Biological Effects of Modulated Radiofrequency Fields

57 Pages·2003·0.672 MB·English

NCRP COMMENTARY No. 18
BIOLOGICAL EFFECTS
OF MODULATED
RADIOFREQUENCY FIELDS
Issued December 31, 2003
National Council on Radiation Protection and Measurements
7910 Woodmont Avenue / Bethesda, Maryland 20814-3095
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LEGAL NOTICE
This Commentary was prepared by the National Council on Radiation Protection and Measurements (NCRP).
The Council strives to provide accurate, complete and useful information in its documents. However, neither the
NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this Commentary,
nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or
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liability.
Library of Congress Cataloging-in-Publication Data
Biological effects of modulated radiofrequency fields.
p.;cm. — (NCRP commentary;no. 18)
“Issued December 2003.”
Includes bibliographical references.
ISBN 0-929600-80-0
1. Radio waves—Health aspects—Congresses.
[DNLM: 1. Radio waves—adverse effects. 2. Environmental
Exposure—adverse effects. 3. Oscillometry. 4. Radiation
Protection—standards. WN 600 B615 2003] I. National Council on
Radiation Protection and Measurements. II. Series
QP82.2.R33B555 2003
616.9’897—dc22
2003025056

Copyright © National Council on Radiation
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Preface
Modulated radiofrequency fields are widely used for radio and television broadcasting,
commercial and military radar systems, and wireless telecommunications. In this Commen-
tary, a summary is presented of scientific literature on the biological interactions and human
health effects of pulsed and amplitude-modulated radiofrequency fields. Conclusions are
drawn on the plausible mechanisms of interaction of these fields with biological tissues, and
the extent to which observed biological effects could influence human health and safety.
Serving on NCRP Scientific Committee 89-4 for the preparation of this Commentary were:
Om P. Gandhi, Chairman
University of Utah
Salt Lake City, Utah
Members
John D’Andrea Don R. Justesen
Naval Medical Research Institute Veterans Administration
Brooks City-Base, Texas Kansas City, Missouri
Kenneth R. Foster Indira Nair
University of Pennsylvania Carnegie Mellon University
Philadelphia, Pennsylvania Pittsburgh, Pennsylvania
Arthur W. Guy Asher R. Sheppard
University of Washington Redlands, California
Seattle, Washington
NCRP Secretariat
Cindy L. O’Brien,Managing Editor
The Council wishes to express its appreciation to the Committee members for the time and
effort devoted to the preparation of this Commentary.
Thomas S. Tenforde
President
iii
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Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Executive Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Present Exposure Standards and Guidelines for Radiofrequency Radiation. . . . . 3
1.1.1 Whole-Body Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Time Averaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.3 Partial-Body Exposure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Pulse-Modulated Fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 American National Standards Institute/Institute of Electrical and
Electronics Engineers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 International Commission on Non-Ionizing Radiation
Protection Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.3 Modulation and Exposure Parameters Other Than
Time-Averaged Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3.1 Peak Electric Field Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3.2 Peak Absorbed Power (Temporal) . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3.3 Amplitude Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Data Base for Existing Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Technical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.2 Effects of Modulation on a Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. In Vitro Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1 Effects at the Organ or Tissue Level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Effects on Cell Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1 Electrical Activity of Excitable Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 Cell Enzyme Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Cell Growth and Cell Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.4 Cell Membrane Functions and Properties . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.5 Cell Calcium Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.6 Cell Physiology and Biochemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 Genotoxicity and DNA Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3. In Vivo Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1 Behavioral Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2 Nervous System Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4. Human Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5. Epidemiological Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
v
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vi / CONTENTS
6. Biophysical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.1 Physical Interaction Mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.1.1 Thermal Mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.1.1.1 Bulk Temperature Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.1.1.2 “Point” Heating or Microthermal Effects . . . . . . . . . . . . . . . . . . . 39
6.1.2 Rate of Temperature Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6.1.2.1 Thermoelastic Expansion and Microwave Hearing . . . . . . . . . . . 39
6.1.2.2 Thermally-Induced Membrane Phenomena . . . . . . . . . . . . . . . . 39
6.1.3 Other Proposed Mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7. Discussion and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
The NCRP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
NCRP Commentaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
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1. Introduction
This Commentary reviews the scientific literature pertaining to possible modulation-
dependent effects of radiofrequency (RF) electromagnetic fields in the frequency range of
3 kHz to 300 GHz.
This effort was motivated by the rapid increase in use of modulated RF energy for commu-
nications and other purposes, which has led to a rapid increase in public exposure to RF
energy from cellular telephones and other wireless technologies. Several widely accepted
standards and guidelines for human exposure to RF energy will be discussed in the next sec-
tion. These standards and guidelines limit the power that is absorbed by the body, averaged
over periods of several minutes, and do not consider modulation in any systematic way.
