et al.       cultured      1, 3, 50, and 200 Hz; 2.3 -    Inhibition of lectin-induced            
 (1983)            human         6.5 mT; square-wave pulses;    mitogenesis by 3- and 50-Hz fields      
                   lymphocyte    3-day exposure                                                         
                                                                                                        
Goodman et al.     sciara-       Single bidirectional pulses    Initiation and increase of RNA          
 (1983)            coprophila    at 72 Hz, or 4-kHz bursts of   transcription at defined loci           
                   salivary      bidirectional pulses with 15                                         
                   giant         Hz repetition rate; 2 mT peak
                   chromosome    intensity; 5- to 90-min
                                 exposures
---------------------------------------------------------------------------------------------------------


Table 10.  (contd.)
---------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results
                   specimen
---------------------------------------------------------------------------------------------------------
Jolley et al.      rabbit        4 kHz bursts of bidirectional  Reduced Ca++ content and efflux;
 (1983)            pancreas      pulses with 15-Hz repetition   reduced insulin release during
                                 rate; 2-mT peak intensity;     glucose stimulation
                                 18-h exposure

Ramirez et al.      Drosophila    0.5-ms square-wave pulses at   Decreased viability of eggs
 (1983)            egg           100 Hz, 1.76 mT peak-to-peak
                                 intensity; or 50 Hz, 1.41 mT
                                 sinusoidal field; 2-day
                                 exposure

Ubeda et al.       chicken       0.5 ms bidirectional pulses    Teratogenic changes in nervous system,
 (1983)            embryo        at 100 Hz (4 different         circulatory system, and foregut
                                 waveforms); 0.4- to 104-µT                
                                 peak intensity; 2-day 
                                 exposure 

Archer &           cultured      1 Hz, 15 - 60 mT square-       Decreased collagenous and non-
 Ratcliffe         chicken       wave pulses; 7-day exposure    collagenous protein synthesis; no
 (1983)            tibia                                        alteration in glycosoaminoglycan DNA
                                                                synthesis

Liboff et al.      cultured      15 Hz - 4 kHz; 2.3 - 560 µT;   Increased DNA synthesis
 (1984)            human         18- to 96-h exposures
                   fibroblast

Cain et al.        cultured      Single bidirectional pulses    Inhibition of cAMP production and
 (1984)            mouse         at 72 Hz, or 4-kHz bursts of   Ca++ release in response to
                   calvarium     bidirectional pulses with      parathyroid hormone
                                 15-Hz repetition rate;
                                 2.5-mT peak intensity;
                                 exposure for 1 - 16 h

Temur'yants        rat           8 Hz, 5.2 µT; daily 3-h        Transient hyperlipaemia in blood-serum
 et al. (1985)                   exposures for up to 45 days
---------------------------------------------------------------------------------------------------------


Table 10.  (contd.)
---------------------------------------------------------------------------------------------------------
Reference          Test          Exposure conditionsa           Results
                   specimen
---------------------------------------------------------------------------------------------------------
Murray &           cultured      15-Hz bidirectional pulses;    Enhanced collagen and total protein
 Farndale          chicken       2.2-mT peak intensity; daily   synthesis, and decreased cAMP after
 (1985)            fibroblast    12-h exposures for 1 -         6 days of exposure
                                 8 days

Cain et al.        cultured      Single bidirectional pulses    Decreased cAMP production and increased
 (1985)            mouse         at 15 Hz; 0.8-mT peak          ornithine decarboxylase activity in
                   calvarium     intensity; 15- to 60-min       response to parathyroid hormone
                                 exposures

Ueno et al.        toad          20 Hz, 2 kHz, and 20 kHz;      Teratogenic effects
 (1985)            embryo        10 - 15 mT; 15-min to 8-h
                    (Xenopus      exposures
                    laevis)

Gundersen &        rat muscle    60 and 70 Hz (linear and       Effects on miniature endplate
 Greenebaum                      circular polarization);        potentials
 (1985)                          0.1 mT; 10-min exposure

Winters et al.     human and     60 Hz, 0.1 mT; 24-h exposure   No effects on mitogen responses, DNA,
 (1985a,b)         dog leuko-                                   RNA or protein synthesis, or levels
                   cytes                                        of cell surface receptors

Phillips et al.    cultured      60 Hz, 0.14 mT; 1-day          Increase in growth rate, number
 (1986a,b)         human         exposure                       transferring receptors, and expression
                   colon                                        of tumour-specific antigens
                   tumour
---------------------------------------------------------------------------------------------------------
a  The magnetic fields were sinusoidal unless otherwise indicated.
    These observations were made with sinusoidal and square-wave 
time-varying magnetic fields and with pulsed magnetic fields that 
had repetition rates in the ELF frequency range.  With few 
exceptions, the peak field intensities that were used exceeded 0.5 
mT and the current density induced in the exposed samples exceeded 
10 mA/m2.  The currents induced within the cellular and tissue 
fluids were therefore at, or above, the upper limit of the 
naturally occurring levels. 

    It is noteworthy that most of the studies with square waveforms 
and with pulsed fields that induced current densities greater than 
10 mA/m2 led to findings of positive bioeffects (Delgado et al., 
1981; Dixey & Rein, 1982; Luben et al., 1982; Norton, 1982; Archer & 
Ratcliffe, 1983; Conti et al., 1983; Goodman et al., 1983; Jolley 
et al., 1983; Ramirez et al., 1983; Cain et al., 1984).  
Developmental effects were observed at lower induced current 
density levels by Delgado et al. (1982) and Ubeda et al. (1983), 
when they  exposed chick embryos to pulsed magnetic fields.  
Juutilainen et al. (1986) and Juutilainen & Saali (1986) found that 
this effect depended on the waveform and frequency of the magnetic 
field.  A large international study is now in progress in an effort 
to replicate these findings.  It has been suggested that the 
currents induced by such fields could exert an electrochemical 
effect at the cell surface (Luben et al., 1982; Jolley et al., 
1983).  This effect, in turn, may influence hormone-receptor 
interactions, adenylate cyclase activity, and the membrane 
transport and intracellular concentration of calcium ions.  All of 
these membrane functions are known to play an important role in 
cell metabolism and growth dynamics. 

    Aarholt et al. (1982) measured the rate of beta-galactosidase 
synthesis in cultures of  E. coli exposed to 50-Hz square-wave 
magnetic fields, in order to investigate the effect of such 
exposure on the lac operon function.  Following a 30-min exposure 
at 0.2 mT - 0.30 mT, a decrease in beta-galactosidase synthesis 
rate of about one-third was reported.  At 0.32 mT, the synthesis 
rate returned to control values, and increased by a factor of 2 at 
0.54 and 0.56 mT.  No differences compared with control values was 
seen at 0.58 mT and higher values up to 0.70 mT.  No measurements 
were made at higher field strengths. 

    Chiabrera et al. (1979) reported a decrease in the chromatin 
density of frog erythrocytes exposed to pulsed magnetic fields, 
such as those used in bone growth stimulation.  This imparted to 
the cells an appearance of earlier maturation stages.  There were 
morphological and cytophotometric changes in chromatin density, 
which suggested gene depression, but such a conclusion does not 
appear to be justified, since RNA, protein, and/or haemoglobin 
synthesis were not investigated. 

    Using biochemical and autoradiographic techniques, Goodman et 
al. (1983) demonstrated the initiation of RNA transcription at two 
different sets of loci in salivary gland giant chromosomes exposed 
to pulsed magnetic fields.  One set of loci became activated 
following 45 min of exposure to single pulses with a 72-Hz 
repetition rate, another set after 15 min of exposure to pulse 

trains with a repetition rate of 15 Hz.  Changes in protein 
synthesis in salivary gland cells exposed under identical 
conditions, reported by Ryaby et al. (1983), offer confirmatory 
evidence.  All the reports quoted above seem to indicate that 
pulsed magnetic fields may affect gene expression.  However, it 
should be noted that these studies were not duplicated or otherwise 
verified by independent teams of research workers. 

    Eighteen of the investigations with ELF sinusoidal magnetic 
fields have involved exposure of rodents to 50-Hz and 60-Hz fields 
with intensities ranging from 0.01 to 0.8 T (Odintsov, 1965; Druz & 
Madiyevskii, 1966; Tarakhovsky et al., 1971; Toroptsev et al., 
1974; Udintsev & Moroz, 1974; Mizu-shima et al., 1975; Udintsev et 
al., 1976; Sakharova et al., 1977, 1981; Kolesova et al., 1978; 
Udintsev et al., 1978; Udintsev & Khlynin, 1979; Chandra & Stefani, 
1979; Kolodub & Chernysheva, 1980; Fam, 1981; Kolodub et al., 1981; 
Toroptsev & Soldatova, 1981; Soldatova, 1982).  With the exception 
of one report in which tumour growth rate was observed not to be 
influenced by brief exposure to a 60-Hz, 0.16-T field (Chandra & 
Stefani, 1979), all of the studies report positive findings of 
cellular and tissue effects from ELF magnetic fields.  The maximum 
current densities induced in the experimental animals by the 
applied field exceeded approximately 10 mA/m2 in these studies, and 
were therefore at, or above, the upper limit of the endogenous 
currents that are normally present within the body (Bernhardt, 
1979). 

    In contrast to the findings of positive biological effects 
listed above, present evidence suggests that animal haematological 
parameters are unaffected by ELF magnetic fields at intensities 
that reportedly influence other cellular and tissue systems.  With 
the exception of one isolated report (Tarakhovsky et al., 1971), 
all of the published studies on haematological parameters in 
exposed animals have shown no consistent field-associated effects 
(Beischer et al., 1973; deLorge, 1974; Mantell, 1975; Goldberg & 
Mel'nik-Guykazyan, 1980; Fam, 1981; Sander et al., 1982).  The 
apparent lack of sensitivity of the haematological system to 
magnetic fields is in distinct contrast to the well-documented 
effects of ionizing radiation and high-intensity microwave fields 
on this particular physiological system. 

    Three of the studies listed in Table 10 involved short-term 
exposures of human volunteers to ELF magnetic fields (Beischer et 
al., 1973; Mantell, 1975, Sander et al., 1982).  With the exception 
of one unconfirmed report of an elevation in serum-triglycerides in 
the exposed subjects (Beischer et al., 1973), none of these 
investigations revealed adverse effects of ELF magnetic fields with 
intensities comparable to or exceeding the levels generally 
encountered by man.  Particularly notable in this regard is the 
report by Sander et al. (1982), who observed that a 4-h exposure of 
human volunteers to a 50-Hz, 5-mT field produced no changes in 
serum chemistry, blood cell counts, blood gases and lactate 
concentration, electrocardiogram, pulse rate, skin temperature, 
hormones (cortisol, insulin, gastrin, thyroxine), and various 
neuronal measurements, including visually evoked potentials 
recorded in the electroencephalogram. 

6.5  Effects of Pulsed Magnetic Fields on Bone Growth and Repair

    Direct current electrical stimulation has been used since the 
nineteenth century for the treatment of bone non-unions and 
pseudarthroses.  Although this procedure has met with some success 
clinically, the use of direct currents has been shown to produce 
several undesirable side-effects including: 

    (a) surgical trauma and a risk of infection through the
        implantation of electrodes in bone;

    (b) the development of electrode polarization with time,
        which leads to increased impedance and decreased
        current for a given applied voltage;

    (c) osteogenesis, which has been found to increase near
        the negative electrode (cathode), but decrease near
        the positive electrode (anode).

