IPCS INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
Health and Safety Guide No. 27
MAGNETIC FIELDS
HEALTH AND SAFETY GUIDE
UNITED NATIONS ENVIRONMENT PROGRAMME
INTERNATIONAL RADIATION PROTECTION ASSOCIATION
WORLD HEALTH ORGANIZATION
WORLD HEALTH ORGANIZATION, GENEVA
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
boundaries.
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.
CONTENTS
INTRODUCTION
1. PHYSICAL CHARACTERISTICS AND APPLICATIONS
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. SUMMARY AND EVALUATION
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. CONCLUSIONS
3.1. Static fields
3.2. Time-varying fields
4. PROTECTIVE MEASURES
4.1. Exposure reduction
4.2. Safety
5. CURRENT REGULATIONS, GUIDELINES, AND STANDARDS
5.1. Static fields
5.2. Time-varying fields
5.3. Magnetic resonance imaging (MRI)
REFERENCES
INTRODUCTION
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
Switzerland
THE INFORMATION IN THIS GUIDE SHOULD BE CONSIDERED AS A STARTING POINT
TO A COMPREHENSIVE HEALTH AND SAFETY PROGRAMME
1. PHYSICAL CHARACTERISTICS AND APPLICATIONS
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 (µ) 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 = µH
Thus, the magnetic field is defined as a vector field of magnetic flux
densityB (B-field). The value of µ (the magnetic permeability) is
determined by the properties of the medium and, for most biological
materials, is equal to µo, the value of the permeability of free space
(air).
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
events.
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
Industry
Aluminium production
Electrolytic processes
Production of magnets and magnetic materials
Transportation
Magnetically levitated vehicles
Medicine
Magnetic resonance
Therapeutic applications
Table 2. Magnetic flux densities at 60 Hz near various appliances
in the USAa
Appliance Magnetic flux density (µT) at various
distances
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
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 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
furnace
50-Hz arc up - 1 2
furnace
10-Hz induction 0.2-0.3 2
stirrer
50-Hz electroslag 0.5-1.7 0.2-0.9
welding
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. SUMMARY AND EVALUATION
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 µT).
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
system;
- 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
range.
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
level.
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 µT) 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. CONCLUSIONS
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. PROTECTIVE MEASURES
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
gradients.
(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. CURRENT REGULATIONS, GUIDELINES, AND STANDARDS
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
Health
Stanford 0.02 T extended (h) whole body Unofficial, occupational
Linear 0.2 T short (min) whole body
Accelerator 0.2 T extended (h) arms, hands
Center (1970) 2 T short (min) arms, hands
US Department of 0.01 T 8 h whole body Recommended to DOE contractors
Energy (DOE) 0.1 T 1 h or less whole body
(Alpen, 1979) 0.5 T 10 min or less whole body
0.1 T 8 h arms, hands
1 T 1 h or less arms, hands
2 T 10 min or less arms, hands
CERN Accelerator 0.2 T minutes whole body Recommended practice
Lab, Geneva 2 T short hands, arms,
(NRPB, 1981) and feet
Lawrence 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
(h)
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
body)
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
or
(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)
and
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.
REFERENCES
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
Manual
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).