United States
Environmental Protection
Agency
Office of
Radiation Programs
Las Vegas, Nevada 89114
ORP/EAD 78-2
April 1978
Radiation
sszEPA
Technical Note
An Analysis Of Radiofrequency
And Microwave Absorption Data
With Consideration Of Thermal
Safety Standards
-------
Office of Radiation Programs Technical Publications
Nonionizing Radiation
Publications of the Office of Radiation Programs are available
from the National Technical Information Service (NTIS), Springfield,
VA 22161. Current prices should be obtained directly from NTIS
using the indicated NTIS Order number. Single copies of some of
the publications listed below may also be available without
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SW Washington, DC 20460.
EPA ORP/SID 72-3
EPA/ORP 73-2
EPA-520/2-73-001
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Reference Data for Radiofrequency Emission
Hazard Analysis CNTIS Order No. PB 220 471)
Environmental Exposure to Nonionizing
Radiation, (Available NTIS only, Order
No. PB 220 851)
Nonionizing Measurement Capabilities: State
and Federal Agencies (Available NTIS only,
Order No. PB 226 778/AS)
RF Pulse Spectral Measurements in the
Vicinity of Several ATC Radars CNTIS Order
No. PB 235 733)
An Evaluation of Satellite Communication
Systems as Sources of Environmental Micro-
wave Radiation (NTIS Order No. PB 257 138/AS)
An Analysis of Broadcast Radiation Levels
in Hawaii (NTIS Order No. PB 261 316/AS)
Radiation Characteristics of Traffic Radar
Systems (NTIS Order No. PB 257 077/AS)
A Measurement of RF Field Intensities in
the Immediate Vicinity of an FM Broadcast
Station Antenna (NTIS Order No. PB 257 698/AS)
An Examination of Electric Fields Under EHV
Overhead Power Transmission Lines (NTIS
Order No. PB 270 613/AS)
An Investigation of Broadcast Radiation
Intensities at Mt. Wilson, California
(NTIS Order No. PB 275 040/AS)
An Analysis of Radar Exposure in the San
Francisco Area (NTIS Order No. PB 273 188/AS)
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AN ANALYSIS OF RADIOFREQUENCY AND MICROWAVE ABSORPTION
DATA WITH CONSIDERATION OF THERMAL SAFETY STANDARDS
Richard A. Tell
April 1978
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RADIATION PROGRAMS
ELECTROMAGNETIC RADIATION ANALYSIS BRANCH
P.O. BOX 15027
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Office of Radiation
Programs, U.S. Environmental Protection Agency, and approved for
publication. Mention of trade names or commercial products does
not constitute endorsement or recommendation for their use.
11
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PREFACE
The Office of Radiation Programs of the U.S. Environmental Protection
Agency carries out a national program designed to evaluate population
exposure to ionizing and nonionizing radiation, and to promote development
of controls necessary to protect the public health and safety. This
report presents an analysis of existing data on radiofrequency and
microwave absorption by humans and examines the absorption of nonionizing
radiation as a thermal load on the body tissues. The thermal viewpoint
presented in this report represents one possible approach to the development
of realistic public health safety standards. Readers of this report are
encouraged to inform the Office of Radiation Programs of any omissions
or errors. Comments or requests for further information are also invited.
Floyd IT. Galpin, Director
Environmental Analysis Division
Office of Radiation Programs
iii
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AN ANALYSIS OF RADIOFREQUENCY AND MICROWAVE ABSORPTION
DATA WITH CONSIDERATION OF THERMAL SAFETY STANDARDS
ABSTRACT
An analysis of existing radiofrequency and microwave radi-
ation absorption data has been performed to examine the frequency
dependent phenomenon of biological tissue heating. This analysis
restricts itself to thermal considerations and examines the
exposure field intensities associated with various levels of RF
and MW induced thermal loading on both the body as a whole and
specific, selectively absorbing tissues in adult humans and
infants. An underlying absorption factor of IW/kg, this being
equivalent to the basal metabolic rate for the adult averaged
over total body mass, is used for comparative purposes in the
analysis. A method of specifying safety standard limits based on
the electromagnetic field energy density rather than the plane
wave, free-space equivalent power density is presented. The
analysis reveals a particularly important resonance frequency
range, 10 MHz <_ f <_1000 MHz, in which RF and MW absorption may
lead to whole body thermal loads several times the whole body
basal metabolic rate for exposures equal to the present safety
standard in use in the United States. A discussion is developed
for applications of this analysis to occupational environments
and short duration exposure conditions. Some implications of
this thermal analysis of RF and MW energy are discussed in terms
of existing safety standards in use in the United States and the
Union of Soviet Socialist Republics (USSR) and to typically
encountered exposures in the United States.
IV
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TABLE OF CONTENTS
Page
Preface iii
Abstract iv
List of Figures vi
List of Tables vii
Acknowledgments viii
Introduction 1
Studies of RF and MW Power Deposition in Man 5
Exposure Fields and Associated Thermal Loading .... 18
Short Duration Exposure 24
Occupational Exposure Considerations 25
Existing Safety Standards 27
Typical Electromagnetic Environments and Safe Levels . 29
Summary 31
References 35
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LIST OF FIGURES
Numbe r
1. Relative microwave energy absorption rate in a two
layer slab model of a tissue system at three different
frequencies 6
2. Average, peak internal, and surface specific absorption
rate in mW/g for an incident microwave field power den-
sity of 10 mW/cm2, as a function of frequency, for an
isolated, multilayered spherical model of the adult
head and child's head 9
3. Whole body specific absorption rate in W/kg for an
average sized man (70 kg, 1.75 m tall) exposed in free
space to an incident power density of 1 mW/cm2 and mo-
deled as an ellipsoid and a prolate spheroid 10
4. Whole body specific absorption rate in W/kg for a sit-
ting Rhesus monkey (3.S kg, 0.40 m tall) exposed in
free space to an incident power dens'ity of 1 mW/cm2 and
modeled as an ellipsoid and a prolate spheroid .... 11
5. Scale model thermograni and measured peak absorbed power
densities for 70 kg, 1.74 m height man frontal plane
exposed to 31.0 MHz electric field 13
6. Distribution of power deposition for a human in electri-
cal contact with ground and in free space 13
7. Specific absorption rates for the whole body and some
intact anatomical parts of man in W/kg for a free space
incident power density of 1 mW/cm2 17
8. Specific absorption rates for the whole body and some
intact anatomical parts of man in W/kg for an incident
power density of 1 mW/cm2 when man is in good electri-
cal contact with a high conductivity ground plane ... 17
VI
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Number Page
9. Electromagnetic field intensity associated with various
levels of thermal loading on selectively absorbing tis-
sues and the whole body 19
10. Accumulative fraction of population of 10 large U.S.
cities exposed to various levels of RF and MW radia-
tion in the frequency range of 54-890 MHz 30
LIST OF TABLES
Number Pag(
1. Electromagnetic field energy densities associated with
various levels of thermal loading in the body 26
VI1
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ACKNOWLEDGMENTS
The author would like to acknowledge the kind cooperation in
providing copies of technical papers, preliminary results of
significance, and informative comment from Dr. Arthur W. Guy at
the University of Washington, Seattle, Dr. Om P. Gandhi,
Dr. Peter W. Barber, and Dr. Carl H. Durney at the University of
Utah, Salt Lake City, and Dr. Claude M. Weil at the Health Effects
Research Laboratory, U.S. Environmental Protection Agency, Re-
search Triangle Park, North Carolina. Stimulating discussions
are gratefully acknowledged with Dr. Elliot Postow, EMR Project
Office, Naval Medical Research and Development Command, Bethesda,
Maryland, Dr. A. Richard Dassler, Naval Medical Research Institute,
Bethesda, Maryland, Dr. Elanor R. Adair at the John B. Pierce
Research Foundation, New Haven, Conneticut, and Dr. Maria A.
