INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 137
ELECTROMAGNETIC FIELDS
(300 HZ TO 300 GHZ)
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.
Published under the joint sponsorship of
the United Nations Environment Programme,
the International Radiation Protection Association,
and the World Health Organization
World Health Orgnization
Geneva, 1993
WHO Library Cataloguing in Publication Data
Electromagnetic fields (300 Hz to 300 GHz)
(Environmental health criteria: 137)
1. Electromagnetic fields - adverse effects 2. Environmental
exposure I.Series
ISBN 92 4 157137 3 (NLM Classification QT 34)
ISSN 0250-863X
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CONTENTS
PREFACE
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES
1.1. Summary
1.1.1. Physical characteristics in relation to biological
effects
1.1.2. Sources and exposure
1.1.2.1 Community
1.1.2.2 Home
1.1.2.3 Workplace
1.1.3. Biological effects
1.1.4. Laboratory studies
1.1.5. Human studies
1.1.6. Health hazard assessment
1.1.6.1 Thermal effects
1.1.6.2 Pulsed fields
1.1.6.3 Amplitude-modulated RF fields
1.1.6.4 RF field effects on tumour induction and
promotion
1.1.6.5 RF-induced current densities
1.1.6.6 RF contact shocks and burns
1.1.7. Exposure standards
1.1.7.1 Basic exposure limits
1.1.7.2 Occupational exposure limits
1.1.7.3 Exposure limits for the general
population
1.1.7.4 Implementation of standards
1.1.8. Protective measures
1.2. Recommendations for further studies
1.2.1. Introduction
1.2.2. Pulsed fields
1.2.3. Cancer, reproduction, and nervous system
studies
1.2.4. Weak-field interactions
1.2.5. Epidemiology
2. PHYSICAL CHARACTERISTICS
2.1. Introduction
2.2. Electric field
2.3. Magnetic field
2.4. Waves and radiation
3. NATURAL BACKGROUND AND HUMAN-MADE SOURCES
3.1. General
3.2. Natural background
3.2.1. Atmospheric fields
3.2.2. Terrestrial emissions
3.2.3. Extraterrestrial fields
3.3. Human-made sources
3.3.1. General
3.3.2. Environment, home, and public premises
3.3.3. Workplace
3.3.4. Medical practice
4. EXPOSURE EVALUATION - CALCULATION AND MEASUREMENT
4.1. Introduction
4.2. Theoretical estimation
4.3. Measurements
4.3.1. Preliminary considerations
4.3.2. Near-field versus far-field
4.3.3. Instrumentation
4.3.4. Measurement procedures
5. DOSIMETRY
5.1. General
5.2. Low frequency range
5.2.1. Magnetic fields
5.2.2. Electric fields
5.3. High-frequency range
5.4. Derivation of exposure limits from basic quantities
6. INTERACTION MECHANISMS
6.1. General
6.2. Electrical properties of cells and tissues
6.2.1. Permittivity
6.2.2. Non-linear effects
6.2.3. Induced fields at the cellular level
6.2.4. Body impedance
6.3. Direct interactions - strong fields
6.3.1. Interactions with excitable tissues
6.3.2. Thermal interactions
6.4. Direct interactions - weak fields
6.4.1. General
6.4.2. Microelectrophoretic motion
6.4.3. Ion-resonance conditions
6.4.4. Calcium ion exchange
6.5. Indirect interactions
7. CELLULAR AND ANIMAL STUDIES
7.1. Introduction
7.2. Macromolecules and cell systems
7.2.1. Effects on cell membranes
7.2.2. Effects on haematopoietic tissue
7.2.3. Isolated cerebral tissue, peripheral nerve tissue,
and heart preparations
7.2.4. Mutagenic effects
7.2.5. Cancer-related studies
7.2.6. Summary and conclusions: in vitro studies
7.3. Animal studies
7.3.1. Nervous system
7.3.2. Ocular effects
7.3.3. Auditory perception
7.3.4. Behaviour
7.3.4.1 Thermoregulation
7.3.4.2 Activity (spontaneous movement)
7.3.4.3 Learned behaviours
7.3.5. Endocrine system
7.3.6. Haematopoietic and immune systems
7.3.7. Cardiovascular system
7.3.8. Reproduction and development
7.3.8.1 kHz studies
7.3.8.2 MHz and GHz studies
7.3.9. Genetics and mutagenesis
7.3.10. Cancer-related studies
7.3.11. Summary and conclusions
8. HUMAN RESPONSES
8.1. Laboratory studies
8.1.1. Cutaneous perception
8.1.2. Other perception thresholds
8.1.3. Auditory effects
8.1.4. Induced-current effects
8.1.5. Thermoregulation
8.1.6. Contact currents
8.2. Epidemiological and clinical comparative studies
8.2.1. Mortality and morbidity studies
8.2.2. Ocular effects
8.2.3. Effects on reproduction
8.2.4. VDU studies
8.2.5. Conclusions
8.3. Clinical case studies and accidental overexposures
9. HEALTH HAZARD ASSESSMENT
9.1. Introduction
9.2. Thermal effects
9.3. RF contact shocks and burns
9.4. Induced current densities
9.5. Pulsed RF fields
9.6. RF fields amplitude modulated at ELF frequencies
9.7. RF effects on tumour induction and progression
10. EXPOSURE STANDARDS
10.1. General considerations
10.2. Present trends
10.3. Recommendations by the IRPA
10.4. Concluding remarks
11. PROTECTIVE MEASURES
11.1. Engineering measures
11.2. Administrative controls
11.3. Personal protection
11.4. Medical surveillance
11.5. Interference with medical devices and safety equipment
GLOSSARY
REFERENCES
RESUME ET RECOMMANDATIONS EN VUE D'ETUDES FUTURES
RESUMEN Y RECOMENDACIONES PARA ESTUDIOS ULTERIORES
WHO/IRPA TASK GROUP ON ELECTROMAGNETIC FIELDS (300 Hz TO 300 GHz)
Members
Prof J. Bernhardta Federal Office of Radiological
Protection, Institute for Radiation Hygiene,
Munich-Neuherberg, Germany
Dr C. F. Blackman US Environmental Protection Agency, Health
Effects Research Laboratory, North
Carolina, USA
Dr L.A. Courta Département de Protection Sanitaire,
Centre d'Etudes Nucléaires,
Fontenay-aux-Roses, France
Mme A. Duchênea Scientific Secretary, International
Non-ionizing Radiation Committee,
Fontenay-aux-Roses, France
Prof M. Grandolfoa Istituto Superiore di Sanità, Rome,
Italy
Dr M.H. Repacholia Royal Adelaide Hospital, Adelaide,
Australia (Chairman)
Dr R.D. Saunders National Radiological Protection Board,
Didcot, United Kingdom (Co-Rapporteur)
Prof M.G. Shandalaa AN Marzeev Research Institute of General
and Communal Hygiene, Kiev, USSR
Dr J.A. Stolwijka Department of Epidemiology and Public
Health, Yale University, New Haven, USA
Dr M.A. Stuchlya Bureau of Radiation and Medical Devices,
Ottawa, Canada
Dr M. Swicord Centre for Devices and Radiological Health,
Food and Drug Administration, Rockville, USA
(Co-Rapporteur)
Dr L.D. Szaboa National Research Institute for
Radiobiology and Radiation Hygiene,
Budapest, Hungary
Dr S. Szmigielski Centre for Radiobiology and Radiation
Safety,Warsaw, Poland (Vice-Chairman)
Observer
Dr A. McKinlaya National Radiological Protection Board,
Didcot, United Kingdom
a From the International Non-Ionizing Radiation Committee of IRPA.
NOTE TO READERS OF THE CRITERIA MONOGRAPHS
Every effort has been made to present information in the criteria
monographs as accurately as possible without unduly delaying their
publication. In the interest of all users of the environmental health
criteria monographs, readers are kindly requested to communicate any
errors that may have occurred to the Director of the International
Programme on Chemical Safety, World Health Organization, Geneva,
Switzerland, in order that they may be included in corrigenda, which
will appear in subsequent volumes.
DEDICATION
This monograph is dedicated to:
Professor Przemyslaw A. Czerski, a charter member of International
Non-ionizing Radiation Committee, who died on 15 April 1990 in Silver
Spring, MD (USA). He was a pioneer investigator into the effects of
non-ionizing radiation on biosystems and the assessment of the
potential hazards associated with such exposure. As a fervent promoter
of international cooperation, Professor Czerski played an active part
in the establishment of the International Non-Ionizing Radiation
Committee and in the development of its activities. His broad
scientific knowledge and his tireless energy made him a major
contributor to the present publication.
PREFACE
The International Radiation Protection Association (IRPA) initiated
activities concerned with non-ionizing radiation by forming a Working
Group on Non-Ionizing Radiation in 1974. This Working Group later
became the International Non-Ionizing Radiation Committee
(IRPA/INIRC), at the IRPA meeting held in Paris in 1977. The
IRPA/INIRC reviews the scientific literature on non-ionizing radiation
and makes assessments of the health risks of human exposure to such
radiation. On the basis of Environmental Health Criteria monographs,
developed in conjunction with the World Health Organization, Division
of Environmental Health, the IRPA/INIRC recommends guidelines on
exposure limits, drafts codes of safe practice, and works in
conjunction with other international organizations to promote safety
and standardization in the non-ionizing radiation field.
A WHO/IRPA Task Group to review the final draft of the Environmental
Health Criteria on Electromagnetic Fields (300 Hz-300 GHz) met at the
WHO Collaborating Centre for NIR in Ottawa, Canada, from 22 to 26
October 1990. Dr A.J. Liston, Assistant Deputy Minister, Health
Protection Branch, opened the meeting on behalf of the Minister for
Health and Welfare Canada. Mr J.R. Hickman, Director General,
Environmental Health Directorate, welcomed the participants. The
support of Health and Welfare Canada and the local organization by the
Environmental Health Directorate are gratefully acknowledged.
The first draft of this publication was compiled by Professor J.
Bernhardt, Professor P. Czerski, Professor M. Grandolfo, Dr A.
McKinlay, Dr M. Repacholi, Dr R. Saunders, Professor J. Stolwijk, and
Dr M. Stuchly. An editorial group comprising Professor J. Bernhardt,
Professor P. Czerski, Professor M. Grandolfo, Mr C. Hicks, Dr A.
McKinlay, Dr R. Saunders, Mr D. Sliney, Professor J. Stolwijk, and Dr
M. Swicord met at the US Army Environmental Hygiene Agency, Edgewood,
MD, in February 1990 to revise the draft. A second editorial group
comprising Professor J. Bernhardt, Mme A. Duchêne, Dr A. McKinlay
(Chairman), Professor B. Knave, Dr R. Saunders, and Dr M. Stuchly met
at the National Radiological Protection Board, Didcot, United Kingdom,
in May 1990 to collate and incorporate the comments received by IPCS
Focal Points, IRPA Associate Societies, and individual experts. Dr M.