    These disadvantages of direct current electrical stimulation 
have been overcome by the recent introduction of pulsed magnetic 
field generators as a means of inducing ELF electrical currents 
within bone tissue (Bassett et al., 1974).  By using magnetic coils 
placed about a limb containing a fractured bone, electric fields 
with a typical strength of 0.2 - 2 V/m can be induced within the 
bone tissue.  In the usual configuration, two coils are placed 
about the limb and positioned such that the bone fracture lies 
along a line joining the centres of the coils, and hence along the 
magnetic field lines.  Assuming the conductivity of bone to be 0.01 
S/m at ELF frequencies (Lunt, 1982), the local current densities 
induced in bone by the pulsed magnetic fields can be estimated to 
lie in the range of approximately 2 - 20 mA/m2.  Initial studies on 
bone fracture reunion in dogs demonstrated that a pulse repetition 
frequency of 65 Hz was more effective than 1 Hz (Bassett et al., 
1974), and several subsequent studies have revealed that 
frequencies of 60 - 75 Hz are the most advantageous in facilitating 
fracture union and preventing pseudo-arthroses (Bassett, 1982). 

    Following the initial demonstration of the efficacy of pulsed 
magnetic fields in achieving bone fracture reunion in experimental 
animals, several successful clinical trials have been reported 
concerning the treatment of bone fractures and arthroses in human 
beings by this method.  In a four-year clinical trial involving 
more than 100 patients, Bassett et al. (1977) reported an 85% 
success rate in the treatment of long-established pseudo-arthroses.  
The successful use of pulsed magnetic fields in the facilitation of 
bone healing in human subjects has subsequently been reported by 
several clinical groups (Watson & Downes, 1978; Bassett et al., 
1982; Hinsenkamp, 1982; Bigliani et al., 1983). 

    Barker et al. (1984) recently published an interim report on a 
double-blind clinical trial in which 9 patients with non-united 
tibial fractures were treated with active magnetic stimulators, 
while a group of 7 control patients were fitted with dummy 
stimulators.  After 24 weeks of treatment, the fracture united in 5 
of the 9 patients with active stimulators, and fractures in 5 of 

the 7 patients with dummy stimulators also united.  Thus, there was 
no statistically significant difference between the treated and 
control groups.  This preliminary result suggests that earlier 
claims of clinical success with pulsed magnetic field applicators 
may have been biased by the use of control groups that were not 
subjected to the same immobilization procedure as the patients 
undergoing active treatment.  Controlled, double-blind studies on 
large numbers of patients are needed to assess this modality of 
treatment. 

    The mechanism by which the weak ELF electric currents induced 
in bone tissues by pulsed magnetic fields could exert an influence 
on fracture repair is also under investigation in a number of 
laboratories.  Evidence from  in vitro studies on osteoblasts and 
chondrocytes indicates that the pulsed fields influence hormone 
binding to receptors at the cell surface, and thereby depress the 
intracellular concentration of calcium ions and cyclic AMP 
(Bassett, 1982; Luben et al., 1982).  These effects, in turn, can 
significantly influence cellular metabolism and stimulate growth.  
Studies by Hinsenkamp & Rooze (1982) with  in vitro cultures of 
limbs from mouse fetuses demonstrated that electromagnetic 
stimulation leads to chondrocyte proliferation and an improved 
alignment of trabeculae and cartilage.  Archer & Ratcliffe (1983) 
reported that cultured tibias from chicken embryos had a reduced 
collagen content following exposure to a pulsed magnetic field for 
7 days.  The observation was also made by these workers that the 
total synthesis of sulfated glycosoaminoglycans, which are major 
components of the extracellular matrix, was not affected by 
exposure to the pulsed magnetic field.  The further elucidation of 
the macromolecular and developmental changes that accompany the 
stimulation of bone tissue by pulsed ELF magnetic fields remains a 
challenging area of research, which will ultimately lend useful 
insight into the mechanisms by which weak ELF fields interact with 
living cells. 

6.6  Conclusions

    A well established and repeatable effect of human exposure to 
ELF magnetic fields is the induction of magnetophosphenes.  This 
effect shows a strong frequency dependence on flux density.  The 
threshold for magnetophosphenes is between 2 and 10 mT in the 
frequency range of 10 - 100 Hz. 

    Much more intense fields are required to directly stimulate 
nerve and muscle tissue.  These effects are also frequency 
dependent with thresholds above 50 mT (10 - 100 Hz). 

    Numerous investigations with ELF magnetic fields with 
sinusoidal, square-wave, and pulsed waveforms have led to reports 
of alterations in cell, tissue, and animal systems, when the 
induced current density exceeded approximately 10 mA/m2.  These 
reported changes have included alterations in cell metabolism and 
growth properties, gene expression, endocrine and immune functions, 
and teratological and developmental effects.  However, several of 
these studies have not been successfully replicated. 

    A large number of laboratory studies have revealed evidence of 
changes in cellular metabolism and growth properties as a result of 
exposure to pulsed magnetic fields.  However, in clinical 
applications of these fields for the facilitation of bone fracture 
reunion, not enough double-blind studies on large numbers of 
patients have been carried out to assess the efficacy of this 
treatment. 

7.   HUMAN STUDIES

    Since epidemiological studies have assumed an important role in 
the assessment of the human health risks of non-ionizing radiation 
exposure, the characteristics of these studies must be considered 
relevant to determining causal relationships.  Although there are 
inherent limitations in an observational method, sufficient data 
can be compiled from epidemiological studies to establish a causal 
relationship, as has occurred, for example, for cigarette smoking 
and lung cancer. 

    The term causality is used when there is a biological 
association, and where a statistical pattern can be inferred.  In 
general terms, a causal relationship is supported by a strong 
association between exposure and disease.  Consistency in 
demonstrating the same association across different populations, 
for example different occupational groups or different regions of 
the country, supports a causal relationship.  Exposure to the 
physical factor prior to the effect is absolutely necessary for the 
association to be interpreted as causal.  A dose-response 
relationship in which risk shows a positive correlation with a 
level of exposure provides a stronger inference of causality.  
Although the mechanism involved does not need to be known exactly, 
it is highly desirable to develop a predictive theory. 

7.1  Studies on Working Populations

7.1.1  Workers exposed to static magnetic fields

    Studies on Soviet workers involved in the manufacture of 
permanent magnets indicated various subjective and physiological 
symptoms: irritability, fatigue, headache, loss of appetite, 
bradycardia, tachycardia, decreased blood pressure, altered EEG, 
itching, burning, and numbness (Vyalov et al., 1964; Vyalov & 
Lisichkina, 1966; Vyalov, 1967).  The strength of the magnetic 
fields causing these symptoms was not reported and there was no 
control group, which significantly reduces the value of the 
reports.  A later study on workers in industries involving magnet 
production and machine building (Vyalov, 1971, 1974), involving 645 
exposed persons and 138 controls, reported subjective complaints 
and minor physiological effects, especially in haematological and 
cardiovascular indices.  The average static magnetic field 
strengths to which these workers were exposed were typically 2 - 5 
mT at the level of the hands and 0.3 - 0.5 mT at the chest and head 
levels.  Unfortunately, no statistical analyses were performed. 

    Marsh et al. (1982) studied workers (320 exposed, 186 controls) 
in the USA employed in industries using electrolytic cells that 
generated large static magnetic fields.  The exposed workers were 
subjected to average magnetic fields of 7.6 mT in operator 
accessible locations and maximum fields of 14.6 mT.  The time-
weighted average field exposures were calculated to be 4 and 11.8 
mT for the mean and maximum field levels, respectively.  Although 
no major health effects were found, minor haematological 
alterations and blood pressure changes were observed. 

    The prevalence of 19 common diseases was studied in 792 workers 
in high-energy accelerator laboratories, bubble chambers, calutrons 
(isotope separation facilities), and high-field magnet facilities, 
compared with the same number of matched controls (Budinger et al., 
1984b).  A subgroup of 198 workers exposed to 0.3 T or higher 
static fields for 1 h or longer was also compared with matched 
controls.  No significant changes were found in the prevalence of 
diseases of the skin; circulation; respiratory tract; male genital 
organs; genito-urinary tract; bone, muscle, and tendon; gastro-
intestinal tract; nervous system; liver and gall bladder; blood; 
and eye.  The prevalences of benign and malignant diseases, 
allergic and metabolic diseases; senility and other ill-defined 
diseases; and accidents including poisonings were also unaffected. 

    In a study on 211 contact welders in the USSR, Abramovich-
Poljakov et al. (1979) showed an increase in nervous system 
disorders and leukocyte counts, and alterations in ECG, compared 
with 113 non-welders.  Although the authors related this to 
exposure to 0.1- to 0.2-s pulsed magnetic fields of strengths 1000 
- 100 000 A/m (1.25 mT - 125 mT), exposure to other hazards, such 
as metal fumes could also be expected to lead to effects on health. 

    Milham (1979, 1982, 1985b) reported that workers in the 
aluminium industry have a significantly elevated mortality from all 
classes of leukaemia and from acute leukaemia.  This conclusion was 
based on a study of the death records of 438 000 males in the state 
of Washington (USA) from 1956-79.  The proportionate mortality 
ratios (PMRs) for all classes of leukaemia and acute leukaemia 
among aluminium workers were 189 and 258, respectively ( P < 0.01).  
This finding was subsequently confirmed by Rockette & Arena (1983), 
though their broader study involving 14 aluminium plants in the USA 
showed only a small overall excess of leukaemia mortality with a 
standardized mortality ratio of 127.9, which was not statistically 
significant.  The study by Rockette & Arena (1983) also revealed a 
trend towards increased pancreatic cancer, lymphohaematopoietic 
cancers, genito-urinary cancer, non-malignant respiratory disease, 
and various unspecified benign neoplasms.  Overall, the elevated 
risk of these various cancers was not statistically significant.  
Milham (1982) suggested that the elevated risk of leukaemia among 
aluminium workers might be associated with exposure to the static 
magnetic fields that result from the high DC electric currents used 
in the electrolytic reduction of alumina to aluminium metal.  
However, at present, there is no clear evidence indicating a link 
between the magnetic fields present in aluminium plants and the 
increased incidence of leukaemia or other cancers.  The process 
used for aluminium production creates coal-tar pitch volatiles, 
fluoride fumes, sulfur oxides, and carbon dioxide.  All of these 
environmental contaminants must be taken into account in any 
attempt to relate magnetic field exposure and cancer risk among 
workers in the aluminium industry. 

    Two other recent studies on persons exposed occupationally to 
static magnetic fields have failed to detect an elevated risk of 
cancer (Budinger et al., 1984b; Barregard et al., 1985).  The 
results of the study by Budinger et al. (1984b) did not reveal any 

elevation in the incidence of benign or malignant neoplasms among 
792 exposed workers compared with an equal number of matched 
controls.  Barregard et al. (1985) studied cancer incidence during 
a 25-year period among a small cohort of workers at a chloroalkali 
plant where the 100-kA DC currents used for the electrolytic 
production of chlorine gave rise to static magnetic fields of 4 - 
29 mT in the working environment.  The observed versus expected 
incidence of cancer among these workers was not significantly 
different. 

    Some of the reported effects in man exposed to magnetic fields 
are summarized in Table 11.  Although these studies are 
inconclusive, they suggest that, if long-term effects occur, they 
are very subtle, since no cumulative gross effects are evident.  In 
general, the available data on cancer incidence among workers in 
occupations that involve exposure to large static magnetic fields 
do not support an association between cancer incidence and exposure 
to these fields. 