Stuchly at the Radiation Protection Bureau, Health and Welfare,
Canada.
viii
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INTRODUCTION
The U.S. Environmental Protection Agency (EPA) has for the
past five years been studying the exposure of the population to
radiofrequency (RF) and microwave (MW) energy. These studies
have focused principally on measuring the exposure of the popula-
tion and determining if presently employed voluntary standards
for the industry as proposed in ANSI C95.4 1966 (1) and as adopted
in the Occupational Safety and Health Administration (OSHA)
standards 29 CFR1910.97 (2), 29 CFR1910.268 (3), 29 CFR1926.54
(4), and as modified in ANSI C95.4 1974 (5) would adequately
provide RF and MW protection to the general population as well as
to RF and MW workers. Such a concern is prompted by the fact
that the original ANSI proposals were developed because of concern
for radiation safety within the industrial and military work
place for those purposefully employed for work with high power RF
and MW sources. Realizing that these groups presumably possess
several important distinctions from the general population as a
whole, such as age, physical fitness for specific jobs, general
health status, and knowledge of predisposing conditions such as
the administration of drugs and medical treatment, that are
generally under the control of the employer, it is rational to
perhaps permit somewhat higher exposure levels to RF workers. It
is not clear that these distinctions from the population as a
whole are properly accounted for when applying such industrial
guidelines to everyone. Mumford (6) has recommended that the
present occupational safety level for continuous exposure be
appropriately reduced according to the ambient heat stress al-
ready present. Mumford's fundamental thermal approach to the
problem is a clear reminder that pre-existing thermal stress
may be viewed as that created by the external thermal environment
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on the individual, or the inherently variable susceptibility to
added heat stress (thermal sensitivity) which may be a character-
istic of particular individuals within the population.
This paper addresses itself to what we presently understand
about the complicated problems of the absorption of RF and MW
energy by humans and how this absorbed energy is deposited spa-
tially as a volumetric heating load on the body tissues. The
resulting frequency dependence of whole body and tissue specific
RF and MW power deposition is presented in terms of exposure
field intensities which will provide a given level of RF and MW
induced thermal loading. The analysis is accomplished for
normal sized human adults and owing to the frequency specific
nature of body dimensions also treats the case of infants.
The present analysis restricts itself to thermal consid-
erations and determines those exposure field intensities which,
in light of present knowledge, are associated with a given value
of RF and MW heating potential as it pertains to specific tissues
within the body as well as the total thermal loading of the body.
For purposes of this analysis, a fundamental, limiting criteria
for power deposition of IW/kg was chosen and applied to the
subsequent determination of fields. The analysis examines the
concept of limiting exposure based on the thermal load provided
to specific tissues which may absorb at a preferentially high
rate, or the thermal load placed on the body as a whole. The
selection of IW/kg power deposition as a limiting criteria is
based on the observation that the metabolic processes of the
human body give rise to a value of about 1.05W/kg when averaged
over the total body mass for the sleeping state (Haines and
Hatch, 7; fielding and Hatch, 8; U.S. Air Force, 9; Guy, 10).
This Basal Metabolic Rate (BMR) for the whole body differs from
those of individual organs; for example, the heart muscle has a
metabolic rate of 33W/kg, the brain HW/kg, the kidney 20W/kg,
the liver 6.7W/kg, skeletal muscle 0.7W/kg, and skin IW/kg (10).
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Though individual tissues may exhibit metabolic rates over one
order of magnitude greater than that averaged over the entire
body mass, it is noted that in laboratory-controlled, heat loss
studies of young men, physically fit and acclimatized to heat
stress studies, the apparent maximum rate of energy expenditure
for humans on a continuous basis, defined as 4-6 hours, is com-
parable to a metabolic loading of about 5W/kg averaged over the
total body (8). Thus the figure of IW/kg maximum power deposition
to any given tissue was viewed as being a very conservative
approach for limiting the externally applied heat stress of RF
and MW radiation. A figure of IW/kg maximum power deposition as
averaged over the whole body, implies a doubling of the BMR on a
whole body basis, or a doubling of total body heat content. It
is interesting to note that on a theoretical basis, an RF or MW
heat load of 1.4W/kg delivered to the total body mass may poten-
tially increase the head core temperature by as much as 0.2°C and
body muscle temperature by as much as 1.5°C (Emery et al.,,11).
This is roughly comparable to the elevation expected due to
physical exercise. Body temperature changes associated with a
total body RF/MW loading of IW/kg are assumed to be noticeable to
most individuals. The conservative approach of limiting exposures
by limiting specific tissue power deposition to IW/kg seems to
incorporate a potentially desirable safety factor by typically
keeping the maximum possible total body heat loading to less than
a factor of two of the BMR since the deposited energy is usually
distributed in a non-uniform manner. An absorption rate of IW/kg
in muscle tissue would lead to a temperature rise of 0.00024°C/sec
if no cooling mechanisms were present, e.g., conduction and blood
circulation. Thus we would tend to protect the total body heat
loading by limiting the absorption rate of certain tissues to
IW/kg while the remainder of the body mass might be absorbing
energy at a significantly lower rate. It is recognized, however,
that such an approach may be overly conservative in that the
normal cooling effect of the blood plays an important role in
maintaining a constant mean body temperature; i.e., the thermal
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impact on specifically heated tissues will tend to be mitigated
by the perfusing blood. But this degree of conservativeness is
also undoubtedly determined, at least in part, by the variance in
magnitude of local hot spots distributed throughout the bodies'
mass.
The philosophy of heat loading as it applies to safe RF and
MW exposure standards is not new. The ANSI standard itself has
been characterized as limiting body heat loading via RF and MW
exposure to a value of two times the BMR of the human adult body
(Michaelson, 12; Schwan, 13). It was argued that since the BMR
of the human body is on the order of 75 watts for a 70kg man with
a body surface area of about 1.9m2, this represents an equivalent
areal heat production rate of about 4mW/cm2. Since half of the
surface area would be available for single sided exposure to a MW
field, by limiting the maximum continuous MW exposure to 10mW/cm2,
we would expect no more than a doubling of the BMR in the body,
even if the incident MW field power were totally absorbed. This
simplistic analysis may be plausible at high microwave frequen-
cies wherein absorption phenomena may be described in terms of a
geometrical optics formulation, but it becomes less convincing at
lower frequencies where recent studies have shown that the human
body will exhibit effective absorption cross sections well in
excess of the geometrical shadow area of the body, being in fact
on the order of 4.2-8.4 times the shadow area (Gandhi, 53; Gandhi
et al., 54) .
The thrust of this paper is to present a thermally based
framework for construction, of realistic RF and MW exposure
limiting values, using the concepts of controlling either specific
tissue volume energy absorption or total body energy absorption.
The resulting analysis reveals that limits on whole body exposure
in the frequency range of 10 to 1000 MHz could be two orders of
magnitude below the presently accepted safe limit (5) if conser-
vative, thermal protection at the specific tissue level is
-------
desired. Less restrictive, and perhaps more realistic, limits
are suggested by controlling total body thermal loading and these
are discussed. The use of lower field intensities than specified
in the current ANSI guide as safety limits for the general popu-
lation would seem to provide the necessary safety factor required
by conservative health oriented controls on radiation and, even
in the case of the most conservative analysis, would not appear
overly restrictive in terms of present day deployment of electro-
magnetic radiation sources.