Repacholi was responsible for the scientific editing of the text and
Mrs M.O. Head of Oxford for the language editing.
This publication comprises a review of the data on the effects of
electromagnetic field exposure on biological systems pertinent to the
evaluation of human health risks. The purpose of the document is to
provide an overview of the known biological effects of electromagnetic
fields in the frequency range 300 Hz to 300 GHz, to identify gaps in
this knowledge so that direction for further research can be given,
and to provide information for health authorities, regulatory, and
similar agencies on the possible effects of electromagnetic field
exposure on human health, so that guidance can be given on the
assessment of risks from occupational and general population exposure.
Most radiofrequency (RF) field standards are based on the premise that
there exists a threshold specific absorption rate (SAR) of RF energy
(for frequencies above about 1 MHz) of 1-4 W/kg, above which there is
increasing likelihood of adverse health effects. Below about 1 MHz,
standards are based on induced currents in the body, causing shocks
and burns. The purpose of updating the original Environmental Health
Criteria monograph on radio frequency (WHO, 1981) is not only to
provide a description of more completely developed RF dosimetry in
humans, but to critically review more recent scientific literature, to
determine if the threshold SAR on which standards are based is still
valid. With the frequency range covered by the document extended down
to 300 Hz, more emphasis is placed on induced currents and other
possible mechanisms of interaction.
In conducting the literature review, earlier reports are not
necessarily included, since these were reviewed in UNEP/WHO/IRPA
(1981). Every effort has been made to distinguish clearly between
biological effects that have been established and those that have been
reported as preliminary or isolated results, or as hypotheses proposed
to explain observed results. The conclusions of this document are
based on peer reviewed and established knowledge of interactions of
electromagnetic fields with biological systems.
Subjects reviewed include: the physical characteristics of
electromagnetic fields; measurement techniques; applications of
electromagnetic fields and sources of exposure; mechanisms of
interaction; biological effects; and guidance on the development of
protective measures, such as regulations or safe-use guidelines.
Health agencies and regulatory authorities are encouraged to set up
and develop programmes that ensure that the maximum benefit occurs
with the lowest exposure. It is hoped that this criteria document will
provide useful information for the development of national protection
measures against electromagnetic fields, as well as serving as a
reliable basis for such reports as environmental impact statements
necessary for proposed electromagnetic field emission facilities.
The WHO Regional Office for Europe has published a second edition of
the book entitled Nonionizing radiation protection, which includes
a chapter on radiofrequency radiation (Suess & Benwell-Morison, 1989).
1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES
1.1 Summary
1.1.1 Physical characteristics in relation to biological effects
This monograph is concerned with the health effects of
electromagnetic fields in the frequency range of 300 Hz-300 GHz, which
includes the radiofrequency (RF) range (100 kHz-300 GHz) covered in
the earlier publication (WHO, 1981). For simplicity, RF is the term
used in this document for electromagnetic fields of frequency 300
Hz-300 GHz. Within these frequencies are microwaves, having
frequencies of between 300 MHz and 300 GHz.
Exposure levels in the microwave range are usually described in
terms of "power density" and are normally reported in watt per square
metre (W/m2), or milliwatt or microwatts per square metre (mW/m2,
µW/m2). However, close to RF sources with longer wavelengths, the
values of both the electric (V/m) and magnetic (A/m) field strengths
are necessary to describe the field.
Exposure conditions can be altered considerably by the presence
of objects, the degree of perturbation depending on their size, shape,
orientation in the field, and electrical properties. Very complex
field distributions can occur, both inside and outside biological
systems exposed to electromagnetic fields. Refraction within these
systems can focus the transmitted energy resulting in markedly
non-uniform fields and energy deposition. Different energy absorption
rates can result in thermal gradients causing biological effects that
may be generated locally, difficult to anticipate, and perhaps unique.
The geometry and electrical properties of biological systems will also
be determining factors in the magnitude and distribution of induced
currents at frequencies below the microwave range.
When electromagnetic fields pass from one medium to another, they
can be reflected, refracted, transmitted, or absorbed, depending on
the conductivity of the exposed object and the frequency of the field.
Absorbed RF energy can be converted to other forms of energy and cause
interference with the functioning of the living system. Most of this
energy is converted into heat. However, not all electromagnetic field
effects can be explained in terms of the biophysical mechanisms of
energy absorption and conversion to heat. At frequencies below about
100 kHz, it has been demonstrated that induced electric fields can
stimulate nervous tissue. At the microscopic level, other interactions
leading to perturbations in complex macromolecular biological systems
(cell membranes, subcellular structures) have been postulated.
1.1.2 Sources and exposure
1.1.2.1 Community
In comprehensive community surveys of background levels of
electromagnetic fields in the USA, a median exposure of the order of
50 µW/m2 was found. Very high frequency broadcasts were identified
as the main contributors to ambient electromagnetic fields. No more
than 1% of the population was exposed to ambient power densities in
excess of 10 mW/m2. Exposure in the immediate vicinity (at a
distance of the order of one half wavelength of the incident fields)
of transmitting facilities, can be higher, and can be enhanced by
nearby conducting objects. Such conditions should be evaluated for
each specific situation.
1.1.2.2 Home
RF sources in the home include microwave ovens, induction heating
stoves, burglar alarms, video display units (VDUs), and television
receivers. Leakage from microwave ovens can be up to 1.5 W/m2 at 0.3
m and 0.15 W/m2 at a distance of 1 metre. Exposure to radiation from
domestic appliances is best limited by design and by monitoring at the
point of manufacture.
1.1.2.3 Workplace
Dielectric heaters for wood fabrication and the sealing of
plastics, induction heaters for the heating of metals, and video
display units, are widely used in a variety of occupational settings.
VDUs create electric and magnetic fields at frequencies in the 15-35
kHz range and frequencies modulated in the ELF range. Personnel
working on, or near, broadcasting towers or antennas can be exposed to
substantial fields of up to 1 kV/m and 5 A/m, respectively. Workers
near radar installations can be exposed to substantial peak power
densities, if they are in the RF beam a few metres from radar antennas
(up to tens of MW/m2). Usually, the average power density in the
vicinity of air traffic control radars, for example, is of the order
of 0.03-0.8 W/m2.
In the occupational environment, the protection of workers is
best assured by referring to the emission specifications for
individual items of equipment, and, where necessary, by monitoring and
surveillance using appropriate instrumentation.
A special case of exposure occurs in the medical environment with
the use of diathermy treatments for pain and inflammation in body
tissues. Diathermy operators are likely to be exposed occupationally
to stray radiation at relatively high levels, which can be reduced by
appropriate shielding or machine design. Field strengths of 300 V/m
and 1 A/m at 10 cm from the applicators have been measured. Similarly,
surgeons using electrosurgical devices operating at frequencies near
27 MHz may be exposed to levels above recommended limits. These field
strengths decline very rapidly with increasing distance from the
applicators.
Most magnetic resonance imaging (MRI) systems use static magnetic
fields with flux densities of up to 2 T, low-frequency gradient fields
up to 20 T/s, and RF fields in the 1-100 MHz frequency range. Although
power deposition in the patient can be substantial, staff exposures
are much lower and are determined by equipment characteristics.
1.1.3 Biological effects
Electromagnetic fields in the frequency range of 300 Hz-300 GHz
interact with human and other animal systems through direct and
indirect pathways. Indirect interactions are important at frequencies
below 100 MHz, but are specific to particular situations. When
metallic objects (such as automobiles, fences) in an electromagnetic
field have electrical charges induced in them, they can be discharged
when a body comes into contact with the charged object. Such
discharges can cause local current densities capable of shock and
burns.
A major interaction mechanism is through the currents induced in
tissues, so effects are dependent on frequency, wave shape, and
intensity. For frequencies below approximately 100 kHz, the
interactions with nervous system tissue are of interest, because of
their increased sensitivity to induced currents. Above 100 kHz, the
nervous tissue becomes less sensitive to direct stimulation by
electromagnetic fields and the thermalization of energy becomes the
major mechanism of interaction.
There is evidence from a number of studies that weak-field
interactions also exist. Different mechanisms for such interactions
have been postulated, but the precise mechanism(s) has not been
elucidated. These weak-field interactions result from exposure to RF
fields, amplitude modulated at lower frequencies.
1.1.4 Laboratory studies
Many of the biological effects of acute exposure to
electromagnetic fields are consistent with responses to induced
heating, resulting either in rises in tissue or body temperature of
about 1 °C or more, or in responses to minimizing the total heat load.
Most responses have been reported at specific absorption rates (SARs)
above about 1-2 W/kg in different animal species exposed under various
environmental conditions. The animal (particularly primate) data
indicate the types of responses that are likely to occur in humans
subjected to a sufficient heat load. However, direct quantitative
extrapolation to humans is difficult, given species differences in
responses in general, and in thermoregulatory ability, in particular.
The most sensitive animal responses to heat loads are
thermoregulatory adjustments, such as reduced metabolic heat
production and vasodilation, with thresholds ranging between about
0.5-5 W/kg, depending on environmental conditions. However, these
reactions form part of the natural repertoire of thermoregulatory
responses that serve to maintain normal body temperatures.
Transient effects seen in exposed animals, which are consistent
with responses to increases in body temperature of 1 °C or more
(and/or SARs in excess of about 2 W/kg in primates and rats), include
reduced performance of learned tasks and increased plasma
corticosteroid levels. Other heat-related effects include temporary
haematopoietic and immune responses, possibly due to elevated
corticosteroid levels. The most consistent effects observed are
reduced levels of circulating lymphocytes, increased levels of
neutrophils, and altered natural killer cell and macrophage function.
An increase in the primary antibody response of B-lymphocytes has also
been reported. Cardiovascular changes consistent with increased heat
load, such as an increased heart rate and cardiac output, have been
observed, together with a reduction in the effect of drugs, such as
barbiturates, the action of which can be altered by circulatory
changes.
Most animal data indicate that implantation and the development
of the embryo and fetus are unlikely to be affected by exposures that
increase maternal body temperature by less than 1 °C. Above these
temperatures, adverse effects, such as growth retardation and
post-natal changes in behaviour, may occur, with more severe effects
occurring at higher maternal temperatures.
Most animal data suggest that low RF exposures that do not raise
body temperatures above the normal physiological range are not
mutagenic: Such exposures will not result in somatic mutation or
hereditary effects. There is much less information describing the
effects of long-term, low-level exposures. However, so far, it does
not appear that any long-term effects result from exposures below
thermally significant levels. The animal data indicate that male
fertility is unlikely to be affected by long-term exposure to levels
insufficient to raise the temperature of the body and testes.
Cataracts were not induced in rabbits exposed at 100 W/m2 for
6 months, or in primates exposed at 1.5 kW/m2 for over 3 months.
A study of 100 rats, exposed for most of their lifetime to about
0.4 W/kg, did not show any increased incidence of non-neoplastic
lesions or total neoplasias compared with control animals; longevity
was similar in both groups. There were differences in the overall
incidence of primary malignancies, but these could not necessarily be
attributed to the irradiation.