7.1.2  Cancer epidemiological studies on workers exposed to ELF 
electromagnetic fields

    Preliminary observations, some published as letters to the 
editor (Milham 1982; Wright et al., 1982; McDowall, 1983; Vagerö & 
Olin, 1983; Coleman et al., 1983; Gilman et al., 1985; Lin et al.,
1985; Milham, 1985a,b; Pearce et al., 1985; Stern et al., 1986) 
reported an epidemiological association of leukaemia and other 
tumours with electrical/electronic occupations involving presumed 
exposure to power-frequency electromagnetic fields (Table 12). 
Table 11.  Studies of workers exposed to static magnetic fields
---------------------------------------------------------------------------
Exposure                 Reported effects             Reference
characteristics          (exposed population)
---------------------------------------------------------------------------
Workers in magnet        Subjective and minor         Vyalov (1974)
production; average      physiological effects
exposure: 2 - 5 mT       (645 exposed, 138 controls,
(hands), 0.3 - 0.5 mT    no statistical analysis)
(chest and head)

Contact welders; 0.1-    Increased nervous system,    Abramovich-
to 0.2-s pulsed magnetic cardiac, and blood           Poliakov et al.
fields of 1.25 - 125     disorders (211 exposed,      (1979)
mT, 8 h/day              113 controls)

Workers in aluminium     Increased risk of leukaemia  Milham (1979, 1982,
plants (no fields        (death records of            1985b)
reported)                438 000 males, but few 
                         cases)
---------------------------------------------------------------------------

Table 11 (contd.)
---------------------------------------------------------------------------
Exposure                 Reported effects             Reference
characteristics          (exposed population)
---------------------------------------------------------------------------
Industries using         Minor haematological         Marsh (1982)
electrolytic cells       alterations, but no major
(average, 7.6 mT;        health effects (320
maximum, 14.6 mT)        exposed, 186 controls)

Workers in aluminium     Small excess of leukaemia    Rockette &
plants (no fields        mortality; non-significant   Arena (1983)
reported)                risk of other cancers

High energy accelerator  No increased prevalence of   Budinger et al.
laboratory (fields up    19 common diseases           (1984b)
to 2 T)                  including cancers (792
                         exposed, 792 controls)

Electrolytic production  No increased incidence of    Barregard et al.
of chlorine (fields      cancer over 25-year period   (1985)
4 - 29 mT)
---------------------------------------------------------------------------

    In an analysis of data for occupational mortality, Milham 
(1982) noted higher than expected proportionate mortality due to 
acute myeloid leukaemia among men "whose occupation requires them 
to be in electric or magnetic fields."  The data base consisted of 
438 000 deaths of men, 20 years of age or older who, from 1950 to 
1979, were residents of Washington state (USA).  Although the 
proportionate mortality ratio (PMR = observed/expected x 100) is a 
useful statistical measure, it has technical limitations that 
should be explored in a complete study.  PMRs significant at the 
 P < 0.01 level were observed for "electricians", TV and radio 
repairmen, power-station operators, and aluminium workers, though 
similarity in field exposure among these groups was not proved and 
is unlikely. 
Table 12.  Cancer incidence and occupational exposure to power frequency
electromagnetic fields
---------------------------------------------------------------------------
Reference               Subject                      Cancer risk
---------------------------------------------------------------------------
Wiklund et al. (1981)   Telecommunication workers    No cancer risk

Milham (1982, 1985b)    Electrical occupations       Increased leukaemia

Wright et al. (1982)    Electrical occupations       Increased leukaemia

McDowall (1983)         Electrical occupations       Increased leukaemia

Coleman et al. (1983)   Electrical occupations       Increased leukaemia

Vagerö & Olin (1983)    Electrical occupations       No leukaemia risk
---------------------------------------------------------------------------

Table 12 (contd.)
---------------------------------------------------------------------------
Reference               Subject                      Cancer risk
---------------------------------------------------------------------------

Swerdlow (1983)         Electrical occupations       Increased eye 
                                                     melanoma

Pearce et al. (1985)    Electrical occupations       Increased leukaemia

Lin et al. (1985)       Electrical occupations       Increased brain
                                                     tumours

Milham (1985a)          Amateur radio operators      Increased leukaemia

Gilman et al. (1985)    Males in underground mines   Increased leukaemia

Vagerö et al. (1985)    Electrical occupations       No leukaemia risk;
                                                     increased urinary
                                                     cancer; increased
                                                     malignant melanoma

Calle & Savitz (1985)   Electrical occupations       No leukaemia risk

Olin et al. (1985)      Electrical occupations       Increased malignant
                                                     melanoma

Stern et al. (1986)     Electrician and welders      Increased leukaemia

Tornqvist et al.        Electric power industry      No leukaemia risk;
(1986)                                               no brain tumour risk
---------------------------------------------------------------------------

    Wright et al. (1982) sought to verify Milham's (1982) results 
by examining a similar statistic, the proportional incidence ratio 
(PIR) of a different and much smaller data base.  They found 
significant increases ( P < 0.05) in the incidence of acute myeloid 
leukaemia, based on a total of 4 cases in power linemen and 
telephone linemen, two groups for which the Washington data yield 
insignificant PMRs.  Calle & Savitz (1985) analysed mortality from 
leukaemia among 81 men in electrical occupations in Wisconsin 
during the period 1963-78.  The classification of occupational 
groups used by these authors was identical to those of Milham 
(1982) and Wright et al. (1982).  PMR was calculated on the basis 
of all deaths occurring during this period in Wisconsin.  No excess 
mortality from leukaemia was found, with the possible exception of 
acute leukaemia in electrical engineers.  The PMR was 257 (one-
sided  P < 0.05).  When the leukaemia mortality data were pooled 
across all 10 electrical occupations, the PMR values were 103 and 
113 for all leukaemia and acute leukaemia, respectively.  Calle & 
Savitz (1985) concluded, on this basis, that there was no 
consistent overall pattern of excess leukaemia risk among workers 
in electrical occupations. 

    Additional data on occupational leukaemia rates in the United 
Kingdom were provided in two letters to the editor.  McDowall (1983) 
found increased evidence of leukaemia in occupationally exposed 
electrical workers using PMRs and also by a case-control study.  
Coleman et al. (1983) also examined the leukaemia incidence for the 
same electrical occupations with evidence for a 17% excess that was 
especially strong for electrical fitters and telegraph operators, 
for whom the extent of electric or magnetic field exposure has not 
been established. 

    The suggestion of a small, but significant, increase in the 
risk of leukaemia in electrical workers in New Zealand with the 
potential for exposure to alternating electrical and magnetic 
fields was found by Pearce et al. (1985).  The authors stated that 
their study would also support that the increased risk of leukaemia 
was due to exposure to metal fumes and substances used in 
electrical component assembly, since the greatest excess of risks 
was found for electronic equipment assemblers and radio and 
television repairers. 

    A recent study by Stern et al. (1986) has led to the 
observation of an elevated incidence of leukaemia among 
electricians and welders in the Portsmouth Naval Shipyard (New 
Hampshire, USA).  A matched case-control study was conducted of 53 
leukaemia deaths and 212 controls identified from a population of 
24 545 workers employed at this naval nuclear shipyard between 1 
January 1952 and 15 August 1977.  No correlations were found 
between leukaemia mortality and exposure to ionizing radiation or 
to organic solvents.  The Mantel-Haenszel odds ratio was 3 for the 
mortality from lymphatic leukaemia among the electricians ( P < 
0.05).  For welders, the odds ratio was 2.25 for myeloid leukaemia 
( P < 0.05).  These elevations in leukaemia mortality were 
attributed by the authors to electromagnetic field exposure among 
workers in the affected groups. 

    Other studies on groups with presumed occupational exposure to 
electromagnetic fields have failed to detect an excess of leukaemia 
cases (Vagerö & Olin, 1983; Vagerö et al., 1985; Tornqvist et al., 
1986).  However, in these studies, a significant increase in the 
incidence of pharyngeal cancer (Vagerö & Olin, 1983), urinary 
cancer (Vagerö et al., 1985), and malignant skin melanoma (Olin et 
al., 1985; Vagerö et al., 1985) was noted.  An excess risk of 
malignant melanoma of the skin was primarily associated with 
occupations that involved soldering. 

    Using years of employment as a measure of exposure to 
electromagnetic fields, Gilman et al. (1985) reported a significant 
increase in the incidence of leukaemia among white male coal miners 
who had worked for more than 25 years underground compared with 
miners who had worked for less than 25 years underground.  It was 
suggested that the electromagnetic fields associated with power 
lines, transformers, etc. were a possible factor in this increased 
risk. 

    In an epidemiological study on telecommunications workers based 
on the Swedish Cancer Environment Registry, Wiklund et al. (1981) 
did not find any increased risk for this occupational group 
compared with the Swedish population as a whole. 

    Swerdlow (1983) suggested an association with an increase in 
the incidence of adult melanoma of the eye in electrical and 
electronic workers and also the non-manual social classes (white-
collar workers). 

    An increased incidence of cancer deaths in male members of the 
American Radio Relay League in California and Washington States was 
found by Milham (1985a).  Lin et al. (1985) recently reported an 
increased number of brain tumour deaths among white male workers in 
3 electrical/electronic occupations in the state of Maryland (USA) 
during the period 1969-82.  The Mantel-Haenszel odds ratio was 2.15 
(with a 95% confidence interval of 1.10 - 4.06) for workers who had 
experienced definite electromagnetic field exposure during the 
course of their work. 

7.1.3  Conclusions

    The association between cancer incidence and occupational 
exposure to power-frequency electric and magnetic fields suggested 
by many of the recent epidemiological studies reviewed here is not 
clearly consistent.  In many of these studies, the ELF field levels 
to which the occupational groups under study were exposed were not 
characterized.  Also, in a number of the investigations, 
confounding variables of high carcinogenic potential, e.g., certain 
organic fumes and hydrocarbon particulates, were not taken into 
account.  Therefore, even if it is concluded that the risk of 
leukaemia or other types of cancer was increased for certain 
occupational groups, it does not follow that the ELF electric or 
magnetic field exposure was the relevant etiological factor. 

    In general, given the limited statistical power of the studies 
reported to date, the reported increase in the incidence of 
leukaemia and other cancers has been less than a factor of 2 (for 
example, from 1 per 106 to 2 per 106) compared with a case-control 
group or the general population.  These epidemiological studies have 
often involved very few disease cases in an occupational category, 
as well as inconsistent category definitions.  As discussed in the 
introduction to this section, epidemiological methods can detect 
associations with a reasonable degree of certainty in studies such 
as these, if appropriate criteria are applied to a large enough 
data base of good integrity.  The suggestion of leukaemia and other 
cancers related to ELF electromagnetic field exposure raises 
important questions that should be addressed by studies of adequate 
statistical power, in which confounding variables are taken into 
account.  There is an urgent need for well-designed experimental 
studies on the carcinogenic effects of ELF electromagnetic field 
exposure, using the time-honoured methods that have been previously 
used for testing the carcinogenic effects of chemical substances. 
Until such data are obtained and additional epidemiological studies 
are carried out, the problem of the carcinogenic effects of ELF 
electromagnetic field exposure should be considered to be unresolved. 

7.2  Epidemiological Studies on the General Population

    Wertheimer & Leeper (1979) reported a 2- to 3-fold increase in 
the incidence of leukaemia among Colorado children, presumably 
exposed to fields from high electric current configurations.  
Magnetic fields (associated with the electric currents) were 
estimated by scoring the type of electrical wiring configuration 
close to the homes into categories of high or low current 
configurations. 

    The same authors (Wertheimer & Leeper, 1982) extended their 
work to a study of the incidence of adult cancer in those living 
near high-current electric wiring.  The associations demonstrated 
were not dependent on age, urbanicity, neighbourhood or socio-
economic level and were most clearly demonstrated where 
urban/industrial factors were not present to obscure the pattern. 
The four types of cancer that appeared to be particularly elevated 
in the exposed adult populations were cancer of the nervous system, 
uterus, breast, and lymphomas.  The authors suggested that 
magnetic fields might have a tumour-promoter effect, since the 
increases were maximal at 7 years from the time of taking up 
residence in the area. 