The frequency range covered by this Commentary is broader than that covered by the
National Council on Radiation Protection and Measurements’ (NCRP) exposure recommen-
dations for RF energy (NCRP, 1986). The frequencies considered here are used for many
broadcast, radar, communications and other applications. The lower frequency limit excludes
power frequency fields and frequencies used by some therapeutic devices. This review also
excludes brief ultra-high field strength electromagnetic pulses such as those emitted by sim-
ulators of nuclear events.
1.1 Present Exposure Standards and Guidelines for
Radiofrequency Radiation
Development of guidelines and limits for exposure to RF electromagnetic fields has been
ongoing since the 1950s. There are presently three generally accepted recommendations. In
the United States, there is the 1991 American National Standards Institute approved
Institute of Electrical and Electronics Engineers standard that was last revised in 1999
(ANSI/IEEE, 1991; 1999). Other recommendations include the 1998 guidelines of the Inter-
national Commission on Non-Ionizing Radiation Protection (ICNIRP, 1998), which operates
in cooperation with the World Health Organization (WHO), and the 1986 NCRP recommen-
dations (NCRP, 1986). In the United States, transmitters licensed by the Federal Communi-
cations Commission are required to comply with the safety guidelines set forth in the 1996
Telecommunications Act (FCC, 1996), which are a combination of NCRP (1986) and
ANSI/IEEE (1991) limits. A fourth standard (technically, a prestandard or prospective
standard) was approved by the European Committee for Electrotechnical Standardization
(CENELEC) in 1995 (CENELEC, 1995) for use by the European Union, but is now being
replaced by limits based on current ICNIRP guidelines.
1.1.1 Whole-Body Exposure
All of the aforementioned standards and guidelines limit the mass-averaged rate of absorb-
tion of RF energy by the body, expressed in units of watts per kilogram (W kg–1) and called
specific absorption rate (SAR). The standards and guidelines are designed to limit the
whole-body-averaged SAR in humans, generally to 0.4 W kg–1 for individuals who are occupa-
tionally exposed. NCRP, ANSI/IEEE, and ICNIRP limits (as well as those of the CENELEC
3
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4 / 1. INTRODUCTION
prestandard) also include a second limit, with a general fivefold reduction of exposure for
members of the public. These two limits are considered to apply to occupational and nonoccu-
pational exposure situations (ICNIRP, 1998; NCRP, 1986) or exposures in controlled and
uncontrolled environments (ANSI/IEEE, 1991; 1999). Although designed to limit SAR, RF
exposure limits are expressed in terms of the intensity of fields incident on the body. Because
of the frequency-dependent coupling of the body to external RF fields (Durney et al., 1986;
Gandhi, 1975), the standards are frequency dependent.
1.1.2 Time Averaging
The exposure limits cited above provide for time-averaging, i.e., they state that brief expo-
sures can be averaged over times ranging from 10 s at 300 GHz in the ANSI/IEEE standard,
to 0.1 to 0.5 h at lower frequencies in the RF/microwave range (ANSI/IEEE, 1991; 1999).
Thus, exposure for brief periods can exceed the general limit (e.g., 0.4 W kg–1) while assuring
that average exposure is consistent with the general limit and subject to an upper limit in the
electric field strength of 100 kV m–1 (ANSI/IEEE, 1991).
1.1.3 Partial-Body Exposure
The standards allow for higher exposures if only parts of the body are exposed. For exam-
ple, ANSI/IEEE (1991; 1999) standards allow partial-body exposure that is higher than the
whole-body-averaged SAR of 0.4 W kg–1 subject to appropriate time averaging.
1.2 Pulse-Modulated Fields
Several exposure standards and guidelines have separate provisions for pulse-modulated
fields.
1.2.1 American National Standards Institute/Institute of Electrical and
Electronics Engineers
For pulsed fields (less than 100 ms pulses) at frequencies between 100 MHz and 300 GHz,
the maximum permissible level as averaged over any 100 ms is reduced by a factor of five rel-
ative to the value permitted by normal time averaging.
However, an additional provision in the ANSI/IEEE (1999) standard limits the RF field
external to the body to 100 kV m–1, over the frequency range 0.1 MHz to 300 GHz. This limit
is considerably lower than the field strength that would cause breakdown of air. It is well
known that the fringing fields at the top of the head of a human for vertically-polarized inci-
dent fields may be five to seven times higher than the free-space incident fields. Even with
this localized enhancement to 500 to 700 kV m–1 in the fringing field region, the fields are con-
siderably lower than the dielectric breakdown strength of air, which is 2,900 kV m–1.