The possibility that exposure to RF fields might influence the
process of carcinogenesis is of particular concern. So far, there is
no definite evidence that irradiation does have an effect, but there
is clearly a need for further studies to be carried out. Many
experimental data indicate that RF fields are not mutagenic, and so
they are unlikely to act as initiators of carcinogenesis; in the few
studies carried out, the search has mainly been for evidence of an
enhancement of the effect of a known carcinogen. Long-term exposure of
mice at 2-8 W/kg resulted in an increase in the progression of
spontaneous mammary tumours, and of skin tumours in animals treated
dermally with a chemical carcinogen.
In vitro studies have revealed enhanced cell transformation
rates after RF exposure at 4.4 W/kg (alone or combined with
X-radiation) followed by treatment with a chemical promoter. The
latter data have not always been consistent between studies. It is
clear, however, that studies relevant to carcinogenesis need
replicating and extending further.
A substantial body of data exists describing biological responses
to amplitude-modulated RF or microwave fields at SARs too low to
involve any response to heating. In some studies, effects have been
reported after exposure at SARs of less than 0.01 W/kg, occurring
within modulation frequency "windows" (usually between 1-100 Hz) and
sometimes within power density "windows"; similar results have been
reported at frequencies within the voice frequency (VF) range (300
Hz-3 kHz). Changes have been reported in: the electroencephalograms of
cats and rabbits; calcium ion mobility in brain tissue in vitro, and
in vivo; lymphocyte cytotoxicity in vitro; and activity of an
enzyme involved in cell growth and division. Some of these responses
have been difficult to confirm, and their physiological consequences
are not clear. However, any toxicological investigations should be
based on tests carried out at appropriate levels of exposure. It is
important that these studies be confirmed and that the health
implications, if any, for exposed people, are determined. Of
particular importance would be studies that link extremely low
frequency, amplitude-modulated, RF or microwave interactions at the
cell surface with changes in DNA synthesis or transcription. It is
worth noting that this interaction implies a "demodulation" of the RF
signal at the cell membrane.
1.1.5 Human studies
There are relatively few studies that address directly the
effects of acute or long-term exposures of humans to RF fields. In
studies in the laboratory, cutaneous perception of fields in the 2-10
GHz range has been reported. Thresholds for just noticeable warming
have been reported at power densities of 270 W/m2 - 2000 W/m2,
depending on the area irradiated (13-100 cm2) and the duration of
exposure (1-180 s). When human volunteers are exposed to SARs of 4
W/kg for 15-20 minutes their average body temperature rises by 0.2-0.5
°C, which is quite acceptable in healthy people. The impact that this
added thermal load would have on thermoregulatory impaired individuals
in environments that minimize the perspiration-based cooling
mechanisms is not known.
The few epidemiological studies that have been carried out on
populations exposed to RF fields have failed to produce significant
associations between such exposures and outcomes of shortened life
span, or excesses in particular causes of death, except for an
increased incidence of death from cancer, where chemical exposure may
have been a confounder. In some studies, there was no increase in the
incidence of premature deliveries or congenital malformations, while
other studies produced indications that there was an association
between the level of exposure and adverse pregnancy outcome. Such
studies tend to suffer from poor exposure assessment and poor
ascertainment and determination of other risk factors.
1.1.6 Health hazard assessment
The following categories of health hazard have been identified in
an overall assessment of the health hazards associated with RF
exposures.
1.1.6.1 Thermal effects
The deposition of RF energy in the human body tends to increase
the body temperature. During exercise, the metabolic heat production
can reach levels of 3-5 W/kg. In normal thermal environments, an SAR
of 1-4 W/kg for 30 minutes produces average body temperature increases
of less than 1°C for healthy adults. Thus, an occupational RF
guideline of 0.4 W/kg SAR leaves a margin of protection against
complications due to thermally unfavourable environmental conditions.
For the general population, which includes sensitive subpopulations,
such as infants and the elderly, an SAR of 0.08 W/kg would provide an
adequate further margin of safety against adverse thermal effects from
RF fields.
1.1.6.2 Pulsed fields
It has been shown, under a number of conditions, that the
thresholds for biological effects at frequencies above several hundred
MHz are decreased when the energy is delivered in short (1-10 µs)
pulses. For example, auditory effects occur when pulses of less than
30 µs duration deliver more than 400 mJ/m2 per pulse. A safe limit
for such pulses cannot be identified on the basis of available
evidence.
1.1.6.3 Amplitude-modulated RF fields
The effects described for this type of field at the cellular,
tissue, and organ levels cannot be related to adverse health effects.
No dose-effect relationships can be formulated that demonstrate
threshold levels; thus, the available information cannot lead to
specific recommendations.
1.1.6.4 RF field effects on tumour induction and promotion
It is not possible, from the reports of the effects of RF
exposure in certain cell lines, on cell transformation, enzyme
activity, and tumour incidence and progression in animals, to conclude
that RF exposure has any effect on the incidence of cancer in humans,
or, that specific recommendations are necessary to limit such fields
to reduce cancer risks.
1.1.6.5 RF-induced current densities
In the frequency range of 300 Hz-100 kHz, the induction of fields
and current densities in excitable tissues is the most important
mechanism for hazard assessment. The thresholds for the stimulation of
nerve and muscle tissue are strongly dependent on frequency, ranging
from 0.1-1 A/m2 at 300 Hz to about 10-100 A/m2 at 100 kHz.
However, with regard to other effects, reported to occur below these
thresholds, there is not sufficient information available to make
specific recommendations.
1.1.6.6 RF contact shocks and burns
Conducting objects in an RF field can become electrically
charged. When a person touches a charged object or approaches it
closely, a substantial current can flow between the object and the
person. Depending on the frequency, the electric field strength, the
size and the shape of the object,and the cross-sectional area of
contact, the resulting current can cause shock through stimulation of
peripheral nerves. If the current is strong enough, burns can result.
Protective measures include the elimination or enclosure of conductive
objects in strong RF fields, or the limiting of physical access.
1.1.7 Exposure standards
1.1.7.1 Basic exposure limits
To protect workers and the general population from the possible
health effects of exposure to electromagnetic fields, basic exposure
limits have been determined on the basis of knowledge of biological
effects. Different scientific bases were used to develop the limits
for frequencies above and below about 1 MHz. Above 1 MHz, biological
effects on animals were studied to determine the lowest value of the
whole body average SAR that caused detrimental health effects in
animals. This value was found to be in the 3-4 W/kg range.
The vast majority of results pertained to exposures in the low
GHz region. Thus, to determine the effects at lower frequencies
requires an assumption concerning the frequency dependence of the
biological response. Since the observed bioeffects in the 1-4 W/kg
range are believed to be thermal, the SAR threshold was assumed to be
independent of frequency. It was considered that exposure of humans to
4 W/kg for 30 minutes would result in a body temperature rise of less
than 1°C. This body temperature rise is considered acceptable.
A safety factor of 10 is introduced, in order to allow for
unfavourable, thermal, environmental, and possible long-term effects,
and other variables, thus arriving at a basic limit of 0.4 W/kg. An
additional safety factor should be introduced for the general
population, which includes persons with different sensitivities to RF
exposure. A basic limit of 0.08 W/kg, corresponding to a further
safety factor of 5, is generally recommended for the public at large.
Derived limits of exposure are given in Tables 34 and 35 of this
publication.
The limitations for the whole body average SAR are not
sufficiently restrictive, since the distribution of the absorbed
energy in the human body can be very inhomogeneous and dependent on
the RF exposure conditions. In partial body exposure situations,
depending on frequency, the absorbed energy can be concentrated in a
limited amount of tissue, even though the whole body average SAR is
restricted to less than 0.4 W/kg. Therefore, additional basic limits
of 2 W/100 g are recommended in any other part of the body, in order
to avoid excessive local temperature elevations. The eye may need
special consideration.
At frequencies below about 1 MHz, exposure limits are selected
that will prevent stimulation of nerve and muscle cells. Basic
exposure limits refer to current densities induced within body
tissues. Exposure limits should have a sufficiently large safety
factor to restrict the current density to 10 mA/m2 at 300 Hz. This
is the same order of magnitude as natural body currents. Above 300 Hz,
the current density necessary for excitation of nervous tissue
increases with frequency, until a frequency is reached at which
thermal effects dominate. For frequencies around 2-3 MHz, the basic
limit for current density is equivalent to the limit for the peak SAR
of 1 W/100g. Since SAR or induced current density values cannot be
measured easily in practical exposure situations, exposure limits in
terms of conveniently measurable quantities must be derived from basic
limits. These "derived limits" indicate the acceptable limits in terms
of the measured and/or calculated field parameters that allow
compliance with the basic limits.
1.1.7.2 Occupational exposure limits
The occupationally-exposed populations consist of adults exposed
under controlled conditions, who are aware of the occupational risks.
Because of the wide frequency range addressed in this publication, a
single limit number for occupational exposure is not possible.
Recommended derived occupational limits in the frequency range 100 kHz
to 300 GHz are provided in Table 34. A conservative approach is
recommended for pulsed fields where electric and magnetic field
strengths are limited to 32 times the values given in Table 34, as
averaged over the pulse width, and the power density is limited to a
value of 1000 times the corresponding value in Table 34, as averaged
over the pulse width.
1.1.7.3 Exposure limits for the general population
The general population includes persons of different age groups,
different states of health, and pregnant women. The possibility that
the developing fetus could be particularly susceptible to exposure to
RF deserves special consideration.
Exposure limits for the general population should be lower than
those for occupational exposure. For example, recommended derived
limits in the frequency range of 100 kHz-300 GHz are provided in Table
35, which are generally a factor of 5 lower than the occupational
limits.
1.1.7.4 Implementation of standards
The implementation of RF field occupational and public health
protection standards necessitates the allocation of responsibility for
measurements of field intensity and interpretation of results, and the
establishment of detailed field protection safety codes and guides for
safe use, which indicate, where appropriate, ways and means of
reducing exposure.
1.1.8 Protective measures
Protective measures include workplace surveillance (exposure
surveys), engineering controls, administrative controls, personal
protection, and medical surveillance. Where surveys of RF fields
indicate levels of exposure in the workplace in excess of limits
recommended for the general population, workplace surveillance should
be conducted. Where surveys of RF fields in the workplace indicate
levels of exposure in excess of recommended limits, action should be
taken to protect workers. In the first instance, engineering controls
should be applied, where possible, to reduce emissions to acceptable
levels. Such controls include good safety design and, where necessary,
the use of interlocks or similar protection devices.
Administrative controls, such as limitation of access and the use
of audible and visible warnings, should be used in conjunction with
engineering controls. The use of personal protection (protective
clothing), though useful under certain circumstances, should be
regarded as a last resort to ensure the safety of the worker. Wherever
possible, priority should be given to engineering and administrative
controls. Where workers could be expected to incur exposures in excess
of the limits applicable to the general population, consideration
should be given to providing appropriate medical surveillance.