    These preliminary studies have limitations common to many 
epidemiological studies involving cohort selection and additional 
problems suggesting possible biases in the techniques for scoring 
the wiring configurations, and in the assumption that the scoring 
technique accurately determines magnetic field strength levels 
among the cases examined.  Further questions are raised, because 
cases were ascertained after death, and therefore no account was 
taken of cancer cases still alive and, because birth and death 
addresses were used, again introducing the potential for observer 
bias.  Considerable interest has been provoked by these findings 
and it is expected that many of the issues will be dealt with in 
future research. 

    The hypothesis that such weak magnetic fields (of the order of 
0.1 - 0.7 µT) produce biological effects has raised questions, such 
as those of Miller (1980), who criticized the Wertheimer & Leeper 
(1979) study on the basis that the magnetic field from electrical 
appliances in the home would far exceed contributions from 
electrical wiring configurations in the environment. 

    Tomenius et al. (1982) and Tomenius (1986) reported an 
increased incidence of tumours (malignant and benign) in children 
living in homes where the magnetic field outside the front door was 
more than 0.3 µT.  The data involved a small number of cases and 
again the field measurement was questionable, because the relation 
of personal exposure to the value of the field measured outside the 
home was not established.  Tomenius (1986) did not find an increased 
incidence of leukaemia but an increased incidence of nervous 
system tumours in residences with magnetic fields greater than 0.3 
µT.  Furthermore, if a cut-off magnetic field strength other than 
0.3 µT was used, no association of tumour incidence and magnetic 
field exposure would occur. 

    These studies, and the preliminary occupational data (see 
above) causing some concern in relation to electric or magnetic 
field exposure, must be investigated further to determine whether 
the suggested link with cancer induction or promotion can be 
established.  Recently, the results of three studies carried out in 
the United Kingdom did not show any association between magnetic 
fields and cancer (Coleman et al., 1985; Myers et al., 1985; 
McDowall 1986).  It should be noted that these studies are open to 
the same criticisms as those above that indicate an association, 
particularly with regard to the limited statistical power and lack 
of quantification of exposure.  A summary of studies on cancer 
incidence and population exposure to electromagnetic fields is 
given in Table 13. 

    Another aspect of ELF magnetic field effects that should be 
considered in the context of behavioural alterations is the report 
of a correlation between the incidence of suicides and the 
intensity of residential 50-Hz magnetic fields from power-line 
sources (Perry et al., 1981).  On the basis of coroner and police 
records from various urban and rural regions within a 5000 km2 area 
in the Midlands of England, a statistically significant increase in 
suicide rate was found among individuals who lived in residences 
where the 50-Hz field intensity exceeded 0.15 µT at the front 
entrance.  A subsequent statistical analysis of the same data 
indicated that the cumulative probability ratio for the incidence 
of suicide increased above the null effect level of unity for 
residential 50-Hz magnetic field intensities exceeding 15 nT 
(Smith, 1982).  However, oscillations occurred in the cumulative 
probability ratio as a function of increasing magnetic field 
intensity, and at 0.2 µT, the ratio for the "urban" study group was 
consistent with the absence of any 50-Hz magnetic field effect.  
From an epidemiological perspective, the lack of a clear-cut 
dependence of the suicide incidence on magnetic field intensity 
suggests that the apparent correlation between these variables may 
be purely fortuitous.  An extension of the studies initiated by 
Perry et al. (1981), using a significantly larger population of 
individuals, will be required before any firm judgement can be made 
regarding the proposed correlation between suicide incidence and 
ELF magnetic field exposure.  Thus, these data cannot serve as a 
basis for the evaluation of possible health effects, particularly 
as McDowall's (1986) data based on an analysis of mortality in a 
group of nearly 8000 persons, identified as living in the vicinity 
of electrical transmission facilities, did not support an 
association with suicide. 

7.3  Studies on Human Volunteers

    A number of research workers (Mantell, 1975; Hauf, 1976, 1982; 
Denisov et al., 1979; Sander et al., 1982; Kholodov & Berlin, 1984) 
have performed  controlled studies on human volunteers in 
laboratories where the field strength and exposure duration were 
accurately known.  The strongest fields and lengths of exposure 
were used by Sander et al. (1982) and Kholodov & Berlin (1984). 


Table 13.  Cancer incidence and population exposure to electromagnetic
fields
---------------------------------------------------------------------------
Reference           Subjects                     Cancer deaths
---------------------------------------------------------------------------
Wertheimer &        Children living near         Increased leukaemia
Leeper (1979)       high current configurations

Fulton et al.       Children living near high    No increased leukaemia
(1980)              current configurations

Wertheimer &        Adults living near high      Increased cancer
Leeper (1982)       current configurations

Coleman et al.      Persons living near          No increased leukaemia
(1985)              high voltage lines

Myers et al.        Children living near         No increased cancer
(1985)              high voltage lines

Rodvall et al.      Persons living near          No increased cancer
(1985)              high voltage lines

Tomenius (1986)     Children living near         No increased leukaemia
                    high voltage lines           Increased nervous
                                                 system tumours

McDowall (1986)     Persons living near          No increased cancer
                    high voltage lines
---------------------------------------------------------------------------

    Sander et al. (1982) exposed human volunteers to 50-Hz magnetic 
fields of 5 mT.  These exposures did not produce any effects with 
the exception of some minor variations in certain haematological 
parameters.  All of the studies on human volunteers exposed to 
relatively weak magnetic fields produced negative results (Table 
14). 
Table 14.  Effects of ELF magnetic fields on man
---------------------------------------------------------------------------
Exposure                  Effect                    Reference
---------------------------------------------------------------------------
0.3 mT, 50 Hz (for 3 h)   No effect on reaction     Mantell (1975); Hauf
                          time or EEG               (1976)

3 mT, 10 Hz               Threshold for             Denisov et al. (1979)
                          perception or sensation

5 mT, 50 Hz (for 4 h/     No effect on many         Sander et al. (1982)
day for 1 week)           physiological parameters

3 mT, 10 Hz               Threshold for             Kholodov & Berlin
                          perception or sensation   (1984)
---------------------------------------------------------------------------

Table 14 (contd.)
---------------------------------------------------------------------------
Exposure                  Effect                    Reference
---------------------------------------------------------------------------

2 - 10 mT, 15 - 20 Hz;    Threshold for perception  Various authors
10 mT, 50/60 Hz           of magnetophosphenes      (Table 7)

60 mT, 50 Hz              Threshold for visually    Silny (1986)
                          evoked potentials
---------------------------------------------------------------------------

    Kholodov & Berlin (1984) exposed the head, arms, and legs of 
human volunteers to determine the thresholds for sensation or 
perception of magnetic fields.  They reported that, for pulsed 
magnetic fields (f = 10 Hz) the threshold was about 3 mT, for 
sweeping magnetic fields, about 0.5 mT, and for static fields, 
about 8 mT. 

    When the hands of human volunteers were exposed to static 
magnetic fields of up to 0.1 T for up to 30 min, skin temperature 
and sensitivity decreased, and capillary spasms were reported 
(Roschin, 1985). 

8.  HEALTH EFFECTS ASSESSMENT

    The process of making a health risk evaluation is quite complex 
and involves consideration of such concepts as numerical values of 
risk, acceptability of risk, reasonable or comparative risk, public 
perception of risk, and cost-benefit analyses (Sinclair, 1981). 

    In making an assessment of the health risks from exposure to 
magnetic fields, criteria must be developed to identify which 
effects are to be considered a hazard for human health.  The 
difficulty in defining the health hazard occurs when value 
judgements are involved that may not be based on scientific 
analysis. 

    Strict guidelines must be established prior to reviewing the 
literature on the biological effects of exposure to magnetic 
fields.  Certain studies are conducted to identify underlying 
mechanisms of interaction.  Many of these will be conducted on 
biological systems exposed  in vitro to magnetic fields.  Health 
effects assessments cannot be based on  in vitro studies alone, 
because effects found  in vitro may not necessarily occur  in vivo.  
 In vitro studies make it possible to determine the toxicity of an 
agent in increasingly complex steps.  For example, effects on 
solutions of biological molecules might be used as a model system 
to study predominant mechanisms of action.  Uncomplicated systems 
can assist in the exploration and evaluation of mechanisms and may 
serve as a useful basis for designing studies at the next level of 
biological complexity, the cellular level.  By restricting the 
complexity of the experimental system, there will be less chance of 
possible subtle effects being masked by gross or dominant effects. 

    Thus, health agencies can place only limited value on  in vitro 
studies.  However, the  in vitro results may indicate that a 
cautious or prudent approach should be adopted when setting 
standards.  Once mechanisms of interaction are understood and found 
to occur in laboratory animals, the next step is to determine if it 
is possible to extrapolate the results to man. 

    Present knowledge of the interaction mechanisms operating when 
biological systems are exposed to magnetic fields is not sufficient 
to predict theoretically the whole range of effects of exposure to 
these fields, particularly the long-term effects.  Thus, care must 
be applied in attempting to predict or extrapolate effects in man 
from effects found in laboratory animals. 

    An approach to making a health risk assessment is to evaluate 
the available data on exposure levels and bioeffects to determine 
if thresholds for effects occur (Repacholi, 1985b).  It should be 
noted that, in undertaking such an evaluation, only reports that 
provide adequate information on experimental technique and 
dosimetry should be used.  Ideally, only data that have been 
reproduced or substantiated by independent laboratories and have a 
direct bearing on health risk should be considered. 

    If possible, the health risk assessment should be based on 
well-conceived, -conducted, and -analysed epidemiological studies.  
Unfortunately, epidemiological studies on human beings exposed to 
magnetic fields tend to suffer from one or more of the following 
deficiencies: small numbers of subjects (resulting in low 
statistical accuracy); a lack of adequate dosimetry or ill-defined 
exposure conditions; lack of information on confounding variables, 
such as exposure to other physical or chemical agents; and a lack 
of a properly matched, stable control group that would provide 
unequivocal interpretation of the data to give a direct causal 
relationship with the hazardous physical agent. 

    Health risk analysis for the development of standards might 
adopt a phenomenological or conservative approach (Kossel, 1982; 
Repacholi, 1983a,b).  In this case, it is assumed, until more 
information becomes available, that exposure to fields that produce 
an adverse biological effect could be hazardous, since later 
studies may reveal that the biological effect was a precursor to 
real injury. 

8.1.  Static Magnetic Fields

    From the available data summarized in section 7, it can be 
concluded that short-term exposure to static magnetic fields of 
less than 2 T does not present a health hazard.  Because of the 
lack of experimental data and from analysis of established 
mechanisms of interaction, exposure to fields above 2 T cannot yet 
be evaluated. 

8.2.  Time-Varying Magnetic Fields

    In evaluating human exposure to time-varying magnetic 
frequencies up to about 300 Hz, it is possible to use an organ-dose 
concept (Bernhardt et al., 1986).  This is based on two 
assumptions: 

    (a)  There are no indications that a specific time-varying 
         magnetic field effect exists at tissue field strengths 
         below the value at which induced eddy currents may cause 
         biological effects.  Reports on calcium efflux (Adey 1981; 
         Blackman et al., 1985b) and on effects in chick embryos 
         (Delgado et al., 1981, 1982; Ubeda et al., 1983; 
         Juutilainen et al., 1986), if confirmed, would appear to 
         be due to other mechanisms. 