1.2.2 International Commission on Non-Ionizing Radiation Protection Guidelines
ICNIRP (1998) guidelines provide “that the equivalent plane wave power density as aver-
aged over the pulse width not exceed 1,000 times the (continuous-wave power) limits or the
field strength not exceed 32 times the field strength limits for frequencies in excess of
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1.2 PULSE-MODULATED FIELDS / 5
10 MHz.” Between 0.1 and 10 MHz, the peak values for the field strength are obtained by
interpolation from the 1.5-fold peak at 0.1 MHz to the 32-fold peak at 10 MHz.
The maximum electric fields inside the body when exposed at the maximum permis-
sible limits of the ANSI/IEEE (1999) standards and ICNIRP (1998) guidelines are given in
Table 1.1.
Thus, the present standards limit the in situ electric fields to less than 100 V m–1 for CW
fields. For pulsed fields with very low duty cycle (brief and infrequent pulses), these electric
field limits are considerably higher [also in the CENELEC (1995) prestandard]. These provi-
sions may be relevant to certain military transmissions and some forms of radar, but not to
modulated fields as used in communications and broadcasting.
1.2.3 Modulation and Exposure Parameters Other Than Time-Averaged Power
The recommendations include additional provisions to limit exposure apart from whole- or
partial-body SAR.
1.2.3.1 Peak Electric Field Strength. The ANSI/IEEE (1999) standard sets a maximum expo-
sure to RF electric fields of 100 kV m–1. This recommendation “is based on the necessity to cap
the allowable electric fields considerably below levels at which air breakdown or spark dis-
charges occur” (ANSI/IEEE, 1991; 1999).
1.2.3.2 Peak Absorbed Power (Temporal). The ANSI/IEEE (1999) standard limits the peak
power in exposure to preclude high-peak SAR for arbitrarily short RF pulses. The temporal
limit is conservative in recognition of uncertainty about the value of spatial-peak SAR, which
could be over 20 times the spatially-averaged SAR (ANSI/IEEE, 1999). Under pulsed condi-
tions (pulse widths less than 100 ms), the maximum permissible exposure averaged over any
100 ms is reduced by a factor of five. For a single pulse, this is equivalent to reducing the
TABLE 1.1Maximum in situ electric fields corresponding to an SAR of 0.4 W kg–1 for CW exposure
and for maximum pulses allowed by ANSI/IEEE (1999) standards and ICNIRP (1998) guidelines.
Electric Field
Conductivity of Maximum In Situ
Strength (V m–1)
Frequency Typical Tissue Electric Fields
Corresponding to an
(S m–1) (very short pulse V m–1)
SARa of 0.4 W kg–1
3 kHz 0.1 63 1,990
1 MHz 0.3 37 1,170
100 MHz 0.7 24 760
1 GHz 1 20 630
10 GHzb 9 7 220
30 GHzb 40 3 95
aBecause of the lower SAR limit for public exposure of 0.08 W kg–1, the electric field strength limit would
be 5–0.5 (0.45) of those listed here.
bAt frequencies greater than ~10 GHz, the depth of penetration of the field into tissue is small, and SAR is
not a good measure for assessing absorbed energy. The incident power density of the field (in W m–2) is a more
appropriate dosimetric quantity at such frequencies.
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6 / 1. INTRODUCTION
maximum permissible peak power density by a factor of five times below the value that nor-
mal time averaging would permit. A maximum of five such pulses are permitted during any
period equal to the averaging time. If there are more than five pulses in any period equal to
the averaging time, normal time-averaging will further reduce the permissible peak power
density.
These limits on peak power were obtained by consideration of a scientific database that
includes the auditory effect in humans (Chou et al., 1982a; Lin, 1980) and RF energy-induced
stun reaction (Guy and Chou, 1982) in rats. “This conservatism is prudent in light of the rel-
ative sparseness of studies for very short high-intensity exposures. Such studies as do exist
are reassuring that this level is indeed far below the threshold for adverse effects”
(ANSI/IEEE, 1999).
ICNIRP (1998) guidelines also limit peak power and recommend that “the equivalent plane
wave power density as averaged over the pulse width not exceed 1,000 times the [CW power]
limits or the field strength not exceed 30 times the field strength limits...for the frequency
concerned...”
The CENELEC (1995) prestandard limits the peak specific absorption to 2 mJ kg–1 for
pulses of duration less than 30 µs at a frequency above 300 MHz (for the general public), with
a five times higher limit for workers. For a 1 µs pulse, this corresponds to a limit in an instan-
taneous peak SAR of 2,000 W kg–1 (public) or 10,000 W kg–1 (workers).