Prevention of health hazards related to RF fields also
necessitates the establishment and implementation of rules to ensure:
(a) the prevention of interference with safety and medical electronic
equipment and devices (including cardiac pacemakers); (b) the
prevention of detonation of electroexplosive devices (detonators); and
(c) the prevention of fires and explosions due to the ignition of
flammable material from sparks caused by induced fields.
1.2 Recommendations for further studies
1.2.1 Introduction
There are concerns about the possible effects of RF fields in the
areas of promotion and progression of cancer, of reproductive
failures, such as spontaneous abortions and congenital malformations,
and of effects on central nervous system function. Knowledge in all
these areas is inadequate to determine whether such effects exist, and
therefore, there is no rational basis for recommendations to protect
the general population from possible adverse effects.
Future research efforts in the areas of weak-interaction
mechanisms on the one hand, and studies of effects on carcinogenesis
and reproduction in animals and humans on the other hand, should be
coordinated to a high degree. This coordination can be brought about
by focusing funding on research proposals of a multidisciplinary and
multi-institutional nature. Studies on RF field effects could well be
coordinated with similar programmes addressing ELF (50/60 Hz) field
effects. A high priority should be placed on research that emphasizes
causal relationships and dose-effect thresholds and coefficients.
The following is a list of priority areas identified by the Task
Group as needing further study.
1.2.2 Pulsed fields
There is a major deficiency in the understanding of the effects
of pulsed fields in which very high peak power densities occur,
separated by periods of zero power. Only a few isolated reports of
pulsed field effects are available and it is not possible to identify
either the frequency or the peak power domain of importance. Data to
assess human health hazard in terms of pulse peak power, repetition
frequency, pulse length, and the frequency of the RF in the pulse, are
urgently needed in view of the widening application of systems
employing high power pulses, (mostly radar), and involving both
occupational and general population exposures.
1.2.3 Cancer, reproduction, and nervous system studies
There is increasing concern about the possibility that RF
exposure may play a role in the causation or promotion of cancer,
specifically of the blood forming organs or in the CNS. Similar
uncertainties surround possible effects on reproduction, such as
increased rates of spontaneous abortion and of congenital
malformations.
Effects of RF exposure on CNS function, with resulting changes in
cognitive function, are also surrounded by uncertainties. In view of
the potential importance of these interactions and the disruptive
effects of the uncertainty on society, a high priority should be
placed on research in this area. It is important that research efforts
be coordinated to clarify rather than increase the level of
uncertainty. Research on possible mechanisms, such as weak-field
interactions, should be closely coordinated with appropriately
designed animal toxicology studies and with human epidemiology.
1.2.4 Weak-field interactions
Very few people are exposed to thermally significant levels of
RF; the vast majority of exposures occur at levels at which weak-
field interactions would be the only possible source of any adverse
health response. A substantial amount of experimental evidence
implicates responses to amplitude-modulated RF fields, which show
frequency and amplitude windows; some responses are dependent on
co-exposure to physical and chemical agents. Establishing the
significance of effects for human health and their dose-response
relationships is of paramount importance. Studies are necessary that
identify biophysical mechanisms of interaction and that extend the
animal and human studies, in order to identify health risks.
1.2.5 Epidemiology
Epidemiological studies on the association between cancer and
adverse reproductive outcomes and RF fields are made difficult by a
number of factors:
- Most members of any population are exposed to levels of RF that
are orders of magnitude below thermally significant levels.
- It is very difficult to establish RF exposure in individuals over
a meaningful period of time.
- Control of major confounders is very difficult.
Some, but not all, of the sources of difficulties can be overcome by
a suitably designed and implemented case-control study. Such studies
are in progress and being planned to study childhood cancer and any
effects of ELF fields. It is important that such studies evaluate any
exposures to RF radiation.
2. PHYSICAL CHARACTERISTICS
2.1 Introduction
The study of the biological effects of electromagnetic fields is
multidisciplinary; it draws from physics, engineering, mathematics,
biology, chemistry, medicine, and environmental health. For this
reason, background information has been included in this publication
that may appear elementary to some readers, but is essential for those
from a different discipline. Much of the confusion and the
controversies that exist in the field today arise from individuals of
one discipline not fully appreciating the basic facts or theories of
another.
In this section, the aim is to summarize briefly the basic
physical characteristics of electric, magnetic, and electromagnetic
fields in the frequency range 300 Hz-300 GHz. The corresponding
wavelengths extend from 1000 km to 1 mm. At low frequencies (below
about 10 MHz) and for near-field conditions (see section 4), the
electric (E) and magnetic (H) fields must be treated separately.
The quantum energies at these frequencies are extremely small and
are not capable of altering the molecular structure or breaking any
molecular bonds. The maximum quantum energy (at 300 GHz) is 1.2
millielectronvolts (meV), while disruption of the weakest hydrogen
bond requires 80 meV; for comparison, the thermal motion energy at 30
°C is 26 meV.
Although there are other definitions of the radiofrequency (RF)
spectrum, its use in this document covers 300 Hz-300 GHz. The region
between 300 MHz and 300 GHz is called microwaves (MW).
2.2 Electric field
Electric charges exert forces on each other. It is convenient to
introduce the concept of an electric field to describe this
interaction. Thus, a system of electric charges produces an electric
field at all points in space and any other charge placed in the field
will experience a force because of its presence. The electric field is
denoted by E and is a vector quantity, which means that it has both
a magnitude and a direction. The force, F, exerted on a point
(infinitely small) body containing a net positive charge q placed in
an electric field E is given by:
F = qE (Equation 2.1)
Various units of the electric field strength are in use; the SI unit
is newton per coulomb (N/C). It is frequently easier and more useful
to measure the electric potential, V, rather than the force and
charge. This is because the potential is much less dependent on the
physical geometry of a given system (e.g., location and sizes of
conductors).
The potential difference V between two points in an electric
field E is defined by V = W/q, where W is the work done by the field
in moving a charge q between the two points. The work done is W = Fd,
where d is the separation between the two points; or using equation
2-1, W = qEd. From V = W/q, it follows that:
E = V/d (Equation 2.2)
In practice, the unit of volt per metre (V/m) is used for the electric
field strength.
Electric fields exert forces on charged particles. In an
electrically conductive material, such as living tissue, these forces
will set charges into motion to cause an electric current to flow.
This current is frequently specified by the current density, J, the
magnitude of which is equal to the current flowing through a unit
surface perpendicular to its direction. The SI unit of current density
is ampere per square metre (A/m2). J is directly proportional to
E in a wide variety of materials. Thus:
J = deltaE (Equation 2.3)
where the constant of proportionality delta is called the electrical
conductivity of the medium. The unit of delta is siemens per metre
(S/m).
2.3 Magnetic field
The fundamental vector quantities describing a magnetic field are
the magnetic field strength H and the magnetic flux density B
(also called the magnetic induction).
Magnetic fields, like electric fields, are produced by electric
charges, but only when these charges are in motion. Magnetic fields
exert forces on other charges but, again, only on charges that are in
motion.
The magnitude of the force F acting on an electric charge q
moving with a velocity v in the direction perpendicular to a magnetic
field of flux density B is given by:
F = qvB (Equation 2.4)
where the direction of F is perpendicular to both those of v and
B. If, instead, the direction of v were parallel to B, then F
would be zero. This illustrates an important characteristic of a
magnetic field: it does no physical work, because the force, called
the Lorentz force, generated by its interaction with a moving charge
is always perpendicular to the direction of motion. The basic unit of
the magnetic flux density can be deduced from Equation 2.4 to be
newton second per coulomb metre [N s/C m]. According to the
International System of Units (SI), this unit is called the tesla (T).
In the literature, both mks and cgs units are also used to express
flux density values. The conversion between the gauss (G), the cgs
unit of flux density, and the tesla is 1 T = 104 G.
The magnetic field strength H is the force with which the field
acts on an element of current situated at a particular point. The
value of H is measured in ampere per metre (A/m).
The magnetic flux density B, rather than the magnetic field
strength, H (where B = µH), is used to describe the magnetic field
generated by currents that flow in conductors. The value of µ (the
magnetic permeability) is determined by the properties of the medium.
For most biological materials, the permeability µ is equal to
µ0, the value of permeability of free space (air) (1.257 × 10-6
H/m). Thus, for biological materials, the values of B and H are
related by the constant µ0.
2.4 Waves and radiation
Maxwell's equations form the theoretical foundation for all
classical electromagnetic field theory. These equations are very
powerful, but for complex systems, such as biological bodies, they are
difficult to solve.
One class of their solutions results in wave descriptions of the
electric and magnetic fields. When the source charges or currents
oscillate and the frequency of oscillation is high enough, the E and
H fields produced by these sources will radiate from them. A
convenient and commonly used description of this radiation is wave
propagation.
The basic ideas of wave propagation are illustrated in Fig. 1.
The distance from one ascending, or descending, node to the next is
defined as the wavelength, and is usually denoted by lamda.
The wavelength and the frequency (the number of waves that pass
a given point in unit time), denoted by f, are related and determine
the characteristics of electromagnetic radiation. Frequency is the
more fundamental quantity and for a given frequency, the wavelength
depends on the velocity of propagation and, therefore, on the
properties of the medium through which the radiation passes.
The wavelength normally quoted is that in a vacuum or in air, the
difference being insignificant. However, the wavelength can change
significantly when the wave passes through other media. The linking
parameter with frequency is the speed of light as expressed in
Equation 2.5 (v = 3 × 108 m/s in air):
lamda = v/f (Equation 2.5)
When RF traverses biological material, its speed is reduced and its
wavelength becomes shorter than in air.
Two idealizations of wave propagation are commonly used:
spherical waves and plane waves (Stuchly, 1983; Grandolfo & Vecchia,
1988). A spherical wave is a good approximation to some
electromagnetic waves that occur. Their wavefronts have spherical
surfaces and each crest and trough has a spherical surface. On every
spherical surface, the E and H fields are constant. The wavefronts
propagate radially outwards from the source and E and H are both
tangential to the spherical surfaces.
A plane wave is another model that approximately represents some
electromagnetic waves. Plane waves have characteristics similar to
spherical waves because, at points far from the source, the curvature
of the spherical wavefronts is so small that they appear to be almost
planar.
The defining characteristics of a plane wave are:
(a) E, H, and the direction of propagation are all mutually
perpendicular.
(b) The quotient E/H is constant and is called the wave
impedance. For free space E/H = 377 OMEGA. For other media and
for sinusoidal steady-state fields, the wave impedance includes
losses in the medium in which the wave is travelling.
(c) Both E and H vary as 1/r, where r is the distance from the
source.
In RF plane wave propagation (far-field), the power crossing a
unit area normal to the direction of wave propagation is usually
designated by the symbol S. When the electric and magnetic field
strengths are expressed in V/m and A/m, respectively, S represents
their product, which yields VA/m2, i.e., W/m2 (watts per square
metre).