    (b)  When possible health risks for man from exposure to time-
         varying magnetic fields are evaluated, the biological 
         effects mainly considered are those that originate from a 
         direct action on the cells in nerve and muscle tissues. 
         The physical quantity determining the biological effect is 
         the induced electric field strength in the tissue 
         surrounding the living cell. 

    There is a considerable amount of experimental data on 
stimulation thresholds for different nerves and muscle cells, often 
expressed in the form of electric current density values and not as 
field strength values.  Only a few papers provide data on field 
strength thresholds.  Therefore, the current density may be used as 
the decisive parameter in the assessment of the biological effects 
at the cellular level.  Field strength and current density are 
related by the conductivity of the medium. 

    Selection of the current density as a measure of an action at 
the cellular level also makes it possible to extrapolate conditions 
in the human body from experimental animal studies or from 
measurements taken on isolated cells, by way of mutual comparison 
of the current densities.  It seems irrelevant whether the electric 
current density surrounding a cell is introduced into the body 
through electrodes or induced in the body by external magnetic 
fields.  However, the current paths within the body may be 
different in the two cases. 

    Several ranges of current densities may be considered.

    (a)   Up to 10 mA/m2

    It can be assumed that a current density of less than 1 mA/m2, 
induced by an external magnetic field, should not produce adverse 
neurological or behavioural effects, since naturally flowing 
currents in the brain are of the same order of magnitude.  Similar 
arguments pertain to fields that produce current densities of less 
than 10 mA/m2 in the heart.  In general, the endogenous current 
densities in major tissue and organ systems, other than the heart 
and brain, are below the 1 mA/m2 level.  Cellular responses in 
various tissues have been observed as shown in Fig. 8, and effects 
on tissue (bone) repair have been noted. 

    (b)   10 - 100 mA/m2

    In this range, electro- and magnetophosphenes are observed.  
Magnetophosphenes can be considered harmless for a short exposure; 
however, the consequences of a long-term exposure with current 
densities at, or above, 10 mA/m2 are not known.  Furthermore, this 
current density will produce a membrane potential of the order of 
0.1 mV (Bernhardt et al., 1986), which may influence the activity 
in other neurons.  The results of electrophysiological studies have 
shown that information can be transferred between neuronal 
elements, even without action potentials (Schmitt et al., 1975).  
It must be expected that current densities that are below the nerve 
stimulation thresholds, may still influence brain function 
associated with electrical activity. 

FIGURE 8

    (c)   100 - 1000 mA/m2

    It has been shown by a number of research groups that current 
densities in this range, which result from electric currents 
applied  in vitro and  in vivo to mammalian central nervous tissue, 
can influence neuron excitability without causing direct 
stimulation.  Much of this work has been carried out using rat 
hippocampal slices (Bawin et al., 1975, 1978, 1984, 1986) and 
guinea-pig hippocampal slices (Jeffries, 1981).  The thresholds for 
stimulation of sensory receptors and of nerve and muscle cells may 
also lie in this range.  It is possible that such stimulation could 
be hazardous.  An unexpected stimulation of muscle tissue may lead 
to a dangerous reaction.  Changes in excitability or the direct 
stimulation of central nervous tissue may lead to adverse changes 
in mental function. 

    (d)   Above 1000 mA/m2

    An increased probability of ventricular fibrillation occurs at 
current densities above 1000 mA/m2.  The probability of this effect 
increases with both duration of exposure and current density 
magnitude.  Continuous (tetanic) muscle contraction may also occur.  
In studies where 50/60-Hz electric currents have been applied to 
human volunteers via electrodes, tetany of the muscles concerned 
with breathing has been produced which, obviously, would be fatal 
if prolonged. 

    A summary of the ranges of induced currents that produce these 
possible effects is given in Table 15. 

Table 15.  Induced current density ranges between 3 and 300 Hz
for producing biological effects
-----------------------------------------------------------
Current density   Effects
(mA/m2)
-----------------------------------------------------------
< 1               Absence of established effects

1 - 10            Minor biological effects reported

10 - 100          Well established effects, visual
                  (magnetophosphenes) and possible nervous
                  system effects; facilitation of bone
                  fracture reunion reported

100 - 1000        Changes in central nervous system
                  excitability established; stimulation
                  thresholds; possible health hazards

> 1000            Extrasystoles, ventricular fibrillation
                  possible; definite health hazards
-----------------------------------------------------------

    In terms of a health risk assessment, it is difficult to 
correlate the internal tissue current densities with the external 
magnetic field strengths.  Calculation of current densities using 
Faraday's law is complicated by the fact that the exact current 
paths depend in a complex way on the distribution and the 
conducting properties of the body tissues.  Current densities 
induced in human beings and animals are extremely non-uniform.  
Current enhancements have been predicted in the human neck, 
axillae, and lower pelvic region for exposure to a horizontal ELF 
magnetic field (Kaune & Curley, 1986).  There are differences in 
the conductivity of the white and grey cerebral matter. 
Furthermore, the effective diameter of the current pathways (loops) 
is not known.  However, using "worst case" assumptions, an 
estimate of the order of magnitude for "safe" and dangerous 
magnetic field strengths and their frequency dependence can be made 
(Bernhardt, 1979, 1985). 

    The threshold field strengths and induced current densities 
required to produce visual effects by exposure to time-varying 
magnetic fields have been studied as a function of frequency 
(sections 4.2 and 6.1).  In addition, the effects of electrical 
stimulation on cell membrane potentials, sensory receptors, and 
cardiac, nerve and muscle tissues have been characterized as a 
function of frequency (section 6.2).  The frequency dependence of 
the thresholds for the direct electrical stimulation of cells and 
tissues, as well as the thresholds for magnetic field generation of 
phosphenes and for altering the VEP, have recently been summarized 
by Bernhardt (1985).  By calculating the magnetic flux density that 

would produce current densities in tissues comparable with those 
produced by direct electrical stimulation, Bernhardt (1985, 1986) 
has constructed a family of curves representing the approximate 
threshold field levels necessary to produce electrical stimulation 
of cells and tissues by time-varying magnetic fields with a 
sinusoidal waveform.  These threshold field levels are plotted in 
Fig. 8 as a function of frequency in the ELF range.  Seven curves 
are shown in this figure, including some experimental data as 
explained in the caption. 

    With the possible exception of production of magneto-
phosphenes, over the entire ELF range, the threshold field levels 
that produce stimulating effects in various target organs and 
tissues are greater than those that induce a current density of 1 
mA/m2 in the brain or heart.  This observation is consistent with 
the results of cell and tissue studies summarized in section 6.3, 
which indicate that the threshold current density for which 
perturbations are consistently observed is approximately 10 mA/m2. 

    The values given in Fig. 8 for the current densities are 
applicable only to the peripheral regions of the heart or the head.  
For zones closer to the centre of the heart or the head (having a 
shorter current path), higher strengths of the magnetic field are 
necessary to induce the same current densities.  From Fig. 8, a 
magnetic field strength that is considered not to produce any 
biological effect is about 0.4 mT for 50 or 60 Hz.  Although some 
experimental data fit satisfactorily into Fig. 8, it must be 
understood that the figure only gives an idea of the magnitude of 
the current density in the body.  Mean values were taken as the 
basis to determine the distribution of the electric field in the 
heart and the head, where the exact current paths are not known. 
Local increases in the internal field strength cannot be precluded.  
The extent of high local field strengths needs further elucidation 
by continued studies. 

    Safety factors may be defined more precisely only after further 
studies.  This has to be considered in the case where curve G in 
Fig. 8 is used to evaluate human exposure to time-varying magnetic 
fields or to provide a basis for discussion on the definition and 
determination of personnel exposure limits. 

8.3.  Conclusions

1.   Only a few mechanisms of the interaction of biological tissue 
with magnetic fields have been established.  Some of the biological 
effects data suggest that other mechanisms may play a role, but 
these have yet to be confirmed experimentally.  Thus, only a 
preliminary assessment of the human health risks from exposure to 
magnetic fields can be made. 

2.   A number of lower organisms have shown a remarkable 
sensitivity to the earth's magnetic field, because of highly 
developed receptors.  Similar receptors have not been found in 
human beings. 

3.   For human exposure to static magnetic fields, it is not 
possible to make any definitive statement about the safety or 
hazard associated with short- or long-term exposure to fields above 
2 T.  Available knowledge suggests the absence of any measurable 
effect of static fields on many major developmental, behavioural, 
or physiological parameters in higher organisms.  Recent medium-
term (days) studies on exposure of animals to static fields of up 
to 2 T have not demonstrated any detrimental effects. 

4.   From the scientific data base on higher organisms exposed to 
magnetic fields, only 4 types of effect can be regarded as 
established.  The first three may be explained by plausible 
mechanisms of interaction and produce a basis for extrapolation to 
man.  These effects are: 

    (a) induction of electrical potentials and magnetohydro-
        dynamic effects within the circulatory system;

    (b) the formation of magnetophosphenes with a time rate
        of change of magnetic field exceeding 0.3 T/s at
        17 Hz; the effect depends strongly on frequency
        (compare Fig. 8);

    (c) direct stimulation of nerve and muscle cells by very
        short (less than 1 ms) pulses of rapidly changing
        magnetic fields (several thousand T/s).  Current
        densities are estimated to exceed 1000 mA/m2.  These
        effects are strongly frequency dependent and may
        exhibit lower thresholds (100 - 1000 mA/m2) under
        more favourable stimulus conditions (10 - 100 Hz).

    (d) other cellular and tissue alterations when the
        induced current densities exceed approximately 10
        mA/m2.

5.   For human exposure to time-varying magnetic fields, it seems 
reasonable to assume that a health risk assessment can be made on 
the basis of significant perturbations of biological functions 
caused by electric currents induced by the fields.  Available data 
suggest that, when current densities less than 10 mA/m2 are induced 
in tissues and extracellular fluids, the induction of adverse 
health effects is unlikely.  However, the possibility of some 
perturbing effects occurring following long-term exposure cannot be 
excluded. 

    The time-varying fields that induce currents in the body depend 
critically on the waveform and pulse shape.  In this regard, the 
peak instantaneous current densities appear to be important.  
Furthermore, the frequency dependence of effects produced by time-
varying fields has to be taken into consideration. 

9.  STANDARDS AND THEIR RATIONALES

    With advances in technology resulting in increasing numbers of 
devices using magnetic fields, the potential for human exposure to 
these fields has increased to the point that valid questions are 
raised concerning safety. 

    Except for the USSR (USSR, 1970, 1978, 1985) and the Federal 
Republic of Germany (1986), no countries have developed, or are 
developing, mandatory standards limiting magnetic field exposure 
because, until recently, there was only a small probability of 
human exposure to magnetic fields strong enough to cause adverse 
health effects.  However, with the advent of high-energy 
accelerators and fusion reactors using strong magnets, magnetic 
levitation systems for transport and, most recently, the 
application of magnetic resonance techniques in diagnostic 
medicine, serious consideration has been given to developing 
exposure limits in various countries. 

    A safety standard is a general term, incorporating both 
regulations and guidelines, and is defined to be a set of 
specifications or rules to promote the safety of an individual or 
group of people.  A regulation is promulgated under a legal statute 
and is referred to as a mandatory standard.  A guideline generally 
has no legal force and is issued for guidance only - a voluntary 
standard.  Safety standards can specify maximum exposure limits and 
other safety rules for personnel exposures, or provide details on 
the performance, construction, design, or functioning of a device. 

    The purpose of this section is to briefly summarize the 
existing standards on magnetic fields and to discuss their 
scientific basis. 

