Health and Safety Guide No. 27






    This is a companion volume to Environmental Health Criteria 69:
    Magnetic Fields

    Published by the World Health Organization for the International
    Programme on Chemical Safety (a collaborative programme of the United
    Nations Environment Programme, the International Labour Organisation,
    and the World Health Organization)

    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
    Radiation Protection Association, or the World Health Organization.

    ISBN 92 4 154348 5
    ISSN 0259 - 7268

    (c) World Health Organization 1989

    Publications of the World Health Organization enjoy copyright
    protection in accordance with the provisions of Protocol 2 of the
    Universal Copyright Convention.  For rights of reproduction or
    translation of WHO publications, in part or  in toto, application
    should be made to the Office of Publications, World Health
    Organization, Geneva, Switzerland.  The World Health Organization
    welcomes such applications.

    The designations employed and the presentation of the material in this
    publication do not imply the expression of any opinion whatsoever on
    the part of the Secretariat of the World Health Organization
    concerning the legal status of any country, territory, city or area or
    of its authorities, or concerning the delimitation of its frontiers or

    The mention of specific companies or of certain manufacturers'
    products does not imply that they are endorsed or recommended by the
    World Health Organization in preference to others of a similar nature
    that are not mentioned.  Errors and omissions excepted, the names of
    proprietary products are distinguished by initial capital letters.



         1.1. Physical characteristics
              1.1.1. Static magnetic fields
              1.1.2. Time-varying magnetic fields
         1.2. Units and quantities
         1.3. Sources of magnetic fields and applications 
              1.3.1. Natural sources
              1.3.2. Man-made sources

         2.1. Human exposure to magnetic fields
         2.2. Mechanisms of interaction
              2.2.1. Magnetic induction
              2.2.2. Magnetomechanical effects
              2.2.3. Electronic interactions
         2.3. Effects on animals and various organisms
         2.4. Effects on human beings
              2.4.1. Static magnetic fields
              2.4.2. Time-varying magnetic fields

         3.1. Static fields
         3.2. Time-varying fields

         4.1. Exposure reduction
         4.2. Safety

         5.1. Static fields
         5.2. Time-varying fields
         5.3. Magnetic resonance imaging (MRI)



    The Environmental Health Criteria (EHC) documents produced by the
    International Programme on Chemical Safety include an assessment of
    the effects on the environment and on human health of exposure to a
    chemical or combination of chemicals, or physical or biological
    agents.  They also provide guidelines for setting exposure limits.

    The purpose of a Health and Safety Guide is to facilitate the
    application of these guidelines in national chemical safety
    programmes. The first three sections of a Health and Safety Guide
    highlight the relevant technical information in the corresponding EHC. 
    Section 4 includes advice on preventive and protective measures and
    emergency action; health workers should be thoroughly  familiar with
    the medical information to ensure that they can act efficiently in an
    emergency.  Within the Guide is an International Chemical Safety Card
    which should be readily available, and should be clearly explained, to
    all who could come into contact with the chemical.  The section on
    regulatory information has been extracted from the legal file of the
    International Register of Potentially Toxic Chemicals (IRPTC) and from
    other United Nations sources.

    The target readership includes occupational health services, those in
    ministries, governmental agencies, industry, and trade unions who are
    involved in the safe use of chemicals and the avoidance of
    environmental health hazards, and those wanting more information on
    this topic.  An attempt has been made to use only terms that will be
    familiar to the intended user.  However, sections 1 and 2 inevitably
    contain some technical terms.  A bibliography has been included for
    readers who require further background information.

    Revision of the information in this Guide will take place in due
    course, and the eventual aim is to use standardized terminology. 
    Comments on any difficulties encountered in using the Guide would be
    very helpful and should be addressed to:

    The Manager
    International Programme on Chemical Safety
    Division of Environmental Health
    World Health Organization
    1211 Geneva 27



    1.1  Physical Characteristics

    A magnetic field can be illustrated by lines of force and always
    exists when there is an electric current flowing.