In free space, electromagnetic waves spread uniformly in all
directions from a theoretical point (isotropic) source. As the
distance from the point source increases, the area of the wavefront
surface increases as a square of the distance, so that the source
power is spread over a larger area.
As power density S corresponds also to the quotient of the total
radiated power and the spherical surface area enclosing the source, it
is inversely proportional to the square of the distance from the
source, and can be expressed as:
S = P/4pi r2 (Equation 2.6)
where P is the total radiated power and r is the distance from the
source.
In the case of plane waves, frequently called far-field
conditions, the power density can be derived from E2/377 or from 377
H (see Table 1). Therefore, in many practical applications only the
E field or the H field needs to be measured when the point of
measurement is at least one wavelength from the source. In this case,
measurement of E makes possible the determination of H and vice-versa.
Table 1. Comparison of power densities in the more commonly used
units for free-space, far-field conditions (Note: values have been
rounded to one or two significant figures, based on the relationships
above)
W/m2 mW/cm2 µW/cm2 V/m A/m
10-2 10-3 1 2 5 10-3
10-1 10-2 10 6 1.5 10-2
1 10-1 102 20 5 10-2
10 1 103 60 1.5 10-1
102 10 104 2 102 5 10-1
103 102 105 6 102 1.5
104 103 106 2 103 5
The region close to a source is called the near-field. In the
near-field, the E and H fields are not necessarily perpendicular;
in fact, they are not always conveniently characterized by waves. They
are often nonpropagating in nature and are sometimes referred to as
fringing fields, reactive near-fields, or evanescent modes.
Near-fields often vary rapidly with distance; the inverse square law
of the dependence with distance does not apply, and the impedance
(E/H) may differ from 377 OMEGA. Objects located near sources may
strongly affect the nature of the fields. For example, placing a probe
near a source to measure the fields may change the characteristics of
the fields considerably (Dumansky et al., 1986).
When RF fields are incident on a conductive object, RF currents
are induced in the object. These currents produce surface fields that
are highly localized to the object and are often referred to as RF hot
spots. RF hot spots are better characterized as electric and magnetic
fields rather than radiation, since, for many conditions, the fields
leading to the hot spot never propagate away from the object. At
higher frequencies, the electric and magnetic fields maintain an
approximately constant relationship in propagating waves. In general,
the lower the frequency, the less coupled the fields become. This is
particularly so when the wavelength is very large with respect to the
physical size of the source. In practice, the fields of concern from
a hazard perspective will be near-fields at frequencies below about 1
MHz.
3. NATURAL BACKGROUND AND HUMAN-MADE FIELDS
3.1 General
In the last few decades, the use of devices that emit
electromagnetic fields has increased considerably. This proliferation
has been accompanied by an increased concern about possible health
effects of exposure to these fields (Grandolfo et al., 1983;
Repacholi, 1988; Shandala & Zvinyatskovski, 1988, Franceschetti et
al., 1989). As a result, throughout the world, many organizations,
both governmental and nongovernmental, have established safety
standards or guidelines for exposure (see section 10).
Electromagnetic devices already in use and the continuous
addition of new sources result in the expansion to new frequencies in
the spectrum and the increasing presence of RF fields. Comprehensive
data on existing emission systems, and evaluation of present levels of
exposure, are essential for the assessment of potential radiation
hazards (Repacholi, 1983a; Shandala et al., 1983; Savin, 1986; Stuchly
& Mild, 1987).
In this section, sources of electromagnetic fields, both natural
and human-made, in the 300 Hz-300 GHz frequency range are surveyed.
The human-made electromagnetic environment consists of fields that are
produced either intentionally or as by-products of the use of other
devices.
Human-made sources in the spectrum considered here, however,
produce local field levels many orders of magnitude above the natural
background. Therefore, for the practical purposes of hazard
assessment, the electromagnetic fields on the earth's surface arise
from human-made sources. According to the treaty of the International
Telecommunications Union (ITU, 1981), the electromagnetic spectrum up
to 3 THz is subdivided into 12 frequency bands. These bands are
designated by numbers as shown in Table 2; only the bands referred to
in this publication are given.
3.2 Natural background
The natural electromagnetic environment originates from processes
such as discharges in the earth's atmosphere (terrestrial sources) or
in the sun and deep space (extra-terrestrial sources).
Table 2. Frequency bands of the electromagnetic spectrum in the
frequency range 300 Hz-300 GHz a
Band Frequency range Metric Description and symbol
number subdivision
3 0.3 to 3 kHz - voice frequency [VF]
4 3 to 30 kHz myriametric very low frequency [VLF]
Table 2 (continued)
Band Frequency range Metric Description and symbol
number subdivision
5 30 to 300 kHz kilometric low frequency [LF]
6 0.3 to 3 MHz hectometric medium frequency [MF]
7 3 to 30 MHz decametric high frequency [HF]
8 30 to 300 MHz metric very high frequency [VHF]
9 0.3 to 3 GHz decimetric ultra high frequency [UHF]
10 3 to 30 GHz centimetric super high frequency [SHF]
11 30 to 300 GHz millimetric extremely high frequency [EHF]
a From: ITU (1981).
3.2.1 Atmospheric fields
Atmospheric fields of frequencies of less than 30 MHz originate
predominantly from thunderstorms. Their strengths and range of
frequencies vary widely with geographical location, time of day, and
season. Some of these variations are systematic and some are random.
Overall, atmospheric fields have an emission spectrum with the largest
amplitude components having frequencies of between 2 and 30 kHz.
Generally, the atmospheric field level decreases with increasing
frequency. The geographical dependence is such that the highest levels
are observed in equatorial areas and the lowest in polar areas.
3.2.2 Terrestrial emissions
The earth emits electromagnetic radiation (black-body radiation),
as do all media, at a temperature T that is different from that at
absolute zero. In the RF range, the black-body radiation follows the
Rayleigh-Jeans law and the thermal noise from the earth (T about 300
K) is 0.003 W/m2 (0.3 µW/cm2), when integrated up to 300 GHz
(Repacholi 1983).
The human body also emits electromagnetic fields at frequencies of up
to 300 GHz at a power density of approximately 0.003 W/m2. For a
total body surface area of about 1.8 m2, the total radiated power is
approximately 0.0054 W.
3.2.3 Extraterrestrial fields
The atmosphere, ionosphere, and magnetosphere of the earth shield
it from extra-terrestrial sources of nonionizing electromagnetic
energy. Electromagnetic waves that are able to penetrate this shield
are limited to two frequency windows, one optical and the other
encompassing radiowaves of frequencies from about 10 MHz to 37.5 GHz.
The short-wave boundary of the RF-window is due to energy absorption
by molecules contained in the atmosphere (primarily O2 and H2O),
whereas the long-wave boundary is related to the shielding action of
the ionosphere.
RF radiation of cosmic origin observed with earth satellites
ranges in magnitude from 1.8 × 10-20 W/m2/Hz at 200 kHz to 8 ×
10-20 W/m2/Hz at 10 MHz (Struzak, 1982).
There are three main types of solar emission. The first is the
so-called background, which is the constant component of the emission
observed during periods of low solar activity. The second is the
component that displays long-term changes, associated with variations
in the number of sunspots. Its main contribution is in the frequency
range from 500 MHz to 10 GHz. The third type of emission arises from
isolated radio flares or radio emission bursts. The intensity of such
emission can exceed the average intensity of the quiet radiation by a
factor of one thousand or more; its duration varies from seconds to
hours.
Natural sources of lesser intensity also exist and include the
moon, Jupiter, Cassiopeia-A, the universal thermal background
radiation at 3 K, hydrogen emissions from ionized clouds, line
emissions from neutral hydrogen, the OH radical and, most recently
observed, from ammonia.
3.3 Human-made sources
3.3.1 General
Radio and television transmitters are examples of human-made RF
sources that intentionally produce electromagnetic emissions for
telecommunication purposes. At frequencies of 3 kHz-3 MHz, normal
service coverage is provided by ground-wave propagation. At VLF,
propagation over distances of thousands of km is possible using this
method. At LF and MF, during night-time, reflections from the
ionosphere make propagation up to 2000 km possible with little
attenuation. At HF, other propagation modes are also possible. At
frequencies of 30 MHz-30 GHz, service coverage is provided by
line-of-sight (short paths), diffraction (intermediate paths), or by
forward scattering (long paths) propagation.
Broadcasting systems vary greatly in terms of their design. This
diversity results in somewhat different approaches in evaluating human
exposure and potential problems. The situations are significantly
different for workers and for the general population. In the case of
some workers, such as those maintaining equipment on broadcasting
towers, there is a potential for exposure to strong RF fields. Workers
may also be exposed to strong fields in the close vicinity of towers
and particularly broadcasting antennas in the VLF, LF, and MF. In
contrast, it is rare for the general population to be exposed to
strong RF fields from broadcasting. However, there is simultaneous
exposure to more than one source.
Some insight on the levels of exposure of the general population
may be gained from data collected in the USA, indicating that, in
large cities, the median exposure level is about 50 µW/m2 (Tell &
Mantiply, 1980). SAR values ranging from 0.05 to 0.3 µW/kg are
expected in the frequency range 170-800 MHz.
There are also human-made sources of electromagnetic fields used
for non-communication purposes, in industry (I), science (S), and
medicine (M). ISM applications are intended to transport and
concentrate electromagnetic energy in a restricted working area to
produce physical, chemical, and/or biological effects.
The frequency bands for ISM applications designated by the ITU
are shown in Table 3. However, in individual countries, different
and/or additional frequencies may be designated for use by ISM
equipment (ITU, 1981; Metaxas & Meredith, 1983).
Table 3. Centre frequencies and frequency bands agreed
internationally and assigned for ISM applications a
Centre frequency Frequency bands Area permitted
70 kHz 60-80 kHz USSR
6.780 MHz 6.765-6.795 MHz subject to agreement
13.560 MHz 13.553-13.567 MHz worldwide
27.120 MHz 26.957-27.283 MHz worldwide
40.68 MHz 40.66-40.70 MHz worldwide
42;49;56;61;66 MHz approx. 0.2% United Kingdom
84;168 MHz approx. 0.005% United Kingdom, Austria,
433.92 MHz 433.05-434.79 MHz Liechtenstein,
The Netherlands, Portugal,
Switzerland W. Germany
Yugoslavia
896 MHz 886-906 MHz United Kingdom
915 MHz 902-928 MHz North and South
America
2.375 GHz 2.325-2.425 GHz Albania, Bulgaria,
Czechoslovakia,
Hungary, Romania,
and USSR
Table 3 (continued)
Centre frequency Frequency bands Area permitted
2.45 GHz 2.4-2.5 GHz worldwide, except
where 2.375 GHz is
used
3.39 GHz 3.37-3.41 GHz The Netherlands
5.8 GHz 5.724-5.875 GHz worldwide
6.78 GHz 6.74-6.82 GHz The Netherlands
24.125 GHz 24.0-24.05 GHz worldwide
40.68 GHz 40.43-40.92 GHz United Kingdom
61.25 GHz 61.0-61.5 GHz subject to agreement
122.5 GHz 122-123 GHz subject to agreement
245 GHz 244-246 GHz subject to agreement
a Adapted from: ITU (1981) and Metaxus & Meredith (1983).