9.1.  Static Magnetic Fields

    Only a few guidelines limiting occupational exposure to static 
magnetic fields have been developed.  The limits of human exposure 
to static magnetic fields in the USSR, US Department of Energy, and 
certain accelerator laboratories in the USA, and the CERN 
Accelerator Laboratory in Geneva are summarized in Table 16.  Only 
one standard (USSR, 1978) has been promulgated to regulate static 
magnetic fields.  A new DIN-VDE draft electromagnetic field 
standard is being discussed in the Federal Republic of Germany 
(1986) and this includes 0 Hz magnetic fields. 

    The earliest static magnetic field guidelines were developed 
as an unofficial recommendation in the USSR (Vyalov, 1967).  
Clinical investigations (Vyalov et al., 1964; Vyalov & Lisichkina, 
1966; Vyalov, 1971, 1974) formed the basis for the Soviet Standard 
(USSR, 1978).  The standard requires that the static magnetic field 
strength at the work-place does not exceed 8 kA/m (0.01 T). 


Table 16.  Limits of occupational exposure to static magnetic fields
------------------------------------------------------------------------------------------------
Author             Field    Exposure time   Body region    Comments
------------------------------------------------------------------------------------------------
USSR (1978)        0.01 T   8 h             whole body     regulation issued by Ministry of
                                                           Health

Stanford           0.02 T   extended (h)    whole body     unofficial, occupational
Linear             0.2 T    short (min)     whole body
Accelerator        0.2 T    extended (h)    arms, hands
Center (1970)      2 T      short (min)     arms, hands

US Department of   0.01 T   8 h             whole body     recommended to DOE contractors
Energy (DOE)       0.1 T    1 h or less     whole body
(Alpen, 1979)      0.5 T    10 min or less  whole body

                   0.1 T    8 h             arms, hands
                   1 T      1 h or less     arms, hands
                   2 T      10 min or less  arms, hands

CERN Accelerator   0.2 T    minutes         whole body     Recommended practice
Lab, Geneva        2 T      short           hands, arms
(NRPB, 1981)                                and feet

Lawrence           0.06 T   day             trunk          maximum average/day in peak fields
Livermore                                                  > 0.5 T
National           
Laboratory         0.06 T   day             trunk          maximum average/week in peak fields
(LLNL,1985)                                                < 0.5 T

                   0.6 T    day             extremities    maximum average/week (in peak fields
                                                           < 0.5 T) or per day (in peak fields
                                                           > 0.5 T)

                   2 T      short (min)     whole body     peak exposure limit
------------------------------------------------------------------------------------------------
    Three sets of guidelines recommending limits of occupational 
exposure to static magnetic fields exist in the USA.  Two of these 
are applicable in high-energy physics laboratories, and the other 
is a US Department of Energy (DOE) guideline. 

    At the Stanford Linear Accelerator Center in California, 
unofficial guidelines were established in 1970.  They suggest that 
the whole body or head of workers should not be exposed to static 
magnetic fields exceeding 0.02 T for extended periods (h) or fields 
exceeding 0.2 T to the arms and hands.  For short periods (min), the 
whole body or head, and arms and hands should not be exposed to 
fields exceeding 0.2 and 2 T, respectively.  The 2-T limit also 
allows film changes at Stanford's bubble chamber. 

    The Lawrence Livermore National Laboratory (LLNL, 1985) has 
drafted a set of policy guidelines for working in magnetic fields 
associated with the high-energy accelerators.  The guidelines 
(Table 16) state that: 

     Maximum exposure: Workers must never be exposed to fields
    exceeding 2 T, regardless of the duration of the exposure
    or the exposed part of the body;

     Fields less than 500 mT: If the peak field to which
    workers are exposed is less than 500 mT, personnel may be
    exposed to a week-long maximum average field strength of
    no more than 60 mT (measured at the torso) or 600 mT
    (measured at the extremities);

     Fields greater than 500 mT: If the peak exposure is
    greater than 500 mT, workers should be exposed to a  daily
    maximum average field strength of no more than 60 mT
    (measured at the torso) or 600 mT (measured at the
    extremities).

    In addition, the following restrictions are made:  Always use 
caution signs indicating the presence of a magnetic field, whenever 
the field strength is 1 mT or greater.  Use additional 
administrative controls or barricades (ropes or fences), whenever 
practical.  Do not allow workers with cardiac pacemakers or other 
medical electronic implants into areas where the magnetic-field 
intensity exceeds 1 mT.  Magnetic fields greater than this level 
can trigger a change in the operating mode of some pacemakers. 
Persons with small metallic implants (such as aneurysm clips) must 
also be stopped from entering an area where the field intensity is 
greater than 1 mT.  Stronger magnetic fields may rotate or even 
remove aneurysm clips from the arteries to which they are attached.  
Workers with large metallic implants, such as hip prostheses, 
should be advised to avoid working anywhere inside the perimeter of 
1-mT field intensity. 

    A rationale supporting the guidelines accompanies the document 
(LLNL, 1985).  The 60-mT limit is set to 1 mV of the 
magnetohydnamic voltage (voltage generated by blood, an ionized 
fluid, moving in a fixed magnetic field) in an obese person engaged 

in moderately heavy work (cardiac output 10 litres/min).  2 T 
limits the rise in blood pressure to 1%. 

    The US Department of Energy (DOE) formed an  ad hoc committee to 
review technologies that use magnetic fields, to make an assessment 
of the scientific literature on biological effects, and to 
establish guidelines for static magnetic fields, field gradients, 
and time-varying magnetic fields.  In July 1979, the Alpen 
Committee (Alpen, 1979) made its recommendations to DOE as shown in 
Table 16.  The guideline in some cases is a factor of 2 lower than 
that for continuous exposure at the Stanford Linear Accelerator 
Center.  This guideline was recommended by the Department of Energy 
to its contractor organizations as an interim measure, until 
official standards are promulgated.  Although the Alpen Committee 
made a review of the literature it has not published a rationale 
supporting the values recommended in their guideline.  According to 
Tenforde, the 0.01 T limit was recommended for continuous exposure, 
because this represented the accepted threshold for 
magnetophosphene production by ELF magnetic fields, and the 
threshold for inducing measurable electrical potentials in the 
central circulatory system during exposure to static magnetic 
fields. 

    A similarly recommended practice for limiting static magnetic 
field exposures of workers exists at the CERN accelerator 
laboratory in Geneva (NRPB, 1981).  CERN recommends that exposure 
of the hands, arms, and feet should not exceed 2 T for periods of 
the order of minutes.  This is reduced by a factor of 10, if the 
head or whole body is exposed. 

    With the advent of magnetic resonance imaging (MRI), the need 
for occupational exposure limits has become more apparent, and 
other organizations that traditionally recommend occupational 
exposure limits have begun to address this need, despite a 
reluctance in the past to recommend limits for magnetic fields 
(Sliney, 1986). 

9.2.  Time-Varying Magnetic Fields

    Except for guidelines limiting patient and operator exposure 
during clinical magnetic resonance imaging, the only standard 
limiting exposure of time-varying magnetic fields in the ELF range 
is the Soviet Standard (USSR, 1985), as shown in Table 17.  The 50-
Hz magnetic field standard (USSR, 1985) issued by the Ministry of 
Public Health of the USSR in January 1985 makes a distinction 
between continuous and pulsed fields and limits the duration of 
exposure, depending on the pulse characteristics.  The limits for 
exposure to continuous wave fields equate to 7.5 mT for 1 h and 1.8 
mT for 8 h.  This standard seems to have been developed for arc 
welding, since pulsed field exposure occurs most frequently in 
welding.  The scientific basis for this standard does not appear to 
have been published. 

    The Federal Republic of Germany (1986) is discussing extention 
of its current electromagnetic field standard (Federal Republic of 
Germany, 1984) down to 0 Hz. 

9.3.  Magnetic Resonance Imaging Guidelines

    Comparing the magnetic field limits in Table 16 with the 
strength of the magnets used in MRI, it is not surprising that 
regulatory and health agencies have begun to look more seriously at 
this imaging modality (Repacholi, 1986).  Hundreds of MRI machines 
have been installed throughout the world and concern about their 
safety has been expressed (Bore, 1985).  Some of these machines use 
superconductive magnets with fields for diagnostic application up 
to about 2.0 T, and there are prototypes with magnets giving fields 
of 4 - 5 T.  These prototypes are being studied to determine the 
feasibility of  in vivo spectroscopy.

    During the imaging procedure, lasting up to tens of minutes, 
the patient lies on a table and all parts of the body are exposed 
to strong static magnetic fields, changing (or time-varying) 
magnetic fields and radiofrequency radiation.  Rapidly switched 
gradient fields are superimposed on the static field to allow 
spatial information to be obtained.  These time-varying fields 
induce electric currents in the body. 

    Table 18 shows the guidelines on static and time-varying 
magnetic field exposure for the clinical examination of patients 
during MRI, recommended by the Center for Devices and Radiological 
Health (CDRH, 1982) of the US Department of Health and Human 
Services, the National Radiological Protection Board (NRPB, 1984) 
in the United Kingdom, the Federal Health Office (FHO, 1984) of the 
Federal Republic of Germany, and Health and Welfare Canada (Health 
and Welfare Canada, 1986). 

    In January 1984, the Health Council of the Netherlands (HCN, 
1984) issued interim advice on the use of magnetic resonance 
imaging.  This general document contains a section on possible 
health risks including statements such as: there are no risks to 
health from static magnetic field exposures up to 0.5 T, and even 
exposures to fields up to 2 T appear safe.  More research is needed 
to determine the safety of fields stronger than 2 T.  The document 
also recommends that limits for time-varying magnetic fields and 
radiofrequency fields accepted in the USA (CDRH, 1982) or the 
United Kingdom (NRPB, 1981) should be followed. 


Table 17.  Maximum permissible levels of magnetic fields with a
frequency of 50 Hza
---------------------------------------------------------------------------
Duration               Magnetic field strength A/m                
of        Continuous and        Pulsed magnetic    Pulsed magnetic
exposure  pulsed magnetic       field              field
(h)       fields with pulse     60 s > tw > 1 s  0.02 s < tw < 1 s
          width tw > 0.02 s     tp >  2 s         tp > 2 s
          and pause tp < 2 s
---------------------------------------------------------------------------
1         6000                  8000               10 000

1.5       5500                  7500               9500

2         4900                  6900               8900

2.5       4500                  6500               8500

3         4000                  6000               8000

3.5       3600                  5600               7600

4         3200                  5200               7200

4.5       2900                  4900               6900

5         2500                  4500               6500

5.5       2300                  4300               6300

6         2000                  4000               6000

6.5       1800                  3800               5800

7         1600                  3600               5600

7.5       1500                  3500               5500

8         1400                  3400               5400
---------------------------------------------------------------------------
a  From: USSR (1985).
Note: The above regimes of pulsed exposures are used in welding.
        tw is the pulse width duration.
        tp is the pulse pause duration.
        Magnetic flux density in mT = Magnetic field strength
        in A/m x 1.256
                  103

Table 18.  Guidelines on magnetic field exposure in clinical MR
---------------------------------------------------------------------------
Country      Static fields               Time-varying fields
---------------------------------------------------------------------------
 USA          Patient - 2 T whole and     Patient - 3 T/s whole and partial
CDRH         partial body exposure       body exposure
(1982)
              Exposure exceeding these limits should be evaluated on an
              individual basis

 United       Operator - 0.02 T (long     Patient and volunteers - 20 T/s
 Kingdom      periods, whole body);       (rms) periods of magnetic
NRPB         0.2 T (long periods,        field change > 10 ms
(1984)       arms, hands);             
             0.2 T (15 min, whole                    or
             body)
             2 T (15 min, arms,          (dB/dt)2t < 4 (rms) for dura-
             hands)                      tion of magnetic field change
                                         < 10 ms where dB/dt in T/s and
                                         t in s
             Patient and volunteers -
             2.5 T (whole and partial
             body exposure)


 Germany,     Patient - 2 T (whole and    Patient - whole and partial body
 Federal      partial body exposure)      exposure: maximum induced current
 Republic of                              density
FHO (1984)                               30 mA/m2 or 0.3 V/m electric
                                         field strength for duration of
                                         magnetic field change of 10 ms
                                         or longer
                                              or
                                         (300/t) mA/m2 or (3/t) V/m for
                                         duration of magnetic field change
                                         (t) shorter than 10 ms (t in ms)


 Canada       Operator - 0.01 T (whole    Patient - 3 T/s (rms)
Health and   body during working day)
Welfare
Canada       - > 0.01 T
(1986)       (keep to minimum)

             Patient - 2 T (whole and
             partial body exposure)
---------------------------------------------------------------------------
9.3.1  United Kingdom

    The NRPB (1984) recommends that the following conditions should 
be fulfilled during the operation of magnetic resonance imaging 
equipment in the United Kingdom. 