    1.1.1  Static magnetic fields

    A static magnetic field is formed around a permanent magnet or by
    direct current flow.

    1.1.2  Time-varying magnetic fields

    These fields are produced by alternating currents having a frequency
    above zero and up to about 300 Hz, and may also be referred to as
    extremely low frequency or ELF magnetic fields.

    In practical considerations regarding protection from radiation, 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 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  Units and Quantities

    The quantities describing a magnetic field are:

    (a) Frequency (f)             hertz (Hz)

    (b) Current (I)               ampere (A)

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

    (d) Field strength (H)        ampere per metre (A/m)

    (e) Flux density (B)          tesla (T) = Wb/m2

    (f) Permeability ()          henry per metre (H/m)

    Magnetic field strength is the force with which the field acts on an
    element of electric current at a particular point.  Magnetic flux
    density is used to describe the magnetic field generated by electric
    currents in a conductor.  The magnetic field strength (H) is related
    to the magnetic flux density (B) by the equation:

         B = H

    Thus, the magnetic field is defined as a vector field of magnetic flux
    densityB (B-field).  The value of  (the magnetic permeability) is
    determined by the properties of the medium and, for most biological
    materials, is equal to o, the value of the permeability of free space

    1.3  Sources of Magnetic Fields and Applications

    1.3.1  Natural sources

    The natural magnetic field consists of a component originating in the
    earth, which acts as a permanent magnet, and several smaller
    time-varying components related to solar activity or atmospheric

    1.3.2  Man-made sources

    The static and time-varying magnetic fields originating from man-made
    sources generally have a much higher intensity than naturally
    occurring fields. This is particularly true for sources operating at
    power frequencies of 50 or 60 Hz (e.g., home appliances), where fields
    occur that are many orders of magnitude greater than natural fields at
    the same frequencies.  Other man-made sources are to be found in
    research, industrial and medical procedures, and other equipment
    related to energy production and transportation.  A list of
    applications that give rise to magnetic fields is given in Table 1. 
    The approximate magnetic flux densities near 60-Hz electrical
    appliances are given in Table 2.

    Some of the sources of, and levels of occupational exposure to,
    magnetic fields are given in Table 3.

    In medicine, magnetic resonance (MR) imaging is used for diagnostic
    purposes and 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 resolution.  Static
    magnetic fields up to 2 T are used and rapid switching of the gradient
    fields produces field changes of up to 20 T/s.

    Pulsed magnetic fields (average field, 0.3 mT; peak field, about
    2.5 mT) are used to enhance wound healing and tissue regeneration, and
    to treat patients suffering from bone fractures that do not heal well.

    Table 1.  Applications that give rise to magnetic fields


    Energy technologies

         Thermonuclear fusion reactors
         Magnetohydrodynamic systems
         Superconducting magnet energy storage systems
         Superconducting generators
         Transmission lines

    Research facilities

         Bubble chambers
         Superconducting spectrometers
         Particle accelerators
         Isotope separation units


         Aluminium production
         Electrolytic processes
         Production of magnets and magnetic materials


         Magnetically levitated vehicles


         Magnetic resonance
         Therapeutic applications

    Table 2.  Magnetic flux densities at 60 Hz near various appliances
              in the USAa


    Appliance                Magnetic flux density (T) at various

                             3 cm             30 cm        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 disposal units   80-250           1-2          0.03-0.1
    Microwave ovens          75-200           4-8          0.25-0.6
      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 sets          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    Readers interested in the sources of this information should
         refer to Environmental Health Criteria 69 :  Magnetic fields,
         Geneva, World Health Organization, 1987.