Because of unavoidable imperfections in the construction,
production, and use of ISM equipment, and of fundamental physical
laws, there is always unintentional leakage of electromagnetic energy
from such equipment. As a result, each ISM generator acts as an
unintentional source producing signals capable of causing harmful
effects, depending upon the amount of leakage.
To date, the total number of ISM installations in the world is
estimated at 120 million (Struzak, 1985). The number of ISM generators
constantly increases at a rate of about 3-7% per year. With such
growth, the number of ISM generators expected by the year 2000 will be
2-4 times greater than it is now.
ISM equipment is usually designed at minimum cost, and,
typically, is reduced to the essentials necessary for operation.
Frequency stability and spectral purity of the power delivered to the
work piece are not normally major objectives. In almost every case,
the work piece is strongly coupled to the oscillator/amplifier, and
since the electromagnetic characteristics of the material change
during the work cycle, the magnitude, phase, and frequency of the
radiation may be affected by these changes.
Electromagnetic energy leaks from ISM equipment mainly from the
applicator and associated leads (e.g., RF heaters and sealers), the
oscillator body/cabinet, and also from surrounding structures in which
RF currents are induced. The amount of energy radiated from the
applicator and associated leads depends on the particular arrangement
of the devices and the work piece, which together act like an antenna
the radiation efficiency of which is usually very low. However, the
radiated power may be considerable if the nominal power is high.
Stray fields are also associated with currents flowing over the
surface of the body/cabinet and over the surrounding structures. The
equipment acts as a complex antenna system consisting of coupled
radiating surface elements resonating at some unspecified frequencies.
Often all the power and control wires are situated close to RF power
circuits with no shielding. As a result, a considerable amount of RF
energy may be fed into these leads and is conducted outwards at a
distance and then reradiated.
Table 4. Typical applications of equipment generating electromagnetic
fields in the range 300 Hz-300 GHz
Frequency Wavelength Typical applications
0.3-3 kHz 1000-100 km Broadcast modulation, medical applications,
electric furnaces, induction heating,
hardening, soldering, melting, refining
3-30 kHz 100-10 km Very long range communications, radio
navigation, broadcast modulation, medical
applications, induction heating, hardening,
soldering, melting, refining, VDUs
30-300 kHz 10-1 km Radionavigation, marine and aeronautical
communications, long-range communications,
radiolocation, VDUs, electro-erosion
treatment, induction heating and melting of
metals, power inverters
0.3-3 MHz 1 km-100 m Communications, radionavigation, marine
radiophone, amateur radio, industrial RF
equipment, AM broadcasting, RF excited arc
welders, sealing for packaging, production
of semiconductor material, medical
applications
3-30 MHz 100-10 m Citizen band, amateur radio broadcasting,
international communications, medical
diathermy, magnetic resonance imaging,
dielectric heating, wood drying and gluing,
plasma heating
30-300 MHz 10-1 m Police, fire, amateur FM, VHF-TV, diathermy,
emergency medical radio, air traffic
control, magnetic resonance imaging,
dielectric heating, plastic welding, food
processing, plasma heating, particle
separation
Table 4 (continued)
Frequency Wavelength Typical applications
0.3-3 GHz 100-10 cm Microwave point to point, amateur, taxi,
police, fire, radar, citizen band,
radionavigation, UHF-TV, microwave ovens,
medical diathermy, food processing, material
manufacture, insecticide, plasma heating,
particle acceleration
3-30 GHz 10-1 cm Radar, satellite communications, amateur,
fire, taxi, airborne weather radar, police,
microwave relay, anti-intruder alarms,
plasma heating, thermonuclear fusion
experiments
30-300 GHz 10-1 mm Radar, satellite communications, microwave
relay, radionavigation, amateur radio
Typical uses of equipment generating electromagnetic fields in
the frequency range 300 Hz-300 GHz are shown in Table 4.
3.3.2 Environment, home, and public premises
A comprehensive evaluation of general population exposure to RF
has been performed by the USA Environmental Protection Agency (Tell&
Mantiply, 1980). Broadcasting services, particularly those usingthe
VHF and UHF bands, have been identified as the main sources of ambient
RF fields (Karachev & Bitkin, 1985). Measurements performed in 15
large cities in the USA led to the conclusions that the median
exposure level was 50 µW/m2 and that approximately 1% of the
population studied was potentially exposed to levels greater than 10
mW/m2.
The presence of conducting objects can give rise to field
strengths higher than those expected from theoretical considerations,
since they act as diffracting elements for the electromagnetic fields.
Consequently, the presence of such objects in the near-field zone of
radio stations makes the area between the radiator and the object
potentially more hazardous and indicates that problems of safety
should be considered carefully (Bernardi et al., 1981).
Although measurements as well as theory indicate that there is no
high-level exposure from broadcasting stations, the existence of
limited areas of relatively high irradiation close to the sources
should be checked (Dumansky et al., 1985a). Such situations can exist
in proximity to very powerful, ground-level transmitters. In several
cases, urban areas are served locally by low-power, in-town repeaters.
These are placed, for convenience, on the top of tall buildings;
unless properly designed, this creates the possibility of stray fields
in a densely populated area directly below the RF source. A typical,
high-power, MF transmitter can have a carrier power of 100 kW, plus up
to 50 kW in the sidebands of the propagated field. This is an example
of how high field strengths can occur in a space open to the public.
Although a broadcasting station's property is usually fenced to
keep out unauthorized individuals, the fence may be close to the tower
base and people may be able to get as close as a few tens of metres or
less from the antenna. Because the wavelengths involved are so long,
a near-field exposure situation may exist and a field strength
considerably greater than the theoretical ground-wave field strength
is to be expected (Bini et al., 1980).
Local MF transmitters find widespread use in cities, where they
provide coverage on "blind spots" or other low-signal receiving areas.
Powers range from 100 to 1000 W at the amplifier output and much less
than that can be expected to be radiated into space. In a typical
arrangement, the transmitting module is located at the top of a
stucture. The radiating system is fed via a coaxial cable. It consists
of a dipole over a ground plane or counterpoise laid directly on the
roof. More than one transmitter can serve the same radiator. Currents
can set up fields in a complicated pattern inside the building (Bini
et al., 1980).
When RF fields are incident on conductive objects, RF currents
are induced in the objects. Because of these currents, the objects
become sources of additional fields that are highly localized and in
some situations can constructively add to original fields.
Among the general population, the most popular application of
microwave power is in the cooking of food. Power levels range from 300
W to 1 kW in consumer microwave models, at a frequency of 2.45 GHz. In
a properly designed microwave oven, a very small fraction of this
power escapes from the oven housing through various leakage paths.
When leakage occurs, it is most frequently through the door seal. It
may increase with use or mechanical abuse of the oven. Small amounts
of leakage can also occur through the viewing screen (Osepchuk, 1979).
Personal exposure from microwave ovens is extremely small because
of the rapid decrease of the power flux density with increasing
distance from the oven. For worst case leakage from the microwave oven
of 5 mW/cm2, the power density at a distance of 0.3 m is less than
0.15 mW/cm2 and, at 1 m it is about 10 µW/cm2. Typical leakage
values, therefore, imply exposure values well below the most
conservative RF exposure standards in the world (Stuchly, 1977;
Dumansky et al., 1980).
Recently, the induction heating stove, a new appliance for
domestic use, has been introduced on the market. This appliance
operates in the kilohertz range. Exposure levels at distances greater
than 0.5 m are low compared with existing exposure limits, being less
than 5 V/m and 0.5-10 A/m, respectively, at a distance of 0.3 m
(Stuchly & Lecuyer, 1987).
Microwave anti-intrusion alarms are typical of low-power devices.
These operate continuously to avoid thermal drift or switching
problems, thus exposing people in the protected area. With a typical
power of 10 mW, power densities of the order of 10 µW/cm2 are
measured at a distance of about 0.5 m. Population exposure to RF
fields from commonly encountered sources, such as airport, marine, and
police radar, is similarly very low (Stuchly, 1977; Dumansky et al.,
1980, 1985b, 1988).
3.3.3 Workplace
Levels of occupational exposure vary considerably, and are
strongly dependent upon the particular application. While most
communication and radar workers are exposed to fields of only
relatively low intensity, some can be exposed to high levels of RF.
Workers climbing FM radio or TV broadcasting towers may be exposed to
E fields up to 1 kV/m and H fields up to 5 A/m (Repacholi 1983a; Mild
& Lovstrand, 1990).
Radar systems produce strong RF fields along the axis of the
antenna. However, in most systems, average field strengths are reduced
typically by a factor of 100-1000, because of antenna rotation and
because the field is pulsed. With stationary antennas, which represent
the worst case, peak power flux densities of 10 MW/m2 may occur on
the antenna axis up to a few metres away from the source.
In areas surrounding air traffic control radars (ATCRs), workers
can be exposed to power flux densities of up to tens of W/m2, but
are normally exposed to fields in the range 0.03-0.8 W/m2. In an
exposure survey of civilian airport radar workers in Australia, it was
found that, unless working on open waveguide slots, or within
transmitter cabinets when high voltage arcing was occurring, personnel
were, in general, not exposed to levels of radiation exceeding the
specified limits in the Australian and IRPA radiofrequency exposure
standards (Joyner & Bangay, 1986a).
Dielectric or RF heaters are widespread in many industries. RF
energy produces heat directly within the processed material. This
unique characteristic is commonly used for such purposes as sealing
plastics or drying glue for joining wood. All RF heaters have a higher
efficiency in comparison with conventional ovens. According to several
surveys (Conover et al., 1980; Stuchly et al., 1980; Grandolfo et al.,
1982; Bini et al., 1986; Joyner & Bangay, 1986b; Stuchly & Mild,
1987), the sealing or welding of polyvinyl chloride (PVC) is the most
common use for RF dielectric heaters. Two pieces of plastic are
compressed between electrodes and RF power is applied. The plastic
material heats, partially fuses, and forms a bond. Plastic heaters
frequently operate at the ISM frequency of 27.12 MHz. However, during
the operating cycle, this value may vary by several megahertz. The RF
output power ranges from fractions of a kilowatt to about 100 kW.
Since the exposure of heater sealer operators and other personnel
working in the same area takes place in the near-field, both E and H
field strengths must be measured to evaluate exposure levels. However,
to demonstrate compliance with basic limits of RF exposure, the
development of body current measurement techniques should prove to be
useful (Allen et al., 1986). In the vicinity of RF sources,
measurements of fields must be made with the operators absent from the
positions that they normally occupy. The stray fields are localized in
the immediate vicinity of the sealers, so that exposure of the body is
highly inhomogeneous.