The analysis permits the ready determination of exposure
levels associated with any selected degree of increased thermal
burden to the human. Though the presented data pertain to a
fixed level of thermal loading which might lead to only slight
temperature elevations of certain tissues, this analysis does not
treat the case of so-called non-thermal effects. In light of the
various reports in the literature that suggest the possibility of
non-thermal biological effects, the available data appear incon-
clusive at present and require additional research to better
define these effects and assess their biological significance
(Cleary, 14). Baranski and Czerski (15) have discussed in depth
considerations of the non-thermal interaction of RF and MW energy
with biological systems. The recent review by Cleary (14) indi-
cates the evidence is persuasive that, in large part, almost all
observed biological endpoints used in experiments to date can be
explained on the basis of a thermal origin. Such evidence lends
weight to a selection of the thermal approach used in this
particular analysis but the potentially more complex problem of
addressing non-thermal interactions still remains.
STUDIES OF RF AND MW POWER DEPOSITION JN MAN
During the last five years extentions of the state-of-the-
art in RF and MW dosimetry have undoubtedly outstripped the
progress made in biological effects research. The literature
-------
revealing RF and MW absorption studies is extensive. Initial
studies by Schwan and his colleagues (Schwan and Li, 15; Schwan
et al., 16; Schwan and Piersol, 17) first showed the phenomenon
of MW field coupling to tissue systems for purposes of evaluating
hazards to radar and the heating properties of diathermy. These
analytical studies showed the variation in localized heating due
to variations in tissue electrical properties and tissue layer
thicknesses for a planar slab model system. This simplified tis-
sue system was studied again later (Tell, 18; Livenson, 19;
Bernardi et al., 20) and extended to other frequencies throughout
the MW region above 1 GHz as well as finitely bounded slab systems
(Livesay and Chen, 21). Additional analytical work with the slab
model was supplemented by actual measurements to experimentally
verify the spatial temperature distributions which would result
from the practical application of diathermy treatment (Lehmann et
al., 22; Brunner et al., 23; Lehmann et al., 24; Guy and Lehmann,
25). Figure 1 illustrates the variation in localized power de-
position obtained from a typical multi-layered slab model analysis
915 MHZ
0123456789 10
TISSUE THICKNESS (cm)
Figure 1. Relative microwave energy absorption rate in a two
layer slab model of a tissue system at three different frequencies;
taken from Tell (18).
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The figure illustrates the dramatic increase in relative heating
which can occur at the boundary separating biological tissues of
highly different dielectric properties. Though it was recognized
that such simple planar slab models could not adequately describe
RF and MW absorption in complicated biologic.al structures with
irregular geometry such as man, the approach indicated that MW
absorption in man would likely reveal nonuniform power deposition
and correlated variations in local tissue heating.
Early analytical and experimental studies by Anne (26) and
Anne et al.(27), this time with spherically shaped dielectric
absorbers, illustrated the severe restraints of the planar slab
approach by pointing out the strongly resonant properties of
spherical objects whose dimensions are comparable to the wave-
length of the incident radiation. These s-tudies presented the
concept of the relative absorption cross section for dielectric
absorbers; the phenomenon by which certain geometrically shaped
and sized objects absorb more MW power than can be accounted for
by the simple product of incident wave power density and the
geometrical shadow cross sectional area. The use of relative
absorption and scattering cross sections has been used for many
years in electromagnetic radiation scattering work. Often it is
referred to as efficiency. The concept of effective electromag-
netic cross section is prominent throughout the literature of RF
and MW absorption effects and has become a popular method of
describing the physical interaction of RF and MW radiation with
absorptive biological systems (Johnson and Guy, 28; Blacksmith
and Mack, 29; Schwan, 30). The question of non-uniform power
absorption in homogeneously constructed spherical models was
answered by Kritikos and Schwan (31), Lin et al.(32,33), and Ho
et al. (34). These early studies of spheres did not concern
themselves with inhomogenity of human type anatomical structures.
Only recently have more sophisticated studies been made of the
more realistic situation (Shapiro et al., 35; Joines and Spiegel,
36; Kritikos and Schwan, 37, 39; Lin, 38; Neuder et al., 40).
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These studies point out the development of internal hot spots due
to spherical body resonances and describe the enhanced coupling
of external fields into the interior absorptive medium commonly
used to represent brain tissue in a model of the human head.
This is due to the effects of varying permittivity of surrounding
tissue layers and supports the somewhat more simplistic studies
of internal energy distribution conducted for homogeneous spheres.
In an attempt to estimate total body absorption of RF and MW
energy by man, several of these studies have considered an adult
human as a spherical mass of high water content tissue, i.e.
muscle. These studies, though attractive because of the sim-
plified analytical formulation of the problem, must be considered
crude as indicators of absorbed power in man but do provide
insight to the rough absorptive properties of humans.
Weil (41) has recently analyzed a multilayered spherical
model of the head composed of a central brain core surrounded
with five layers of cerebral spinal fluid, bone, fat, and skin-
dura tissues. This analysis, similar to that of Shapiro (35),
has presented the important results in a convenient format for
application to practical problems. Weil's computed results for
the frequency range of 0.1 to 10 GHz for head dimensions with
radii of 10 and 6cm have been taken as representative of head
absorption in human adults and small children respectively. Data
are provided for determining the average, maximum, and surface
absorption power densities throughout the frequency range for
these models and are shown in Figure 2. These data are incor-
porated in the analysis of limiting exposure values presented in
this paper.
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\ I I I I II IIII I
ujf- OUTHKAOIUJ -iue«
•aoumun-uiii/w'
I I I I I I I I I I I I I I I I I I II
IN Ml MM W S40 180 10011UD 20M Ml 4000MM IM
urn
.J I I/I 111 I 11 III I III II
IN M nO 100 400 SM TO WO MO nOOMO 4000 HM
PMQUCMCT.IttU
A B
Figure 2. Average, peak internal, and surface specific absorp-
tion rate in mW/g for an incident microwave field power density
of 10 mW/cm2, as a function of frequency, for an isolated, multi-
layered spherical model of the adult head, r=10cm(A) and the
small child's head, r=6cm(B); taken with permission from Weil
(41).
Improved analytical techniques have been developed and ap-
plied to the numerical analysis of absorption properties of more
realistic models of man in the form of prolate spheroids (Durney
et al., 42; Johnson et al., 43; Lin and Wu, 44; Massoudi et al.,
45,47; Barber et al., 46). Very recent formulations have even
modeled man using a cell approach to best fit the actual contour
of the human body (Chen et al., 48; Hagmann et al., 49). It has
been found that the ellipsoidal model, although more complicated
than most numerical approaches to date, seems to more conve-
niently represent the actual form of man and certain experimental
test animals (Allen et al., 50). Extensive computations of
ellipsoidal and prolate spheroidal models of man and animals have
been compiled by Durney et al.(Sl) for conditions of free space
irradiation and are presented in Figures 3 and 4 for models
corresponding to an adult man and the Rhesus monkey which has
been taken for the purposes of this analysis as representative of
a human infant. These data conform closely to the experimental
9
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work of Gandhi et al.(63). These data, utilizing results from
the ellipsoidal analysis for adults below 30 MHz and for the
Rhesus monkey below 100 MHz and from the prolate spheroidal
analysis above these frequencies, are subsequently utilized in
this analysis. The ellipsoidal results are considered more
accurate but due to precision restraints imposed by presently
available computing machinery, the numerical results are not
available for adult human models above 30 MHz or the Rhesus model
above 100 MHz. Experimentally derived absorption rates have been
obtained for ellipsoidal models for frequencies up to 500 MHz
(Gandhi et al., 63). A major result of these detailed calculations
is the finding of a whole body resonance frequency for a model
height of about 0.4X. Experimental results which support these
findings have been obtained which not only confirm the presence
and magnitude of the resonance absorption property but also
7
10- iff
FREQUENCY (MHZ)
10'
ib1 ib' 10'
F(MHZ)
B
10'
o1
Figure 3. Whole body specific absorption rate in W/kg for an
average sized man (70 kg, 1.75m tall) exposed in free space to an
incident power density of ImW/cm2 and modeled as an ellipsoid (A)
and a prolate spheroid (B); taken with permission from Durney et
al. (51).
