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