RF industrial heaters (plastic sealers and other devices) have
been found to produce exposure fields exceeding the limits recommended
in various countries and by the IRPA. Furthermore, direct current
measurements have confirmed coupling of the RF energy from the device
to its operator. Various methods have been developed to ameliorate the
situation, such as shields, grounding strips, and others. Potential
overexposure to RF radiation is probably one of the most common
occurrences in the case of RF heaters, unless protective measures are
employed.
Magnetic fields below a few tens of megahertz are used in
industry for the induction heating of metals and semiconductors and in
arc welding. Surveys of the magnetic field strength to which the
operators are exposed have shown that these exposures are, in many
instances, high compared with recommended exposure limits (Stuchly &
Lecuyer, 1985; Conover et al., 1986; Stuchly, 1986; Andreuccetti et
al., 1988; Stuchly & Lecuyer, 1989).
In many practical situations, exposure can be reduced either by
administrative measures (Eriksson & Mild, 1985) or by the use of
protective screening. Screening may be intentional (wire fences) or
incidental (walls of buildings) and may function by reflection or by
absorption. In general, both contribute to the total attenuation
provided.
Thin metal sheets are adequate for the attenuation of RF electric
fields. However, in many cases, it is usual to use wire screens or
perforated sheets, since these have the advantages of transparency,
ventilation, light weight, etc. In all cases, surveys are desirable to
verify the integrity of such screens or shields. Faults in screens
could, in some circumstances, be secondary sources of significant
radiation or reactive fields (White, 1980).
The applications of video display units (VDUs) are numerous and
their use widespread. Even more applications are anticipated in the
future. In the RF region, they emit electric and magnetic fields from
the cathode ray tubes (CRTs). The dominant sources are the horizontal
and vertical sweep generators (fly-back transformers) operating at
frequencies of some 15-35 kHz. VDUs produce fields that have complex
waveforms. Typical electric field stengths at the operator position
(0.5 m from the screen) range from less than 1 to 10 V/m (RMS).
Magnetic flux densities range typically from less than 0.01 µT to 0.1
µT (RMS). In most VDUs, both fields are produced at the lower end of
these ranges. VDUs also produce weak, electric and magnetic fields at
the power line frequency (50 or 60 Hz) and its harmonics. All surveys
have concluded that VDUs do not present any hazard for human health
within the context of existing guidelines for exposures to
non-ionizing electromagnetic fields (see section 10) (BRH, 1981;
Stuchly et al., 1983b; Harvey, 1984; Repacholi, 1985; Elliott et al.,
1986).
A statement has been published by the International Non-Ionizing
Radiation Committee of the International Radiation Protection
Association (IRPA, 1988b). Conclusions in this and other documents
(WHO, 1987; ILO, 1991) are that, on the basis of current biomedical
knowledge, there are no health hazards associated with radiation or
fields from VDUs and that there is no scientific basis to justify
radiation shielding or regular monitoring of the various radiations
and fields emitted by VDUs.
3.3.4 Medical practice
Shortwave and microwave diathermy treatments are used to relieve
pain through the non-invasive application of non-ionizing
electromagnetic energy to body tissues. Several surveys have been
published (EHD, 1980; Ruggera, 1980; Grandolfo et al., 1982; Stuchly
et al., 1982; Delpizzo & Joyner, 1987), with the primary purpose of
measuring the field strengths to which diathermy operators are exposed
during typical treatments. Measurements of the magnitude of fields
near diathermy electrodes (applicators) were made from shortwave
diathermy units operating at 27.12 MHz, and from microwave diathermy
units operating at 434 MHz and 2.45 GHz. They indicate emissions of
high field and radiation levels in directions other than those
intended for treatment. Operators, physiotherapists, and personnel
performing service and maintenance tasks are exposed to stray fields
and radiations. Reduction of unnecessary exposure of both operator and
patient during microwave and shortwave diathermy treatments is
technically achievable through adequate shielding of existing units,
careful design of new equipment, and judicious planning of the
treatment area (Bonkowski & Makiewicz, 1986).
Hyperthermia devices are used in cancer adjuvant therapies (Storm
et al, 1981; Stuchly et al., 1983a; Hagmann et al., 1985). Treatments
have been based on biological studies that suggest hyperthermia
effectiveness in conjunction with radiotherapy and with chemotherapy.
The evaluation of hyperthermia efficacy is proceeding through the
development of therapeutic trials for specific tumours (Arcangeli et
al., 1985; Perez et al., 1991). A few international and national
organizations have independently determined and developed randomized
trials (Lovisolo et al., 1988). For the purposes of safety evaluation,
hyperthemia devices can be classified as: (a) those irradiating
external to the body and intended for superficial and deep
hyperthermia, and (b) those irradiating from inside the body and
intended for interstitial and endocavitary hyperthermia.
All devices present, to a greater or lesser extent, problems of
patient health protection. Adverse effects on patients have included
pain, discomfort, burn, ulceration, and, for deep hyperthemia,
tachycardia and faintness. These are due to an overheating of
superficial tissues, tissues surrounding the tumour, and, in deep
hyperthermia, other tissues far from the tumour and irradiated region
(Myerson et al., 1989; Petrovich et al., 1989). The magnitude of the
electromagnetic field around superficial, interstitial, and
endocavitary applicators is relatively low and does not cause any
health risk to the operators, though the possibility of leakage of RF
energy from generators and connecting cables has to be considered in
some models. Capacitive and phase array devices, however, may leak RF
energy (Storm et al., 1981).
Hagmann et al. (1985) measured the stray electric and magnetic
fields of angular phased array and helical coil applicators for limb
and torso hyperthermia at 70.93 MHz. Field strengths were measured in
excess of 300 V/m and 1A/m, respectively, at a distance of about 10 cm
from the applicator. At 0.5 m, these values were reduced to 14 V/m and
0.1 A/m, respectively. In general, manufacturers of hyperthermia
devices pay too little attention to minimizing the leakage of RF
fields from generators, cables, and applicators, and each new model
generator should be tested for RF emissions.
Magnetic resonance imaging (MRI) is now an established diagnostic
technique while in vivo spectroscopy is undergoing rapid
development. The complexity of exposure associated with MRI requires
the safety consideration of three different fields (Tenforde &
Budinger, 1986; Budinger, 1988). During clinical imaging, patients or
volunteers, and operators are exposed to static magnetic fields,
time-varying magnetic fields, and radiofrequency electromagnetic
fields. RF fields in the frequency range 1-100 MHz, are deposited in
patients, principally as heating associated with eddy currents induced
by the RF magnetic field (Grandolfo et al, 1990). For MRI systems with
static magnetic flux densities below 2 T, power deposition from
electric fields associated with RF transmitter coils is relatively
low, when efficient transmitter coil designs are employed. The power
deposited by transient magnetic field gradients is similarly low
(Bottomley et al., 1985). Staff operating the equipment are
intermittently exposed to weaker fields that are present in the
vicinity of the imaging equipment. Guidelines on "Protection of the
patient undergoing magnetic resonance examinations" have been
published by the International Non-Ionizing Radiation Committee of the
International Radiation Protection Association (IRPA, 1991).
4. EXPOSURE EVALUATION - CALCULATION AND MEASUREMENT
4.1 Introduction
Exposure evaluation provides information necessary to perform
risk assessments. Two methods are available: (i) a theoretical
estimation; and (ii) measurements of the fields or related parameters,
such as energy absorption rates and currents.
Estimates of exposures are necessary before an installation is
constructed. Whenever possible, estimates of radiation fields should
be made before detailed surveys of potentially hazardous exposures are
carried out. This procedure is needed to select suitable survey
instruments, and to determine whether potentially hazardous exposure
of the surveyor could occur, if the choice of the instrument were
inappropriate or if the instrument were faulty.
4.2 Theoretical estimation
Electromagnetic waves may be harmonic, i.e., the electric and
magnetic fields oscillate as sine waves, and power is generated as a
continuous wave (CW) at essentially a single frequency. The waves may
be also modulated, i.e., the amplitude, phase, or frequency may be
changed in a chosen manner, for example, if pulse modulation,
short-duration electromagnetic pulses are emitted at certain time
intervals. The duration of the pulse (pulse length or pulse width),
which may be of the order of small fractions of a second, is
designated by t. Its reciprocal, the pulse repetition frequency (pulse
repetition rate), is expressed in hertz. The product of pulse length
and repetition rate is referred to as the duty cycle, D. In case of
pulsed-wave generation, the emitted power increases rapidly, reaches
a peak pulse power, and rapidly decreases.
This may be averaged for pulse length or per unit time, which
introduces the concept of mean (average) power emitted, according to:
Pp = Pa/tfr or Pa = Ppfr t (Equation 4.1)
where Pp is the peak power, Pa the average power, fr the
repetition frequency, and t the pulse length.
In practice, average power is usually measured, and, for safety
purposes, mean power density is used. The peak pulse power may be many
times higher than the average power output. The average and peak power
flux densities (Sa and Sp) are given by:
Sa = DSp (Equation 4.2)
Universally used sources with moveable antennas and/or beams, such as
scanning or rotating radars, introduce an additional complication from
the safety viewpoint. Electromagnetic power from such installations
arrives intermittently.
The power flux density for a scanning antenna in motion can be
estimated from the power flux density measured with the antenna
stationary using the expression:
Wm = ksWs (Equation 4.3)
where Wm is the power flux density for the antenna in motion, ks
is the antenna rotational reduction factor, and Ws is the power flux
density measured on the axis of the stationary antenna at a given
distance.
In most radar installations, the antenna rotates and therefore an
occupied position is exposed only when the radar beam sweeps it. The
average exposure level is obtained by multiplying the measured or
estimated level from a stationary antenna by the rotational reduction
factor (RRF). In the far-field, RRF equals the ratio of the half power
beam width to the antenna scan angle.
The rotational reduction factor (ks) for the near-field region
is equal to:
a/Rk (Equation 4.4)
where "a" is the dimension of the antenna in the scan (rotation) plane
and Rk is the circumference of the antenna scan sector at the given
distance r, at which the measurements have been made.
The region close to a source antenna is called the near-field. As
shown in Fig.2, the near-field can be divided into two subregions: the
reactive near-field region and the radiating near-field region. The
region of space surrounding the antenna in which the reactive
components predominate is known as the reactive near-field region. In
the radiating near-field region, the radiation pattern varies with the
distance from the antenna. The near-fields often vary rapidly with
distance and mathematical expressions generally contain the terms 1/r,
1/r2, ...., 1/rn, where r is the distance from the source to the
point at which the field is being determined. At greater distances
from the source, the 1/r2, 1/r3, and higher-order terms are
negligible compared with the 1/r term and the fields are called
far-fields. These fields are approximately spherical waves that can,
in turn, be approximated in a limited region of space by plane waves.
Measurements and calculations are usually easier in far-fields than in
near-fields.