10
-------
io*
FREQUENCY (MHZ)
0'
F(MHZ)
A B
Figure 4. Whole body specific absorption rate in W/kg for a
sitting Rhesus monkey (3.5kg, 0.40m tall) exposed in free space
to an incident power density of ImW/cm2 and modeled as an ellip-
soid (A) and a prolate spheroid (B); taken with permission from
Durney et al.(51).
illustrate the important dependence of absorption on the polari-
zation of the body with respect to the incident field (Gandhi,
52,53; Gandhi et al., 54). Neukomm (102) has recently provided
direct experimental confirmation of the full body resonance fre-
quency of about 75 MHz in his studies with actual human subjects.
Several modifications and extensions to these mathematical
techniques have been proposed (49), (Rowlandson and Barber, 55;
Massoudi et al., 56). Recent results by Barber et al.(99) show
that a more sophisticated, multilayered model of man may exhibit
slightly enhanced absorption characteristics over the homogeneous
model in the post resonance region. This is presumably due to
the impedance matching properties of the various body tissue
layers. A major effort now appears to be the experimental veri-
fication of the numerical results obtained for man in free space
and in the presence of a ground plane and nearby reflective
11
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structures which indicate the possibility of significantly en-
hanced RF and MW energy absorption rates (54), (Gandhi et al.,
57; Hagmann et al., 58). Gandhi (53) obtained some experimental
verification of these ground interaction absorption phenomena in
scale models of the human body. A significant aspect to the
study of power deposition within the human is the extensive
thermographic investigation of the spatial distributions of the
heating potential in scale models by Guy and his colleagues (Guy,
59; Guy et al., 60;62; Chou and Guy, 61; Gandhi, 54;63). These
studies demonstrate the extreme importance of knowing the spatial
distribution of absorbed power density, illustrating the areas of
the body susceptible to high localized heating effects which may
exceed by a factor of 30 the power absorption rates when averaged
over the total body mass (54,60). Presently utilized thermographic
measurement techniques employed by Guy (64) provide a limiting
volume heating resolution of about 2 cm3 when applied to the full
sized adult. Consequently, the limiting volumetric heating load
used in this analysis is interpreted to mean IW/kg averaged over
no greater than 2 cm3 tissue volume. The neck and ankle regions
exhibit enhanced local absorption for free-space and ground plane
irradiation conditions, respectively. Figures 5 and 6 taken from
the work of Guy et al.(60) and Gandhi et al.(S4) illustrate
typical variations of localized field intensification found in
figurine scale models of man when exposed to free-space fields
and ground plane conditions. Figure 5 from Guy's work shows that
under the right conditions the ankle, as an example here, may
exhibit a local absorption rate of 26 times the whole body
average. It should be noted that the experimental measurement of
high specific absorption rates (SAR) in models presents a dif-
ficult problem in that the high localized heating in the modeled
tissue tends to smear with time, creating the potential of under-
estimating the actual energy absorption rate. A rough index of
the spatial variability in local hot spot occurrence has been
found by forming the ratio of the standard deviation of localized
SAR values in scale models of man to the mean value of the SAR
12
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MAN
FRONT h = l.74m E2=IV2/m2 sf^4.62
f = 3I.OMHz
INTENSITY SCAN
F-F'
= 50.2PW/kg
G-G'
W=
H-H
W=ll.7/iW/kg
Figure 5. Scale model thermogram and measured peak absorbed
power densities for 70kg, 1.74m height man frontal plane exposed
to 31.0 MHz electric field: Vertical division = 2 C; horizontal
divisions = 4cm. Note that an equivalent volume ellipsoid would
have a whole body specific absorption rate of 5.8yW/kg and here
the ankle has a value of 149yW/kg and the neck, not shown here,
has a value of 29.6yW/kg; taken with permission from Guy et
al.(60).
l/X - 0.103
0.417
o.4H
0.4*
O.JM
0.147
O.W7
4.7H
B
Figure 6. Distribution of power deposition for a human in elect-
rical contact with ground (A) and in free space (B). The numbers
are relative to a whole body average specific absorption rate of
4.5W/kg for a 1.75m tall man exposed to 10mW/cm2 incident fields;
taken with permission from Gandhi et al.(54).
13
-------
as determined by averaging of the local values. Under non-
resonant conditions this ratio is fairly constant at a value of
about 1 while under conditions of resonance this value may in-
crease to 1.5. It is clear that limiting exposure on the basis
of total body absorption can underestimate the significance of
specific tissue heating within the body. Recent numerical re-
sults by Hagmann et al.(58), have suggested even higher localized
tissue energy deposition rates, but these findings remain subiect
to experimental verification and analysis from the standpoint of
practically achievable ground plane situations for man. An
estimate of practicably obtainable total body power absorption by
man at resonance on a high conductivity ground plane appears to
be on the order of two times the whole body absorption under free
space conditions, i.e., when the individual is not grounded
(Gandhi, 65). Additionally, local internal field enhancement
leading to a maximum power deposition of approximately 8-10 times
the average whole body absorption has been found for ground plane
conditions and appears to be a good practical estimate of this
phenomenon (Gandhi et al., 54; Gandhi, 65). The further exam-
ination of achievable ground plane enhancement in power absorp-
tion must be stressed since man may assume, under certain occupa-
tional circumstances, body configurations similating a monopole
on conducting towers in the presence of substantial field inten-
sities (Tell, 66).
For purposes of this analysis a potential localized power
deposition of 30 times the averaged whole body absorption rate is
assumed for free space irradiation conditions in order to account
for possible localized tissue heating effects. This assumption
proves conservative in that it provides for resulting whole body
thermal loads that will be generally significantly less than
twice the BMR of the body, i.e., whole body thermal loads may be
limited to as much as 1/30 of the resting body BMR. For cases of
ground plane exposures localized internal power deposition en-
hancement of ten times the average for the whole body is assumed.
14
-------
A complicating factor in the analysis of absorbed RF and MW
energy is the existence of the near-field zone about radiating
structures in which the wave impedance varies from its intrinsic
value of IZOir ohms in the far field. Such changes in the wave
structure, wherein the electric and magnetic field components do
not have a necessarily known relationship, can account for dif-
ferent specific absorption rates in man. These differences have
been examined by Guy et al.(60) and Durney et al.(51) and have
been determined to account for as much as a factor of three times
greater absorption for pure H field exposures as opposed to pure
E field exposures in an ellipsoidal model of man at a frequency
of 30 MHz. Durney et al.(Sl) show on the basis of limited numeri-
cal results that the absorption rates of prolate spheroidal
models of man can be 1.4, 4.3, or 3.5 times greater than that
under plane wave conditions at 10 MHz depending on body orienta-
tion for electric, cross, and magnetic polarizations of the body,
this being for a wave impedance of one half the free space value.