    Table 3. Occupational sources of exposure to magnetic fieldsa


    Source                   Magnetic flux          Distance (m)
                             densities (mT)

    VDTs                     up - 2.8  10-4        0.3

    Welding arcs             0.1-5.8                0-0.8
      (0-50 Hz)

    Induction heaters        0.9-65                 0.1-1
      (50-10 Hz)

    50-Hz ladle              0.2-8                  0.5-1

    50-Hz arc                up - 1                 2

    10-Hz induction          0.2-0.3                2

    50-Hz electroslag        0.5-1.7                0.2-0.9

    Electrolyte process      7.6 (mean)             operator
      (0-50 Hz)                                     position

    Isotope separation       1-50                   operator
      (static fields)                               position

    a    Readers interested in the sources of this information should
         refer to Environmental Health Criteria 69 :  Magnetic fields,
         Geneva, World Health Organization, 1987.


    2.1  Human Exposure to Magnetic Fields

    Apart from the natural background exposure from the earth and
    atmosphere, everyone near a source of electricity (electric current
    flow) is exposed to magnetic fields.  The general population is
    exposed to magnetic fields from domestic appliances, electric power
    distribution systems, and specialized medical devices.  Workers are
    exposed in all industries using electric power, especially those using
    large electric currents for fabrication.  Certain energy production
    plants, research facilities, kinds of transport, and medical
    applications have the potential to expose people to relatively strong
    magnetic fields.

    2.2  Mechanisms of Interaction

    There are three established physical mechanisms through which static
    and time-varying magnetic fields interact with living matter.

    2.2.1  Magnetic induction

    This mechanism is relevant to both static and time-varying fields, and
    may result from various types of interaction.

    (a)   Electrodynamic interactions with moving electrolytes

    Both static and time-varying fields exert forces on moving carriers of
    an ionic charge, and thereby give rise to induced electric fields and
    currents.  This interaction is the basis of the magnetically-induced
    blood flow potentials that have been studied under the influence of
    both static and time-varying fields.

    (b)   Faraday currents

    Time-varying magnetic fields induce currents (eddy currents) in living
    tissue in accordance with Faraday's law of induction.  The available
    evidence suggests that this mechanism may underlie various effects on
    electrically excitable tissues, including the visuo-sensory
    stimulation that produces magnetophosphenes.  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.

    2.2.2  Magnetomechanical effects

    A static magnetic field exerts two types of mechanical effect on
    biological objects.

    (a)   Magneto-orientation

    In a uniform static field, both diamagnetic and paramagnetic molecules
    experience a torque that tends to orientate them with the field.

    (b)   Magnetomechanical translation

    Variation in the strength of a static magnetic field with distance
    produces 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.

    2.2.3  Electronic interactions

    Certain classes of chemical reaction 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.

    2.3  Effects on Animals and Various Organisms

    Some organisms are sensitive to a static magnetic field with a low
    intensity comparable with that of the geomagnetic field (about 50 T). 
    Phenomena for which there is substantial experimental evidence of
    sensitivity to the earth's field include:

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

    -    orientation and swimming direction of magnetotactic bacteria;

    -    kinetic movements of molluscs;

    -    migratory patterns of birds; and

    -    waggle dance of bees. 

    The available experimental information on the response of organisms,
    including land-dwelling mammals, to static and time-varying magnetic
    fields indicates that the three biological effects indicated below can
    be regarded as established phenomena:

    -    the induction of electrical potentials within the circulatory

    -    magnetophosphene induction by pulsed and time-varying  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

    -    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, a
    body of experimental data indicates the absence of irreversible
    effects on many developmental, behavioural, and physiological
    variables in higher organisms.  Broadly summarized, the available
    evidence suggests that the following nine classes of biological
    function are not significantly affected by static  magnetic fields at
    levels up to 2 T: cell growth, reproduction, pre- and post-natal
    development, bioelectric activity of isolated neurons, behaviour,
    cardiovascular functions (acute exposures), the blood-forming system
    and blood, immune system functions, and physiological regulation and
    circadian rhythms.