    (a)   Static fields

    For people (patients and volunteers) exposed to the imaging 
process, the static magnetic field should not exceed 2.5 T for the 
whole or a substantial portion of the body.  The NRPB Advisory 
Group formulating the guidelines suggested that static fields have 
been shown to affect certain chemical reactions  in vitro and that 
reproducible changes in primate behaviour have been found in fields 
of several tesla.  Although flow potentials are generated across 
blood vessels by the flow of blood perpendicular to the field, 
their biological significance at fields of a few tesla remains 
unclear.  However, at 2.5 T, the peak flow potential is calculated 
to be approaching the depolarization threshold for myocardial 
muscle.  Although only a fraction of this potential occurs across 
each cell, it was considered prudent to limit acute exposure to 2.5 
T, until further information becomes available. 

    Occupational static field exposure limits are recommended for 
staff operating MRI equipment.  Exposure for prolonged periods to 
more than 0.02 T for the whole body or 0.2 T for the arms or hands 
should be avoided.  NRPB (1984) recommends that these limits may be 
increased to 0.2 T for the whole body and 2 T for the arms and 
hands for periods totalling less than 15 min at a time, provided 
intervals of about 1 h occur between such exposures. 

    These operator limits are essentially the same as those 
recommended by the Stanford Linear Accelerator Center (Table 16), 
where no adverse symptoms have been reported from staff working at 
the facility, since the introduction of their guidelines in 1970. 

    (b)   Time-varying fields

    For the time-varying fields, excluding radiofrequency fields, 
the NRPB (1984) recommends limits based on the duration of magnetic 
flux density changes (i.e., the time during which electric currents 
are being induced).  When the duration of exposure exceeds 10 ms, 
exposures should not exceed root mean square (rms) rates of change 
of magnetic flux density (dB/dt) of 20 T/s for all persons 
(patients exposed to the imaging process, volunteers).  For 
durations of change of less than 10 ms, the relationship (dB/dt)2t 
less than 4 should be observed where dB/dt is in T/s, and t is the 
duration of the change of the magnetic field in seconds.  For 
continuously varying magnetic fields, such as sinusoidal fields, 
the duration of the change can be considered as half the period of 
the waveform. 

    The rationale for the NRPB guidelines is given in a publication 
by Saunders & Smith (1984).  The NRPB Advisory Group recognized 
that rapidly changing magnetic fields can induce electric currents 

in tissues that could be sufficiently large to interfere with the 
normal functioning of nerve cells and muscle fibres.  These conduct 
electrical impulses in the form of localized membrane 
depolarization produced by the flow of ions and, above a certain 
threshold, give rise to sensation or muscle contractions.  From 
experimental data it was inferred that the threshold would be 
lowest when the current pulse width (or duration of magnetic flux 
density change) exceeded about 10 ms. 

    It was felt that, although the sensation of magnetic phosphenes 
occurred at a threshold in man of about 1.3 T/s (at 20 Hz), this 
sensation of light flashes in the eye has not been shown to be 
hazardous.  However, excitation of nerves and muscles could be 
hazardous, but requires exposure to high rates of change of 
magnetic flux density.  The threshold for excitation depends on the 
pulse length and pulse repetition frequency of the induced current.  
Since insufficient information is available to define safe limits, 
they must be derived from effects of electric currents applied by 
electrodes.  The threshold current density to induce ventricular 
fibrillation is 3 A/m2.  Thus, to achieve a factor of 10 safety 
margin, it was decided that MRI operating conditions should be such 
as to induce current densities that did not exceed 0.3 A/m2 for a 
duration of magnetic flux density change greater than 10 ms. 

    For durations of the current pulse of half period (t) of less 
than 10 ms, the evidence suggests that, when t decreases, the 
threshold rms current density for inducing ventricular fibrillation 
increases.  Experimental data suggest that the square of the rms 
current density multiplied by the duration (t) remains constant.  
The magnetic field vector in most MRI equipment is parallel to the 
longitudinal axis of the body (z-axis).  The current density 
induced by time variation of the z-gradient is proportional to the 
conductivity, the inductive loop radius, and the rate of change of 
the magnetic field.  Assuming the average value for tissue 
conductivity to be 0.2 S/m, the radius of the body to be 0.15 m, 
the limit applied to the rms current density of 0.3 A/m2 for pulses 
or half periods of induced current exceeding 10 ms restricts the 
rms rate of change of the z gradient magnetic flux density to 20 
T/s, when the duration of magnetic field change exceeds 10 ms.  For 
durations shorter than 10 ms, (Saunders & Smith, 1984), the 
relationship for determining the limit for the time-varying field 
can be derived: 

    (dB/dt)2t < 4,

    where dB/dt is in T/s, and t is in seconds

    It was assumed that the current densities induced in the body 
by variation of the anterior-posterior (y) and lateral (x) 
gradients would not be significantly greater than for the z 
gradients. 

    (c)   Other guidelines

    (i)   RF exposure of the patient and staff must be 
          restricted so that the rise in temperature does not
          exceed 1 °C, as shown by skin and rectal temperature,
          or more than 1 °C in any mass of tissue not exceeding 
          1 g in the body.

    (ii)  Patients should be exposed only with the approval
          of a registered medical practitioner or research
          ethics committee.

    (iii) Patients must be fully informed of the procedure
          and consent freely to it.

    (iv)  Only medically assessed suitable volunteers should
          be used in trials.

    (v)   Frequently exposed volunteers should have regular
          ECG checks.

    (vi)  It is prudent to exclude women in the first three
          months of pregnancy.

    (vii) Special care is needed for patients with cardiac
          pacemakers or large metallic implants.

    (viii)Warning notices should be posted indicating that
          magnetic and RF fields may affect pacemakers and
          electronic equipment.

9.3.2  USA

    The recommendations issued by the Center for Devices and 
Radiological Health (CDRH, 1982) in the USA are intended to assist 
the medical profession and manufacturers in making health risk 
benefit assessments.  Based on information available in the 
literature, it was suggested that, in the case of diagnostic 
magnetic resonance applications involving exposure to static 
magnetic fields not exceeding 2 T or time-varying fields not 
exceeding 3 T/s, the benefits outweigh the risks, within the 
current medical indications and contra-indications.  Pregnant women 
should not be exposed as the safety of such exposure has not been 
established.  It should be noted that the CDRH guidelines are not 
limits for patient exposure in MRI imaging investigations.  The 
recommendations are essentially criteria that provide a demarcation 
between devices exceeding the magnetic field levels stated in the 
guidelines and therefore requiring further evaluation to determine 
if any health risk exists for the patient, and devices operating 
below the levels given in the guidelines. 

    The recommendations for the magnetic field levels were 
determined after consideration of existing unofficial standards and 
recommendations and their rationales, and a review of the 
scientific literature.  The scientific rationale for the guidelines 

is essentially that proposed by Budinger (1981).  Budinger 
concluded, after a review of the bioeffects literature and a 
theoretical analysis of the known interaction mechanisms of static 
magnetic fields with biological systems, that harmful effects on 
human beings or reproducible cellular, biochemical, and genetic 
effects have not yet been observed and are not expected at fields 
of less than 2 T.  For changing magnetic fields, Budinger concluded 
that the thresholds for effects of induced currents is above that 
produced by 1 - 100 Hz sinusoidal fields of strength 5 mT.  However, 
he did note that potential biological effects due to differences in 
waveform, repetition rate, peak magnetic field, and duration of 
exposure required further study. 

    The CDRH also recommends that the radiofrequency field exposure 
of the patient should be limited, so that the specific absorption 
rate (SAR) does not exceed 0.4 W/kg, averaged over the whole body, 
or 2 W/kg, averaged over any gram of tissue. 

9.3.3  Federal Republic of Germany

    The Federal Health Office (FHO, 1984) has made recommendations 
to physicians who work with clinical MRI devices.  It is stated that 
no adverse health effects on patients, operators, or any other 
persons in the vicinity of MRI equipment have been detected so 
far.  However, possible effects on the body can be estimated from 
induced currents and potentials in the body.  The guidelines for 
static and time-varying magnetic fields are based on these 
estimations and study of the literature.  It is stated that, if 
there is compliance with these recommendations, any detrimental 
effects will be detected at the earliest possible time.  A 
translation of the original guidelines from German to English is 
provided in Bernhardt & Kossel (1985). 

    (a)   Static fields

    The FHO recommends that patients imaged in an MRI facility 
should not be exposed to static magnetic fields exceeding 2 T.  If 
patients are exposed to fields higher than 2 T, they should be 
monitored for cardiac and circulatory function. 

    The rationale for determining this value is as follows: 
orientation effects are observed in such systems as DNA, retinal 
rods, and sickle cells at static field strengths above 1 - 2 T.  
Electric potentials induced in flowing blood exposed to static 
fields above 0.3 T have been noted in ECG measurements in animals, 
but no adverse health effects have been observed in animals exposed 
to fields up to 10 T.  However, the potential differences induced 
by cardiac contractions in a magnetic field exceeding 2 T may impair 
the excitation stimulation or conduction of excitation. 

    (b)   Time-varying fields

    Time-varying magnetic fields induce electrical potentials, the 
size of which depends on the magnetic field strength, pulse 
duration and frequency.  Using essentially the reasoning outlined 

by Bernhardt (1985) for estimating the values of induced electric 
potentials and currents that are likely to cause biological 
effects, the FHO (1984) recommends that patients should not be 
exposed to time-varying magnetic fields having a duration of 
magnetic field change equal to or greater than 10 ms, which induce 
electric fields greater than 0.3 V/m or current densities exceeding 
30 mA/m2.  If the duration of the magnetic field change of the 
time-varying fields is less than 10 ms, then the maximum induced 
electric field is 3/t V/m and maximum induced current density is 
300/t mA/m2 (0.3/t A/m2), where t is the duration of magnetic field 
change in milliseconds.  The MRI machine manufacturer must inform 
the purchaser of the operating conditions that will result in the 
induced field strength and current density remaining below the 
recommended values.  If these values are capable of being exceeded 
by the machine, the manufacturer must prove that it is safe. 

    A brief rationale for these recommended values is given in FHO 
(1984).  Compared with natural currents, induced current densities 
of 1 mA/m2 have no detectable effect on the body.  Current densities 
of 10 mA/m2 induce effects that depend on the frequency of the 
time-varying magnetic field, but do not pose hazards.  At 
frequencies between 10 and 50 Hz, magnetic fields above 5 mT 
produce magnetophosphenes.  Ventricular fibrillation may be caused 
if the magnetic field induces current densities exceeding 1000 
mA/m2 or electric fields exceeding 100 mV/cm. 