This characterizes an exposure in which the magnetic field is
predominant. Results for higher frequencies are yet to be com-
puted but the trend of the low frequency data suggests a propor-
tionately greater increase in the absorption rate for this wave
impedance. The practical significance of intense low impedance
field exposure must not be overlooked even at frequencies in the
VHP region of the spectrum since such occupational exposure
situations do exist Tell (66). Durney et al.(51) have also
determined for the same prolate spheroidal model of man that high
impedance fields, that is, those in which the electric component
is predominant, will generally cause lower total body absorption
than that predicted for free space plane wave irradiation. For
the cases of electric, cross, and magnetic polarization of the
body they have computed, for fields with a wave impedance of 240ir
ohms or double the free space value, that the whole body power
absorption rates are 0.9, 0.3, and 0.3 of the free space values
respectively, again at 10 MHz. These types of findings when
extended to the VHP region will also provide a method for practi-
15
-------
cally indexing the potential hazard zones about the ends of
dipolar OT similar radiating structures -which are characterized
by extremely high impedance electromagnetic fields (Tell, 67).
These non-plane wave absorption results are particularly appli-
cable to the occupational environment but probably have less
significance in terms of the consideration of safety criteria for
the general population. Accordingly, no -wave impedance modifica-
tions have been used in the analysis in this paper; such consid-
erations should be made when applying the results of this analy-
sis to occupational exposure conditions.
A final confounding factor in RF and MW dosimetry is the
recent demonstration of "proximity effects1' (Gandhi et al.,63),
body reflections which give rise to increased power deposition in
the head compared to predictions from the isolated head model or
multiple target interactions. Gandhi et al.(68) have reported
results of their experimental and numerical studies using more
realistic figurine models indicating that body reflections can
account for perturbations of the whole body specific absorption
rate. Their data show that the intact adult head resonates near
350 MHz rather than at 470 MHz for the isolated head reported by
Weil (41). An enhancement in total power deposition to the
intact head was found to exceed total absorption determined for
an isolated model by a factor of about 3 at the resonance fre-
quency of 350 MHz and to be about equal to or slightly less than
the isolated value at 470 MHz. This suggests that analytical
results obtained for isolated models of the head, at least on a
total absorption basis, may inadequately describe the potential
for hazardous exposure conditions. According to Gandhi (65),
however, there would appear to be no major change in the relative
amplitudes of the internal field distributions of the head. Thus
these findings have been taken into account in this analysis.
Figures 7 and 8 taken from Gandhi et al.(68) are provided to
reveal the kind of variations in localized tissue absorption
rates for both free space and grounded conditions of man.
16
-------
I
.
LJ7
- O
OC *t
i >
! .
al
K.C,
82
03 "
u
. i
en -*:
a
a:
u
o
10'
FREQUENCY (MHZ)
10'
Figure Specific absorption rates for the whole body and some
intact anatomical parts of man in W/kg for a free space incident
power density of ImW/cm2; taken from Gandhi et al.(68).
103
FREQUENCY (MHZ)
u
Figure 8. Specific absorption rates for the whole body and some
intact anatomical parts of man in W/kg for an incident power den
sity of ImW/cm2 when man is in good electrical contact with a
high conductivity ground plane; taken from Gandhi et al.(68).
17
-------
The indicated specific absorption rates are those integrated over
basic anatomical structures and do not reveal the worst case,
very localized absorption which can occur.
The question of near-body interaction with the absorption
properties of man are interesting and have been investigated in
large arrays of experimental animals by Kinn (69) and Gandhi et
al.(68). These observations though somewhat preliminary, suggest
a reasonably low probability of significant alteration of iso-
lated body studies when used in practical exposure situations,
due to critical separation distances required. Accordingly, no
modifications due to multiple target absorption enhancement are
used in this analysis.
EXPOSURE FIELDS AND ASSOCIATED THERMAL LOADING
Results from several of the above described investigations
have been massed together to form an overall picture of the
frequency dependence of RF and MW absorption in man. The data
relating to absorption have been used to compute the field inten-
sity which is associated with a maximum power deposition in any
body tissue of IW/kg at any frequency in the range of 10 KHz -
lOGHz for adults and infants assuming whole body exposure. These
data have been presented graphically in Figure 9. The units for
specification of biologically significant field intensities have
received much discussion in the literature (Youmans and Ho, 70;
Bowman, 71; Schwan, 13). These authors have pointed to the
unfortunate choice of using the areal surface power density, or
Poynting vector notation, for indicating exposure field amplitude,
The uses of absorbed power density in tissue proposed by Johnson
and Guy (28) or internal tissue current density by Schwan (13)
are obviously preferred methods of quantifying dose-rate-effect
results in biological research but such concepts must be replaced
by other means of practically specifying the external field for
control purposes. That the parameter of power density is inap-
18
-------
1000
ELECTROMAGNETIC FIELD INTENSITY ASSOCIATED WITH
VARIOUS LEVELS OF THERMAL LOADING ON SELECTIVELY
ABSORBING TISSUES AND THE WHOLE BODY
100
2 10
Infant, frw space, 0.1 BMR whole body
Infant, free space, max. 1 W/kg any tissue
Adult, free space. 0.1 BMR whole body
Adult, free space, max. 1W/kg any tissue
t±±1 tltl
ANSI-OSHA Standard
Adult head, max. 1W/kg any tissue
4 • Surface heating predominates
beyond this frequency
USSR Occupational T < 20 mln.
H''
Adult, ground plane,
0.1 BMR whole body
Surface heating predominates ,,.
beyond this frequency
1000
10000
100000
FREQUENCY (MHZ)
Figure 9. Electromagnetic field intensity associated with various levels of thermal
loading on selectively absorbing tissues and the whole body. Note that U=U£+UH and
that in the far field Ur^U,,.
-------
propriate for many exposure situations has been reinforced,
notably by Bowman (71), who has, among others (Wacker, 72),
proposed that fields be specified in terms of the electric field
energy density UE or more generally in terms of the total electro-
magnetic field energy density U in units of Joules/cubic meter or
more practically nJ/m3. In the interest of encouraging the
sensible and wide spread acceptance of Bowman's recommendation,
the field amplitudes on Figure 9 are given in units of electro-
magnetic field energy density, this being closely related to the
quantity measured by most available field meters. This is in-
nately logical since these instruments do not respond to the
field power density itself but in most instances (Bowman, 73;
Asian, 74; Hopfer, 75) are sensitive to the square of the electric
field strength, which is directly proportional to UE< Bowman's
argument for using a basic quantizing term of the field, i.e.,
electric or magnetic field energy density, that does not depend
on the proximity of an individual to the radiation source is a
fundamentally attractive proposition and provides for less ambi-
guity, particularly when specifying near field exposures. An-
other advantage to using the total energy density U of the expo-
sure field is that the important hazard potential of the magnetic
field, which is typically more predominant at lower frequencies,
particularily below 30 MHz, is taken into account. Because
almost all work relating to RF and MW hazard analyses has re-
ferred to the field power density Figure 9 also carries for
convenient reference, plane wave, free space equivalent power
density in units of m¥/cm2 on the right hand ordinate. For far
field conditions, the areal power density S is related to the
electric field energy density UE and the magnetic field energy
density UH by the relation
Up (nJ/m3) UH(nJ/m3) (1)
m2) = 1(?.v = H16.7 •
20
-------
In the far field U=UE+UH. Alternatively, in the near field as
well as the far field, U- and UH are related to the electric
field and magnetic field strengths E and H by the relations
UE(nJ/m3) = 0.00443 E2(V2/m2), (2)
UH(nJ/m3) = 628 H2(A2/m2). (3)
Bowman (71) has discussed these relationships in detail.