    For time-varying magnetic fields, few systematic studies have been
    carried out to define the threshold field characteristics in relation
    to the production of significant perturbations of biological
    functions. Nevertheless, the available evidence suggests that
    time-varying magnetic fields must induce current densities in tissues
    and extracellular fluids that exceed 10mA/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 make use of magnetic fields in this

    2.4  Effects on Human Beings

    2.4.1  Static magnetic fields

    Studies in the USSR on workers involved in the manufacture of
    permanent magnets indicated various subjective symptoms and functional
    disturbances.  However, the 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 do 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 up to 2 T.

    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 which carcinogenic factors within the work
    environment are responsible.

    2.4.2  Time-varying magnetic fields

    Time-varying magnetic fields generate internal electric currents. For
    example, fields with a time rate of change of 3 T/s can induce current
    densities of about 30 mA/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

    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 with whole-body exposure:

    -    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.

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

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

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

    Laboratory studies have been conducted with human subjects exposed to
    sinusoidally time-varying magnetic fields.  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 hours.

    Of particular concern are recent epidemiological reports that present
    preliminary data indicative of an increase in the incidence of cancer
    among children, adults, and occupational groups.  In other
    epidemiological studies, 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 (0.1-1 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 time-varying 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.


    3.1  Static Fields

    The available evidence 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 definite statement about the possible
    hazards 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.

    3.2  Time-Varying Fields

    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 10-100mA/m2 (from fields higher than
    5-50 mT at 50/60Hz), it has been established that short-term exposure
    (few hours) to these induced current densities 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
    100mA/m2 (greater than 50 mT at 50/60 Hz), various stimulation
    thresholds are exceeded and hazards to health may occur.


    4.1  Exposure Reduction

    In general, there are two types of technique available to minimize
    needless exposure to high intensity magnetic fields.

    (a)  Distance and time

    Limit human access to and/or the duration of stay in locations where
    field strengths are high.  Since the external magnetic flux density
    decreases with distance from the source, separation distance is a
    fundamental protective measure.

    (b)  Magnetic shielding

    The use of ferromagnetic core materials restricts the spatial extent
    of the external flux lines of a magnetic device.  External enclosures
    of ferromagnetic materials can also "capture" flux lines and reduce
    external flux densities.  However, shielding is normally expensive and
    of limited use for scientific instruments.  Furthermore, it has not
    generally been shown to be cost-effective for large installations in
    comparison with the use of separation distance.

    4.2  Safety

    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

    (a)  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
    40mT/s.  Certain pacemaker models also operate abnormally as a result
    of the closure of a reed relay switch in static magnetic fields that
    exceed 1.7-4.7mT.  Magnetic fields can also affect the functioning of
    other medical electronic monitoring devices, such as
    electroencephalograph and electrocardiograph equipment.

    (b)  Metallic implants

    The sensitivity of implanted surgical devices to magnetic fields
    depends 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 lead to serious

    consequences as a result of their displacement.  All persons entering
    magnetic field environments should be screened carefully and, if
    necessary, prohibited from access.

    (c)  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 hand tools from the vicinity of strong
    magnetic field sources.


    5.1  Static Fields

    The limits of occupational exposure to static fields in the USSR and
    various national accelerator laboratories are given in Table 4.

    5.2  Time-Varying Fields

    The only national standard for time-varying magnetic fields is in the
    USSR.  This standard, issued by the Ministry of Health in 1985, is
    shown in Table 5.  The limits for exposure to continuous-wave 50-Hz
    fields are equivalent to 7.5 mT for 1 hour and decrease with
    increasing time to 1.8 mT for an 8-hour stay in the field.

    5.3  Magnetic Resonance Imaging (MRI)

    During the imaging procedure, which may last more than 1 hour, the
    patient lies on a table and all parts of the body are exposed to
    strong static magnetic fields, changing (or time-varying) magnetic
    fields, and radio-frequency radiation.  Rapidly switched gradient
    fields are superimposed on the static field to allow spatial
    information to be obtained.