    The final values of induced electric field and current density 
represent estimates, based on studies and theoretical calculations 
(described by Bernhardt, 1985), that are thought to provide a wide 
margin of safety. 

    (c)  Other recommendations

    1.  Exposure to radiofrequency fields should be such that
        the SAR does not exceed 1 W/kg (whole body) or 5 W/kg
        (partial body - per kg of tissue, except the eyes).

    2.  Prior to patient examination, care must be taken with
        regard to implants made of ferromagnetic materials,
        implanted cardiac pacemakers, dislocation of catheters, 
        vascular clips, and the like.

    3.  Patients must undergo additional medical examinations
        (described in FHO, 1984), if the recommended exposure
        limits are exceeded.

9.3.4  Canada

    The Bureau of Radiation and Medical Devices of Health and 
Welfare Canada (Health and Welfare Canada, 1986) has published a 
safety code containing guidelines on exposure to electromagnetic 
fields from magnetic resonance clinical systems.  The document 
contains information on levels of exposure for typical devices, 
exposure guidelines from various countries, a summary of health 
effects from magnetic and radiofrequency fields, and guidance on 

exposure of patients and operators.  Details of the guidelines are 
given in Table 18. 

    Health and Welfare Canada (1985) have also published 
recommendations to ensure the protection of patients and 
operational personnel from potential hazards in MRI.  This report 
contains recommendations on magnetic fields as shown below: 

    (a) static magnetic fields must be below 0.5 mT in
        unrestricted areas;

    (b) entrance to areas in excess of 1.5 mT must be
        strictly controlled, to prevent introduction of
        magnetic material by patients, operational personnel,
        and visitors;

    (c) equipment for cardiopulmonary resuscitation must be
        available and usable within the imaging room and, if
        possible, in areas where the field exceeds 10 mT;

    (d) static magnetic fields should not exceed 2.5 T (this
        differs from the 2 T recommended in Health and
        Welfare Canada (in press)); and

    (e) time-varying magnetic fields should not exceed 3 T/s.

10.  PROTECTIVE MEASURES AND ANCILLARY HAZARDS

    Protective measures for the industrial and scientific use of 
magnetic fields can be categorized as engineering design measures, 
the use of separation distance, and administrative controls. 
Another general category of hazard control measures, namely 
personal protective equipment (e.g., special garments and face 
masks) do not exist for magnetic fields.  However, protective 
measures against ancillary hazards from magnetic interference with 
emergency or medical electronic equipment and for surgical and 
dental implants are a special area of concern regarding health 
aspects of magnetic fields.  The mechanical forces imparted to 
ferromagnetic implants and loose objects in high-field facilities 
require that precautions be taken. 

    The techniques to minimize needless exposure to high intensity 
magnetic fields around large research facilities generally fall 
into three types: 

    (a)   Distance and time

    Limit human access and/or occupancy duration in locations where 
field strengths are high.  Since the external magnetic flux density 
decreases with distance from the source, separation distance is a 
fundamental protective measure.  For example, at large distances 
from a static magnetic field dipole source, the field decreases 
approximately as the reciprocal cube of the separation distance. 

    (b)   Magnetic shielding

    The use of ferromagnetic core materials restricts the spatial 
extent of external flux lines of a magnetic device.  External 
enclosures of ferromagnetic materials can also "capture" flux lines 
and reduce external flux densities.  However, shielding is normally 
an expensive control measure and of limited use for scientific 
instruments.  Furthermore, it has not generally been shown to be 
cost-effective for large installations when compared to the use of 
separation distance (Hassenzahl et al., 1978). 

    (c)   Electromagnetic interference (EMI) and cardiac pacemakers

    Certain types of modern cardiac pacemakers exhibit malfunction 
in response to EMI produced either by endogenous myopotentials or 
by external sources such as high-voltage systems.  The modern 
implantable pacemakers are microprocessor-controlled and function 
in a "demand" mode in which stimulatory pulses are delivered to 
the heart, only if it fails to exhibit intrinsic electrical 
activity.  The endogenous cardiac activity is detected by a signal-
sensing circuit, in order to avoid competitive pacing between the 
pacemaker's stimuli and the heart's intrinsic activity.  The modern 
pacemakers also contain a noise detection circuit that can 
discriminate electric fields with different frequencies and 
waveforms from those associated with the heart's bioelectrical 
activity.  When EMI is sensed, the demand pacemaker reverts to a 
fixed-rate pacing mode, which may be asynchronous with the normal 

cardiac activity.  This pacing mode is frequently referred to as 
the "reversion" or "noise" mode of operation, and can be 
undesirable if the pacemaker signals are competitive with the 
intrinsic cardiac electrical activity. 

    Two different configurations of electrode leads are used in 
pacemakers, and these have very different sensitivities to EMI.  In 
one type, termed the "bipolar" design, both leads are implanted 
within the heart at a typical separation distance of 3 cm.  In the 
second type, termed the "unipolar" design, the cathode lead is 
implanted in the heart and the pacemaker case serves as the anode.  
Because of the considerable physical separation of the anode and 
cathode leads in the unipolar design, this type of pacemaker 
provides a large antenna for the reception of EMI.  Of the two 
designs for pacemaker electrode configurations, only the unipolar 
type has been found to be sensitive to EMI.  Among the 350 000 - 
500 000 individuals in the USA who have implanted pacemakers, 
approximately 50% have models with the unipolar electrode design. 

    During the past decade, several laboratory tests and studies on 
pacemaker patients have been conducted to assess the response of 
different pacemaker models to power-frequency electric and magnetic 
fields (Jenkins & Woody, 1978; Butrous et al., 1983; Griffin, 1985; 
Moss & Carstensen, 1985).  Two types of pacemaker malfunction have 
been observed in response to EMI: (a) an aberrant pacing rate, with 
irregular or slow pacing; and (b) reversion to fixed-rate 
(asynchronous) pacing.  The probability that a malfunction will 
occur in the presence of an external electromagnetic field is 
strongly dependent on the pacemaker model, since some manufacturers 
have incorporated a feature into their pacemaker models that 
automatically decreases the sensitivity of the amplifier circuit 
when EMI is sensed.  These specific brands of pacemaker thereby 
avoid reversion to an asynchronous mode in response to EMI. 

    Griffin (1985) estimated the total population of pacemaker 
patients in the USA who might be at serious risk from the effects 
of EMI.  He assumed that: (a) 350 000 - 500 000 individuals wear 
pacemakers; (b) 50% of the pacemakers are of the unipolar design; 
(c) 10 - 20% of the unipolar pacemakers are highly sensitive to 
EMI; and (d) 20 - 25% of the patients with sensitive pacemakers are 
totally dependent on the pacemaker to sustain their cardiac 
rhythm.  With these assumptions, it can be calculated that 
approximately 3500 - 12 000 pacemaker wearers might be at serious 
risk from EMI.  However, it must be borne in mind that only a small 
fraction of the individuals at risk are likely to encounter a 
source of EMI during the time periods when their cardiac function 
is totally dependent on an implanted pacemaker.  The above estimate 
of the population at risk must therefore be regarded as an upper 
limit that perhaps greatly overestimates the actual probability of 
the occurrence of a potentially fatal pacemaker malfunction in 
response to EMI. 

    Both power-frequency electric and magnetic fields have been 
found to introduce EMI that can alter the functioning of many 
commercially available pacemakers.  In studies on 11 patients with 

7 different implanted pacemaker models from 4 manufacturers, Moss & 
Carstensen (1985) observed alterations in pacemaker function during 
exposure to 60-Hz electric fields ranging from 2 - 9 kV/m.  Only 
models produced by 2 out of the 4 manufacturers were sensitive to 
EMI from fields of this strength.  A similar set of observations 
was made by Butrous et al. (1983). 

    A total of 26 pacemaker models were examined by Jenkins & Woody 
(1978).  Twenty of these units were found to revert to an 
asynchronous mode of pacing or to exhibit abnormal pacing 
characteristics in 60-Hz fields ranging from 0.11 to 0.4 mT, with 
the average threshold field level for an effect being 0.21 mT.  The 
minimum value of dB/dt producing an effect was therefore 41.5 mT/s 
(for the 60-Hz, 0.11-mT field).  Pacemaker malfunctions produced by 
power-frequency magnetic fields require field levels that are 
greater than those associated with high-voltage transmission lines 
and most other types of electrical systems.  However, the fields in 
the immediate vicinity of various types of industrial machinery and 
appliances (section 3) are sufficiently strong to represent a 
potential source of EMI that could alter pacemaker functioning. 

    Pavlicek et al. (1983) found that a rapidly switched gradient 
field used in magnetic resonance imaging with a time variation of 3 
T/s could induce potentials up to 20 mV in the loop formed by the 
electrode lead and the case of a unipolar pacemaker.  This signal 
amplitude is sufficiently large to avoid rejection by the 
pacemaker's EMI discrimination circuitry, and could therefore be 
recognized as a valid cardiac signal. 

    Pacemaker malfunctions can also be caused by static magnetic 
fields, which produce closure of a reed relay switch used to test 
the pacemaker's performance while operating in a fixed rate pacing 
mode.  On the basis of a study of pacemakers produced by 6 major 
manufacturers, Pavlicek et al. (1983) found the most sensitive 
model to exhibit reed switch closure and reversion to fixed-rate 
pacing in a 1.7-mT static field.  Field levels of 1.7 - 4.7 mT were 
observed to produce closure of the reed switches in all of the 
models tested.  All of the models were also found to experience 
forces and torques when placed in MRI devices operated at fields of 
up to 0.5 T.  Two of the pacemakers experienced a torque that was 
strong enough to produce significant movement of these units within 
tissue. 

    (d)   Administrative measures

    The use of warning signs, and special access areas to limit 
exposure of personnel near large magnet facilities has been of 
greatest use to control exposure.  Administrative controls, such as 
these, are generally preferable to using magnetic shielding, which 
can be extremely expensive.  In some circumstances, for example MRI 
facilities, a combination of shielding, restricted access, and the 
use of metal detectors may be appropriate to avoid detrimental 
effects from exposure to high strength magnetic fields.  Loose 
ferromagnetic and paramagnetic objects can be converted into 
dangerous missiles when subjected to intense magnetic field 

gradients.  Avoidance of this hazard can only be achieved by 
removing loose metallic objects from the area and personnel.  Such 
items as scissors, nail files, screwdrivers, and scalpels should be 
banned from the immediate vicinity. 

    Of particular concern in MRI, are the forces exerted by static 
magnetic fields on implanted metal objects such as aneurysm clips 
and pacemakers.  Even the most modern pacemakers will malfunction 
when placed in an MRI machine (Erlebacher et al., 1986).  New et 
al. (1983) also measured the magnetic torques exerted on 21 types 
of haemostatic clips and various other materials such as dental 
amalgam.  Of the 21 clips, 19 of which were aneurysm clips, 16 
showed a deflection near the portals of two magnets operating at 
0.147 T and 1.44 T, respectively.  Of the remaining materials 
tested, only a shunt connector demonstrated significant 
ferromagnetic properties.  The non-magnetic materials were 
primarily composed of austenitic stainless steel.  Surgical clips 
composed of tantalum or titanium are also non-ferromagnetic.  Clips 
composed of martensitic stainless steels are ferromagnetic and 
experience significant forces and torques in static magnetic 
fields.  These findings indicate a clear requirement for strict 
administrative controls in determining whether patients bearing 
medical implants could be adversely affected by the fields present 
in MRI devices. 

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    See Also:
       Toxicological Abbreviations
       Magnetic fields (HSG 27, 1989)