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
joint venture of the United Nations Environment Programme, the
International Labour Organisation, and the World Health
Organization. The main objective of the IPCS is to carry out and
disseminate evaluations of the effects of chemicals on human health
and the quality of the environment. Supporting activities include
the development of epidemiological, experimental laboratory, and
risk-assessment methods that could produce internationally
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toxicology. Other activities carried out by the IPCS include the
development of know-how for coping with chemical accidents,
coordination of laboratory testing and epidemiological studies, and
promotion of research on the mechanisms of the biological action of
chemicals.
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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. Duchêne, Commissariat à 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 à l'Energie Atomique, Commissariat
à l'Energie Atomique, Institut de Protection et de Sûreté
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
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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
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).
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).
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).
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
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Reference Principal findings
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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
---------------------------------------------------------------------------
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