    Guidelines on exposure to static and time-varying magnetic fields for
    the clinical examination of patients during MRI have received special
    attention by various national authorities and are shown in Table 6.

        Table 4.  Limits of occupational exposure to static magnetic fields


    Author                   Field         Exposure time       Body region            Comments

    USSR (1978)              0.01 T        8 h                 whole body             Regulation issued by Ministry of

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

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

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

    Lawrence Livermore       0.06 T        day                 trunk                  Maximum average/day in peak fields >0.5 T
    National Laboratory      0.06 T        day                 trunk                  Maximum average/week in peak fields <0.5 T
    (LLNL, 1985)             0.6 T         day                 extremities            Maximum average/week (in peak fields
                                                                                      <0.5 T) or per day (in peak fields >0.5 T)
                             2 T           short (min)         whole body             Peak exposure limit

        Table 5.  Maximum permissible levels of magnetic fields with a frequency of 50 Hza


    Duration                      Magnetic field strength (A/m)
    of exposure                                                                      
                    Continuous and          Pulsed magnetic     Pulsed magnetic
                    pulsed magnetic         field               field
                    fields with pulse                                                
                    width tw > 0.02 s       60 s > tw > 1 s     0.02 s < tw <1 s
                    and pause tp <2 s       tp >2 s             tp > 2 s

    1               6000                    8000                10000
    1.5             5500                    7500                9500
    2               4900                    6900                8900 
    2.5             4500                    6500                8500 
    3               4000                    6000                8000 
    3.5             3600                    5600                7600 
    4               3200                    5200                7200 
    4.5             2900                    4900                6900 
    5               2500                    4500                6500 
    5.5             2300                    4300                6300 
    6               2000                    4000                6000 
    6.5             1800                    3800                5800 
    7               1600                    3600                5600 
    7.5             1500                    3500                5500 
    8               1400                    3400                5400 

    a  Note:  The above regimes of pulsed exposures are used in welding.
                tw is the pulse width duration,
                tp is the pulse pause duration.

    Table 6.  Guidelines on magnetic field exposure in the clinical use of magnetic resonance


    Countrya       Static fields                       Time-varying fields

    USA            Patient - 2 T whole and             Patient - 3 T/s whole and partial
    (CDRH)         partial body exposure               body exposure

                        Exposure exceeding these limits should be evaluated
                        on an individual basis

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

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


                                                       (300/t) mA/m2 or (3/t) V/m for
                                                       duration of magnetic field change
                                                       (t) shorter than 10 ms (t in ms)

    Table 6. (contd)


    Countrya       Static fields                       Time-varying fields

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

                   Patient - 2 T (whole and
                   partial body exposure)

    a  CDRH = Center for Devices and Radiological Health, Rockville, Maryland, USA.
       NRPB = National Radiological Protection Board, United Kingdom.
       FHO = Federal Health Office, Federal Republic of Germany.

    ALPEN, E.L. (1979) Magnetic field exposure guidelines. In: Tenforde,
    T.S., ed.  Magnetic field effects on biological systems. New York,
    London, Plenum Press, pp. 25-32.

    LLNL (1985)  Working in magnetic fields. Berkeley, University of
    California, Lawrence Livermore National Laboratory (Health and Safety

    NRPB (1981) Exposure to nuclear magnetic resonance clinical imaging.
    Radiography, 47 (563): 258-260.

    STANFORD LINEAR ACCELERATOR CENTER (1970)  Limits on human exposure to
     static magnetic fields. Palo Alto, California.

    USSR (1978)  [Maximum permissible levels of exposure to static
     magnetic fields at work with magnetic installations and magnetic
     materials.] Moscow, Ministry of Public Health (Document No. 1742-77)
    (in Russian).


    See Also:
       Toxicological Abbreviations
       Magnetic fields (EHC 69, 1987)