There are several curves which appear in Figure 9 and each
of these are discussed. The data points used in plotting the
data have been shown. The adult free space curves are developed
from Durney's work (51) based on Figure 3. An inhomogeniety
intensification factor of 30 has been used in computing the re-
quired free space exposure field energy densities to limit
maximum tissue absorption to IW/kg. The infant free space
curves are taken again from the same source (51) but are based on
computed results for a Rhesus monkey, Figure 4, which has been
taken to represent the human infant in terms of size. Curves are
shown for both specific tissue power deposition of IW/kg and
whole body deposition of 0.IW/kg averaged over the body. It is
apparent that for all intermediate sizes of individuals there
will exist an envelope, indicated by the dashed line connecting
the resonance dips for man and infant, which defines the limiting
exposure levels for in-between cases.
Weil's isolated head absorption data (41) in Figure 2(A)
have been used to determine a ratio of peak to average SAR in the
adult head model. This ratio has a maximum value of 8 and has
been used in conjunction with Gandhi's data (68) found in Figure
7 pertaining to average SAR values for the intact adult head to
determine the exposure fields that would be associated with a
peak internal SAR of 1 W/kg. The peak, as opposed to average,
SAR has been selected as the more important criterion of RF and
21
-------
MW absorption by the head since tissues of the central nervous
system may exhibit higher thermal sensitivities than other tis-
sues of the body.
Lacking information on whole head absorption by the intact
child's head, Weil's data (41) on peak internal SAR of the iso-
lated head, Figure 2(B), have been plotted directly with the
inclusion of an absorption enhancement factor of 3 suggested by
Gandhi's data (68) for the intact adult head compared to the
isolated model at resonance. It is noted that above 1200 MHz
surface heating predominates for the adult, i.e., internally
induced heating will not exceed that on the surface. The same
surface heating predominates in the small child's head above 2300
MHz. Continued increase in the frequency would be characterized
by the surface heating load being confined to thinner and thinner
dermal layers until prohibitively high skin tissue power deposi-
tion would occur with increasing frequency except for the fact
that surface thermal radiation and blood diffusion preclude
excessive skin surface temperatures. It is inconceivable that,
on the basis of body surface heat escape properties, the radiation
intensity, which limits specific tissue absorption to IW/kg at
the point of predominately surface heating, could create a bio-
logically significant thermal load at the skin surface. For this
reason, the limiting values determined at 1200 and 2300 MHz, for
adults and small children respectively, are suggested as appli-
cable for all higher frequencies. Data are plotted for adults
and small children as designated on the curves.
The curve labeled "Adult, ground plane, 0.1 BMR whole body"
is based on the experimental and theoretical data of Gandhi et
al.(57) and Gandhi (63) and has been determined by limiting the
whole body power absorption to 0.1 BMR. This also limits specific
tissue absorption to IW/kg since a local intensification factor
of 10, used by Gandhi et al.(54) exists for the grounded condition.
22
-------
A similar curve for infants would fall above the other curves and
consequently has been eliminated from presentation.
It should be pointed out that at frequencies below about 30
MHz the contribution to RF induced thermal loading due to the
magnetic field component can become significant. If the field
specification were in terms of UE only, this important aspect of
the potential hazard would not be accounted for.
It is apparent now, that if one limits the maximum tissue
energy absorption rate anywhere in the body to a value equivalent
to the resting body BMR, that a pronounced region of maximum
susceptibility has been identified for man which spans the fre-
quency range of about 10 MHz to 1000 MHz and requires the exter-
nal field to be limited to about two orders of magnitude below
the generally accepted safety limit in the U.S. of 10 mW/cm2.
It is tempting to draw an envelope of suggested values of
electromagnetic field energy density in Figure 9 which would
provide a practical definition of thermally safe exposure. Such
suggested values have not been indicated but it is clear that the
phenomenon of thermal loading is frequency sensitive and as such
limiting exposure levels could properly incorporate such a
dependence. It is particularly obvious that below 10-20 MHz
substantially higher limiting fields are appropriate from a
thermal viewpoint. It must be pointed out, however, that at
these lower frequencies the likelihood of dangerous transient
spark discharge effects due to sizeable voltage inductions on
conducting objects in the irradiation field can become signifi-
cant. In fact there is evidence that good safety practice would
dictate an even lower acceptable field level, perhaps signifi-
cantly below 400 nJ/m3 (Honolulu Advertiser, 76; Hammett and
Edison, 77). In the frequency range 10 MHz <. f <. 10000 MHz
curves are provided which limit specific selectively absorbing
tissues to a maximum of IW/kg and which limit total body power
23
-------
absorption to 1/10 the BMR. This latter set of curves is intro-
duced to permit rapid estimation of total body heat loading for
any suggested exposure field value. It is doubtful that notice-
able body temperature increase would occur at RF or MW induced
whole body thermal loads less than 1/10 the BMR.
From a thermal point of view safety criteria based on
limiting the SAR of selectively absorbing tissues to the resting
body BMR would probably be viewed as unnaturally conservative.
On the other hand limiting whole body power absorption to several
times the BMR would seem to be questionable, depending on other
environmental parameters and an individual's thermal sensitivity.
Though the subject of multiple frequency components in the
exposure field is not treated here, further consideration is
necessary for proper specification of acceptable field intensi-
ties when more than one frequency is present with significant
field amplitude.
SHORT DURATION EXPOSURE
It is reasonable to make some distinction between long and
short term exposure limits. The incorporation of exposure time
in such a consideration is attractive in that it provides a
reasonable approach to dealing with intermittently operated RF
and MW sources which for brief periods of time may produce
relatively high exposure in their vicinity. Such a proposal has
recently been discussed (78) and forms a part of the philosophy
of the ANSI standard (1) (5). For application to the general
population, it is plausible to assume that the emphasis, for
short term exposure, should be on individual tissue protection at
some higher intensity, and that this maximum acceptable RF
induced thermal load on any selected body tissue should not
exceed some multiplicative factor of the BMR. A defining rela-
tionship of total body exposure energy density and time would be
24
-------
of the form
Q = U(hJ).T(hr) <_ Q'nJ-hr/m3 (4)
iP"
where Q' represents the time weighted tissue heating potential in
units of nJ-hr/m3. Maximum total body exposure energy density
would be necessarily limited to a value which would insure not
exceeding a total body heat load of several times the BMR.
Table 1 provides a summary of some of the critical points on
Figure 9. In the case of partial body exposure, i.e., those
instances wherein particular parts of the body are predominately
exposed when compared to the body as a whole, it is reasonable to
permit even higher maximum energy densities when it can be
determined that those body tissues subject to maximum absorption
could not, because of source limiting restraints, absorb more
than perhaps lOW/kg. With the lack of near-field knowledge there
is no obvious way to prescribe in general what these exposures
will be and therefore this problem remains subject to a case-by-
case evaluation.
It is not apparent that relaxation of field amplitude limits
below 10 MHz for short duration exposures would be prudent for
application to the general population. For occupational RF and
MW exposure, a somewhat relaxed controlling level would seem
appropriate in view of the presumably greater knowledge of the
exact irradiation circumstances.