When the longest dimension (L) of the source antenna is greater
than the wavelength (lamda), the distance from the source to the
far-field is 2L2/lamda. For L<lamda, this distance is lamda/2pi
(see Fig. 2). In practice, the distance from the source that
represents the boundary between the near-field and far-field regions
is often taken to be the greater of the two quantities, lamda and
2L2/lamda. However, the appropriate empirical relationship depends
on the type of aperture of the source and, for example, for a circular
aperture, such as on a microwave relay tower, the relationship
L2/lamda may be more appropriate. In this case, with a frequency of
2 GHz (lamda = 15 cm), L is approximately 3 m and, consequently, the
quantity L2/lamda = 60 m. Because 60 m is much greater than 15 cm,
this is the distance that can be assumed as a boundary between the
near- and far-field regions.
The boundary between the near-field and far-field regions,
however, is not sharp, because the near-fields gradually become less
as the distance from the source increases.
In free space, electromagnetic waves spread uniformly in all
directions from a theoretical point source. In this case, the
wavefront is spherical. As the distance from the point source
increases, the area of the wavefront surface increases as a square of
the distance, so that the source power is spread over a larger area.
If the exposure takes place in the far-field of a well
characterized antenna in free space, then S is calculated by the
formula:
S = GPt /4pi r2 (W/m2) (Equation 4.5)
where G is the far-field power gain, Pt is the power transmitted (W)
and r is the distance (m) from the antenna.
For a horn or reflector type antenna:
G = 4pi Ae/lamda2 (Equation 4.6)
where Ae is the effective area of the antenna.
If G is not known, a useful approximation of S can be obtained by
substituting the physical area A for Ae in equation 4.6. This gives
a somewhat larger value for S, since A is generally larger than Ae.
Although the equations are far-field relationships, i.e., correct
for distances greater than approximately 2L2/lamda (L > lamda),
they can be used with an acceptable error for distances greater than
0.5 L2/lamda. The error is on the safe side, since the equations
predict greater values of S. However, at distances closer than 0.5
L2/lamda, the values of S predicted by the equations become
unrealistically large and radiating near-field estimates must be used.
For commonly encountered horn and reflector type antennas, the maximum
expected radiating, near-field, power flux density Sm can be
estimated (Hankin, 1974) from:
Sm = 4Pt/A (Equation 4.7)
Unfortunately, there are no equivalent reactive near-field
formulae for small radiators. The radiating near-field behaviour of
horn and reflector type antennas is discussed in detail elsewhere
(Bickmore & Hansen,1959; SAA, 1988). A detailed discussion of the
reactive near-field of small radiators can be found in Jordan &
Balmain (1968).
In the near-field, the situation is somewhat complicated, because
the maxima and minima of E and H fields do not occur at the same
points along the direction of propagation as they do in the the case
of the far-field. In this region, the electromagnetic field structure
may be highly inhomogeneous and typically, there may be substantial
variations from the plane wave impedance of 377 OMEGA; i.e., in some
regions, almost pure E-fields may exist and, in other regions, almost
pure H-fields. Field strengths in the near-field are more difficult to
specify, because both the E and H fields must be measured and because
the field patterns are more complicated; the power density tends to
vary inversely with r instead of r2 (as in the far-field), and may
display interference patterns. Near-field exposures become
particularly important when considering fields from microwave
diathermy equipment, RF sealers, broadcasting antennas, and microwave
oscillators under test.
4.3 Measurements
4.3.1 Preliminary considerations
Several steps are necessary for the accurate assessment of RF
exposure. The source and exposure situation must be characterized, so
that the most appropriate measurement technique and instrumentation
can be selected (ANSI, 1990; Tell, 1983). The correct use of this
instrumentation requires knowledge of the quantity to be measured and
the limitations of the instrument used. A knowledge of relevant
exposure standards is essential.
In the following sections, information is given concerning
preliminary RF survey considerations, measurement procedures, and
calibration facilities.
Prior to the commencement of a survey, it is important to obtain
as much information as possible about the characteristics of the RF
source and the exposure situation. This information is required for
the estimation of the expected field strengths and the selection of
the most appropriate survey instrumentation.
Information about the RF source should include:
- frequencies present, including harmonics;
- power transmitted;
- polarization (orientation of E field);
- modulation characteristics (peak and average values);
- duty cycle, pulse width, and pulse repetition frequency;
- antenna characteristics, such as type, gain, beam width and scan
rate.
Information about the exposure situation must include:
- distance from the source;
- existence of any scattering objects. Scattering by plane surfaces
can enhance the E field by a factor of 2, hence, S, by a factor
of 4. Even greater enhancement may result from curved surfaces,
e.g., corner reflectors.
4.3.2 Near-field versus far-field
For the practical purposes of measurement, the reactive
near-field exists within 0.5 lamda from the source with a practical
outer limit of several wavelengths (Jordan & Balmain, 1968). Both E
and H field components must be measured within the reactive
near-field. At present, no instruments are available commercially for
the measurement of H-fields above 300 MHz, which imposes a de facto
frequency limit on the measurements.
4.3.3 Instrumentation
An electric or magnetic field-measuring instrument consists of
three basic parts; the probe, the leads, and the monitor. To ensure
appropriate measurements, the following instrumentation
characteristics are required or are desirable:
- The probe must respond to only the E field or the H field and not
to both simultaneously.
- The probe must not produce significant perturbation of the field.
- The leads from the probe to the monitor must not disturb the
field at the probe significantly, or couple energy from the
field.
- The frequency response of the probe must cover the range of
frequencies required to be measured.
- If used in the reactive near-field, the dimensions of the probe
sensor should preferably be less than a quarter of a wavelength
at the highest frequency present (see next section).
- The instrument should indicate the root mean square (rms) value
of the measured field parameter.
- The response time of the instrument should be known. It is
desirable to have a response time of about 1 second or less, so
that intermittent fields are easily detected.
- The probe should be responsive to all polarization components of
the field. This may be accomplished, either by inherent isotropic
response, or by physical rotation of the probe through three
orthogonal directions.
- Good overload protection, battery operation, portability, and
rugged construction are other desirable characteristics.
- Instruments provide an indication of one or more of the following
parameters:
(a) Average power density (W/m2, mW/cm2);
(b) Average E field (V/m) or mean square E field (V2/m2);
(c) Average H field (A/m) or mean square H field (A2/m2).
However, no instrument actually measures average power density
and this quantity is not useful in the near-field of antennas. Power
density is measured in the far-field by E-field or H-field probes. The
surveyor should be aware of the field parameter (E or H) to which the
instrument responds, and that exposure standards generally stipulate
limits corresponding to both field parameters. Equivalent plane wave
power density is certainly a convenient unit, but in the reactive
near-field, E and H field components must be measured and compared
with the corresponding exposure limits.
Some factors that can influence the signal levels of the
instruments (e.g.,influence of multiple signals, pulse modulation,
lead pick-up, coupling into probes) are discussed in detail in ANSI
(1981) and Joyner (1988).
4.3.4 Measurement procedures
If information on the RF source and exposure situation is well
defined, then a surveyor, after making estimates of the expected field
strengths and selecting appropriate instruments, may proceed with the
survey using a high-range probe to avoid inadvertent probe burnout and
a high-range scale to avoid possible over-exposure.
In the reactive near-field of radiators operating at frequencies
of less than 300 MHz, an electrically small (largest dimension <0.25
lamda) probe sensor is required since large gradients in field
components exist. Spatial resolution is critical (large probes will
yield spatially averaged values) and the use of an isotropic probe is
strongly recommended. E and H field measurements should not be made
closer than a distance of 20 cm from metallic objects. In some such
cases, it may be possible to assess compliance with exposure standards
by making contact current measurements.
Non-uniform field distributions result from reflections from
various structures. Peaks in the field distribution are separated by
at least one-half wavelength with the maximum levels of E and H fields
occurring in different locations. Temporal variations occur also as a
result of scanning antennas, scanning radiation beams, and changes in
frequency. Therefore, it is imperative that any survey include a
sufficiently large sample of data to preclude omission of hazardous
combinations of conditions. When surveying sources of leakage
radiation, such as waveguide flanges, equipment cabinet doors, and
viewing or shielding screens, a "sniffing" procedure in the immediate
vicinity of the equipment is required. A low-power probe and
high-range setting should first be used to determine leakage sources
from a distance, and lower-range settings used as a closer approach is
made. Usually, leakage power varies as the inverse square of the
distance.
When surveying radar antennas, it is necessary to have the
antenna or the beam stationary, because the response time of the
instruments is generally not short enough to indicate the maximum
levels for common beam sweep and scan speed. It is important to
estimate the peak exposure level, in order to ensure that the probe
chosen can withstand such a peak level. Also, instruments that
time-sample the field at insufficiently low sample rates should not be
used for radar applications (Tell, 1983). Appropriate equations are
then used to convert back to time-averaged levels for a rotating
antenna.
All occupied and accessible locations should be surveyed. The
operator of the equipment under test and the surveyor should be as far
away as practicable from the test area. All objects normally present,
which may reflect or absorb energy, must be in position. The surveyor
should take precautions against RF burns and shock, particularly near
high-power, low-frequency systems.
With careful measurement techniques and the correct choice of
instrument, overall measurement uncertainties that are acceptable can
be achieved. Direct field measurements frequently do not provide
reliable means for exposure evaluation at distances from the field
source (an antenna, or a re-radiating surface) of less than about 0.2
m or lamda/2, whichever is smaller. In such a case, it may be
necessary to evaluate the specific absorption rates (SARs) in a model
of the human body using one of the dosimetric measures (Stuchly &
Stuchly, 1986), or to measure directly the RF current flowing through
the person (Blackwell, 1990; Tell, 1990a).
5. DOSIMETRY
5.1 General
Time-varying electric and magnetic fields induce electric fields
and corresponding electric currents in biological systems exposed to
these fields. The intensities and spatial distribution of induced
currents and fields are dependent on various characteristics of the
exposure field, the exposure geometry, and the exposed biological
system. The exposure field characteristics that play a role include
the type of field (electric, magnetic, or electromagnetic radiation),
frequency, polarization, direction, and strength. Important
characteristics of the exposed biological body system include its
size, geometry, and electrical properties. The electrical properties
of biological systems described by the complex permittivity and
electrical conductivity differ for various tissues.
The biological responses and effects due to exposure to
electromagnetic fields generally depend on the strength of induced
currents and fields. However, only the external fields can be measured
easily and dosimetry has been developed to correlate the induced
currents and fields with the exposure conditions. Induced currents, as
a measure of dose, have been used in the quantification of
experimentally induced effects in animals and the results have been
extrapolated to humans.
In the broad range of frequencies considered in this publication,
i.e., 300 Hz-300 GHz, two different, but interrelated, quantities are
commonly used in dosimetry. At lower frequencies (below approximately
100 kHz), many biological effects can be quantified in terms of the
current density in tissue. Therefore, this parameter is most often
used as a dosimetric quantity. At higher frequencies, where many (but
not all) interactions are due to the rate of energ