OCCUPATIONAL EXPOSURE CONSIDERATIONS
Although it is not the intent of this paper to address the
evaluation of limiting safety criteria for occupational appli-
cations, the basic analysis provides a reasonable approach to
evaluating such criteria. And while the definition of absolute
limits for safe occupational exposure could be made rather
25
-------
TABLE 1. ELECTROMAGNETIC FIELD ENERGY DENSITIES
ASSOCIATED WITH VARIOUS LEVELS OF THERMAL LOADING IN THE BODY
Electromagnetic Field Energy Density CnJ/m3) for Limiting
Thermal Loading to Various Levels
Free Space Resonance Grounded Resonance
Nature of Loading Adult Infant Adult
1 X BMR Whole Body 140.0 115.0 107.9
1 X BMR Specific Tissue 4.68* 3.83* 10.79**
0.1 X BMR Whole Body 14.0 11.5 10.79
* A possible field intensification factor of 30 is used for trans-
forming from whole body to specific, selectively heated tissue
for free space conditions.
** A possible field intensification factor of 10 is used for trans-
forming from whole body to specific, selectively heated tissues
for grounded conditions.
26
-------
arbitrarily, it seems intuitive that considerable relaxation of
the limiting values considered for the general population is in
order if care is exercised in determining potentially hazardous
exposure areas. That is to say, when detailed knowledge of the
exposure situation is available, one can then predict more
reliably the actual potential for significant power absorption in
the body and therefore, reassessment of the limiting criteria
would be appropriate. For example, in physically fit working
adults, it does not seem unreasonable to impose, during a working
day of 8 hours, a significantlv greater thermal load on these
individuals compared with the general population. Partial body
exposure, particularly of the extremities such as the hands,
would seem to also warrant substantial increases in allowable
power deposition; accordingly, partial body exposure needs
careful additional evaluation.
Proposals for a model occupational safety guide are not
given but it is worthy of note to compare the absorption frequency
response curves of Figure 9 with the existing ANSI standard for
continous exposure of 10mW/cm2 plane wave power density (5). it
is seen that above 1C MHz there are regions wherein this allow-
able exposure level could introduce power deposition in selective
tissues of the body 100 times greater than the resting whole body
BMR. It is questionable whether this possibility is prudent for
the work place let alone the uncontrolled exposure of the general
population, since it has been recognized that a tissue power
deposition of 50W/kg is associated with a vigorous diathermy
treatment Guy (10).
EXISTING SAFETY 'STANDARDS
A number of intercomparisons of the various RF and MW expo-
sure standards throughout the world can be found in the litera-
ture (Michaelson, 12; Cleary, 14; IRPA, 79; Tell, 80). Such
intercomparisons are not the subject here but it is worthwhile to
27
-------
make note of several significant features when examining some of
these standards in view of the analysis which is the subject of
this paper. For reference, a number of the existing RF and MW
safety standards are graphically illustrated on Figure 9.
The U.S. Air Force standard (81) for use at frequencies
below 10 MHz would appear to be reasonably conservative in terms
of possible body thermal loading. It is noted, however, that the
possible influence of ground plane phenomena which might signifi-
cantly enhance power deposition, particularly in the lower portion
of the body, may need to be assessed in practical implementation
of this standard in the field to determine the actual margin of
thermal safety.
The ANSI standard (5), as previously pointed out, could
be viewed as defining an upper bound for RF and MW induced ther-
mal burdens insofar that certain body tissues, under the right
conditions, could experience selective power deposition at defi-
nitely thermalizing levels. Such allowances in the uncontrolled
general population seem questionable.
The US microwave oven performance standard (82) promul-
gated by the U.S. Department of Health, Education and Welfare
(DHEW) in its Bureau of Radiological Health (BRH) to control MW
leakage from these electronic products appears at first glance to
be similar in character to the higher ANSI standard (5). But as
pointed out by Czerski (83), Osepchuck et al.(84), and IRPA (79),
for practical distances encountered in routine operation of these
devices, whole body exposures are in essence limited to about 5-
20yW/cm2 and thus the oven standard must obviously be considered
very conservative from a thermal standpoint. When examined from
even a partial body exposure viewpoint, the BRH oven performance
standard is unlikely to produce specific tissue power deposition
greater than lOW/kg, but detailed analysis of this possibility
has not been accomplished.
28
-------
One of the more interesting findings is that were the
approach used in this analysis used to reduce the maximum accept-
able possible RF and MW power deposition in any given tissue to
no more than an additional 10 percent of the inherent BMR load-
ing, one would determine corresponding exposures equal to the
present Soviet occupational exposure standard (85). Though there
is essentially no evidence that the Soviet safety guides for the
work place (85), and as proposed for the population (Gordon, 86),
were developed from a thermal analysis of the problem, the adopted
limits in the USSR, it may be speculated, might in fact be re-
lated to very subtle manifestations of thermal loading in RF and
MW workers. It remains to be determined whether the many reported
subjective observations, described in the Soviet literature as
the asthenic syndrome (Gordon, 86; Letavet and Gordon, 87;
Pressman, 88), can be correlated with very slight, tissue specific,
internally generated thermal excursions, which because of a lack
of sensitivity in experimental designs have been labeled as non-
thermal.
TYPICAL ELECTROMAGNETIC ENVIRONMENTS AND SAFE LEVELo
There are no obvious indications in the literature that the
lowest limiting safe values of electromagnetic radiation energy
density, as might be suggested by an analysis of this type, would
allow undesirable RF or MW thermal stress in man; i.e., the
limiting levels of energy density would appear reasonably conser-
vative, biologically speaking. How would these conservative
limits then, compare with the presently determined levels of RF
and MW exposure typically encountered in the environment? A
significant part of the answer to this question can be found in
the extensive field measurements made by EPA in large metro-
politan areas. Preliminary results of these measurements made
with an EPA developed mobile field intensity measurement system
(Tell et al., 89) have been discussed previously (Janes et al.,
90; Athey et al., 91) and imply that less than 1 percent of the
29
-------
population in 10 large US urban areas are exposed in excess of
about 34 pJ/m3 or 1 uW/cm2. Figure 10 indicates the latest
results of an EPA analysis of 10 U.S. cities (92) and shows that
the median exposure of this population group is about 0.2 pJ/m3
or 0.007 uW/cm2. Thus ground level exposures do not in general
approach the most conservative thermal limitations suggested by
this analysis, being typically more than 100 times lower in
value.
FRACTION OF POPULATION EXPOSED AS A
FUNCTION OF POWER DENSITV
.99
.95
.9 •
•
.7 •
.6 -
.5 .
.4 .
.3 .
.2 •
.1 .
.05 .
.01 .
CITIES: tOSTON
ftTUMTft *
niimi
PH1LAOELPHIK +
MM VMK
CHICMM +
HMN1NOTON
Utt VEWS *
MM DIEM
; MMTUMD *
| +
+
i i i i i
»
*
•
-5 -4 -3-2-1 01 2
LOG 5) S • POWER DENSITV IN UW/CM/CM
Figure 10. Accumulative fraction of population of 10 large U.S.
cities exposed to various levels of RF and MW radiation in the
frequency range of 54-890 MHz. The median exposure is 0.007yW/cm2
and 0.7 percent of the population are exposed to levels above
lyW/cm2.
Field intensity measurements inside of tall buildings sit-
uated adjacent to other buildings which support high power broad-
cast transmission facilities have shown, however, the presence of
much higher radiation intensities, in some instances being equal
30
-------
to about 4 nJ/m3 or .12 mW/cm2. These higher intensities are due
to main lobe illumination from the transmitting antenna and
situations involving such proximity effects have been described
by Tell and Janes (93). So, in limited areas it appears that
broadcast fields might exceed the most conservatively set limits
but these cases are probably relatively small in number and there
exist economically attractive methods whereby such fields may be
substantially reduced in intensity (Tell, 94).
The consideration of other RF and MW transmitting facilities,
besides broadcast, has not yet identified many areas wherein a
very conservative environmental limit of even 4 nJ/m3 time
averaged energy density would seem difficult to comply with but
this subject is still being analyzed within EPA. It is not
certain, for example, that major impacts on existing military and
civilian radar operations would occur. Presently employed hazard
analysis procedures, normally used in the military, often use
very conservative methods of defining exclusion zones about
radars (95,96,97,98). It is noted that in many instances actual
exposures measured at the exclusion zone boundary must be signifi-
cantly below the predicted, main beam, worst case values providing
an inherent degree of latitude in the ability to adjust to a
lower acceptable environmental level. The economic impact of any
standard setting activity must include a thorough evaluation, but
in view of what is presently seen of the deployment of RF and MW
radiation emitting sources, it would appear that an environmental
exposure limit of even 4 nJ/m3 would not be unnecessarily restric-
tive.
SUMMARY
This paper has discussed an analysis of the existing RF and
MW absorption data for man as these data relate to specifying
electromagnetic power deposition in tissues of the body leading
to various levels of thermal loading. A limiting energy absorp-
31
-------
tion rate of IW/kg for any tissue within the body was assumed as
a conservative criterion for the development of a thermally based
RF and MW radiation protection guide, this specific tissue absorp-
tion rate being equal to the basal metabolic rate of a human on a
whole body basis. The analysis examines the frequency dependent
nature of total body absorption, includes the important informa-
tion on the distribution of absorbed power throughout the body
and determines values of the exposure field intensity correspond-
ing to a maximum possible thermal load of IW/kg placed on selec-
tively absorbing body tissues or on the body as a whole. Ground
plane and body reflection effects which can lead to enhanced
power absorption are incorporated in the analysis. Base curves
are provided, relating given exposure intensities to thermal
loading at both the specific tissue and whole body levels, which
may be used for convenient thermal evaluation of safety standards,
By describing field intensities in terms of the electromagnetic
field energy density the ambiguity of specifying near field
exposure in terms of power density is removed and the importance
of the magnetic field component, particularly at low frequencies
is taken into account. A method for modification of limiting
values Is suggested which incorporates the duty cycle of the
irradiation field for short term exposure.
The results illustrate the strong frequency dependent nature
of electromagnetic energy absorption in man and the existence of
resonance frequencies for the body as a whole and anatomical sub-
structures. For the human adult, at a resonance frequency of
about 70 MHz, a free space exposure field of 140 nJ/m3 electro-
magnetic field energy density, a far-field equivalent of 4.2
mW/cm2 power density, will induce a doubling of the heat load due
to the BMR. This would probably be detectable as a rise in body
temperature by most individuals at rest. While the whole body
heat loading is only doubled, the nonuniform nature of the absorp-
tion process could potentially lead to localized absorption rates
32
-------
as high as 30 W/kg. Such a high local absorption rate is probably
not advisable for the general population.
The results reveal serious reservations for applying the
currently used ANSI standard (5) to the population as a whole in
that localized power deposition could, under proper conditions of
exposure, apparently lead to substantial thermal burdens in
various parts of the body. It is noted that the adoption of more
conservative limits would not appear in general to impose undue
hardships on existing facilities inasmuch as environmentally
encountered RF and MW intensities are rarely above 4 nJ/m3.
The formulation presented provides one possible approach to
considering limitations on RF and MW exposure of the population.
It is founded on a relatively sound base of analytically and
experimentally derived dosimetric data for man and uses the
fundamentally attractive thermal concept as a basis for describing
electromagnetic radiation effects in man. It must be pointed out
that the dosimetric data used is the best available but even so,
must be considered as only an approximation to the complex nature
of electromagnetic field distribution in man and consequently,
the results obtained by using these data undoubtedly contain
inherent vagaries when conceptualizing exact human absorption.
Using a thermal concept raises questions of a biological
nature which are beyond the scope of the present paper but which
must be answered to arrive at a meaningful understanding of the
ultimate biological significance of different possible safety
levels of RF and MW exposure. At least three biological con-
siderations are important: (a) the heat dissapation properties
of the tissues, (b) the tissues' thermal sensitivities, and (c)
the thermal regulatory feedback mechanisms of the biological
system. These considerations interact in a complex fashion. It
is not clear whether thermal protection of the human from RF and
MW radiation should be directed at a whole body level or at the
33
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specific tissue or organ level or a combination of both. For
example, it is conceivable that total body energy absorption in a
given field intensity might not perturb the system at all if the
deposition were uniform. On the other hand, at the same given
field intensity and because of the inherent nonuniform nature of
RF and MW absorption by the body throughout much of the frequency
spectrum, certain tissues or organs might absorb at a deleteriously
high rate. Balancing the possibly detrimental biological impact
of heating certain critical tissues against whole body heating is
difficult. For example, the extreme thermal sensitivity of the
anterior hypothalamic region of the brain wherein a temperature
difference of as little as 0.2 °C can ellicit behavioral changes
in experimental animals (100) suggests caution in viewing RF and
MW absorption from the more simplistic whole body approach. Be-
cause the absorption of RF and MW energy by the human body is
characterized by significant nonuniformity, it would appear at
first glance that attention should be concentrated on examining
specific tissue interactions. It has been suggested (101) that
using the local BMR values for certain tissues and/or organs
might be a first approach to identifying such thermally suscep-
tible targets.
The author would like to indicate that the approach used in
this paper treats the absorption of RF and MW fields totally from
the viewpoint of the conversion of this energy into heat within
the tissues of the body; the significance, if any, of high peak
intensity, pulsed fields has not been treated. The literature
which suggests that pulsed fields may be more efficacious in
inducing biological effects than continuous wave fields demands
careful evaluation in that other than direct thermal effects may
be involved (Cleary, 103).
34
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45
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
3. RECIPIENT'S ACCESSION NO.
TITLE AND SUBTITLE
An Analysis of Radiofrequency and Microwave
Absorption Data with Consideration of Thermal
Safety Standards
5. REPORT DATE
April 1978
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Richard A. Tell
ORP/EAD 78-2
. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Radiation Programs
Electromagnetic Radiation Analysis Branch
P.O. Box 15027
Las Veeas. NV 89114
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as above
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
An analysis of existing radiofrequency and microwave radiation absorption data
has been performed to examine the frequency dependent phenomenon of biological
tissue heating. This analysis restricts itself to thermal considerations and
examines the exposure field intensities associated with various levels of RF and MW
induced thermal loading on both the body as a whole and specific, selectively
absorbing tissues in adult humans and infants. An underlying absorption factor of
IW/kg, this being equivalent to the basal metabolic rate for the adult averaged over
total body mass, is used for comparative purposes in the analysis. A method of
specifying safety standard limits based on the electromagnetic field energy density
rather than the plane wave, free-space equivalent power density is presented. The
analysis reveals a particularly important .resonance frequency range, 10 MHz <_ f
£1000 MHz, in which RF and MW absorption may lead to whole body thermal loads severa
times the whole body basal metabolic rate for exposures equal to the present safety
standard in use in the United States. A discussion is developed for applications of
this analysis to occupational environments and short duration exposure conditions.
Some implications of this thermal analysis of RF and MW energy are discussed in
terms of existing safety standards in use in the United States and the Union of
Soviet Socialist Republics (USSR) and to typically encountered exposures in the
United States.
KEY WORDS AND DOCUMENT ANALYSIS
a.
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Croup
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (Thilpage)
Unclassified
22. PRICE
EPA Perm 2220-1 (R»v. 4-77) PREVIOUS EDITION n OBSOLETE
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