1.2.3.3 Amplitude Modulation. The NCRP (1986) recommendation is the only one of these
RF standards that has a separate provision for AM fields. It incorporates a fivefold reduction
in the occupational exposure limit if the carrier frequency is “modulated at a depth of 50 per-
cent or greater at frequencies between 3 and 100 Hz.” This reduction is based on complex
amplitude and frequency dependence (“frequency and amplitude windows”) reported in some
studies of biological effects of RF energy in birds and mammals (Adey, 1980). Neither ICNIRP
(1998) guidelines or other contemporary standards and recommendations such as those of the
National Radiological Protection Board of the United Kingdom (NRPB, 1993) and CENELEC
(1995) make separate provision for AM fields based on such effects.
1.3 Data Base for Existing Standards
All of the above standards and guidelines except NCRP (1986) are based on data up to the
early 1990s. NCRP (1986) recommendations relied on scientific data existing as of late 1982
(with a few reports included from 1983). This Commentary examines some of the more recent
scientific data as well.
1.4 Technical Background
1.4.1 Modulation
Modulation is “the process of encoding signal information onto a carrier frequency (f) for
c
purposes of transmitting information” (Dorf, 1997). Several modulation schemes are pres-
ently used for broadcasting and communications. Commercial broadcasting systems, however,
use the following principal types of modulation:
• frequency modulation (FM), in which f is changed in response to the information to be
c
sent (signal), usually by a small fraction of the carrier frequency;
• amplitude modulation (AM), in which the amplitude of the carrier is varied, once again
in response to the information to be sent; or
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1.4 TECHNICAL BACKGROUND / 7
• pulse modulation, which is a special form of AM characterized by abrupt shifts of
signal from zero to full strength. This form of modulation is generally used for radar
applications where pulse widths are measured in microseconds and the f usually is
c
above 300 MHz in the microwave frequency range. For use in communications, pulse
widths are usually much longer than for radar and the ratio between peak pulse ampli-
tude and average signal amplitude is less extreme.
Modern wireless communications, including cellular telephones, use several modulation
schemes that are variants of frequency, amplitude and pulse modulation and, in some
instances, a communications scheme may incorporate all three of the basic modulation types.
From the point of view of bioeffects studies, pulse modulation is particularly important.
Radar emissions are characterized by short (microsecond) pulses with low duty cycle (typi-
cally on the order of 0.0005 to 0.001). A duty cycle of 0.001 means that the peak power or
energy is 1,000 times greater than the average value or the total energy is reduced 1,000
times from the energy content if it were not pulsed. However, since the power is proportional
to the square of the field strength when comparing the peak field strengths in a pulsed wave
(PW) to those in a CW field of the same average energy content (intensity), one finds the peak
field strength of the pulsed field is greater only by a factor of approximately 32 (square root
of 1,000) for a duty cycle of 0.001. As will be discussed below, some bioeffects have been well
established to be elicited by pulsed fields that are not produced by CW fields of the same aver-
age intensity, suggesting the importance of the rapidity of energy transfer to a biological sys-
tem or, in other cases, the possible importance of peak field strength rather than average
intensity. Moreover, some hazards can be foreseen from pulsed fields with high-peak values
but low-average intensities because of the direct physical interaction caused by high-peak
electric field strengths.
In part, because of the availability to researchers of high-power pulsed sources originally
built for pulsed radar applications, many bioeffects studies have been conducted using pulse
modulated fields, most of them at 915 or 2,450 MHz. Indeed, the only extensive database that
allows one to examine possible modulation-dependent effects consists of studies comparing
PW and CW microwave fields.
Several commonly used forms of modulation, and the technologies that employ them, are
summarized in Table 1.2.
1.4.2 Effects of Modulation on a Signal
Modulation introduces changes in a signal that might be relevant to biological effects. In
particular, modulation introduces new frequencies that are in addition to the frequency of the
carrier. An unmodulated sinusoidal carrier contains only one CW frequency component (f).
c
Both AM and FM produce a signal whose frequency spectrum (f) extends above and below f.
s c
For standard AM, if the signal has a bandwidth from 0 to f the spectrum ranges between
s,
approximately (f – f) and (f + f). For FM, the range is from (f – kf ) to (f + kf), where k is a
c s c s c s c s
factor on the order of 5 to 10. Pulse modulation, with pulses of width τ seconds, introduces
frequency components which lie (approximately) between f – (2 πτ)–1 and f + (2 πτ)–1.
c c
It should be noted that, with the exception of ultrawideband pulses that are presently
being considered for military and some communication applications, the spread in the band-
width of a modulated RF signal is very small (typically much less than one percent of the car-
rier frequency). For example, the frequency spread of the signal from an FM broadcast station
is 180 kHz, less than 0.2 percent of the carrier frequency for a station operating at 100 MHz.
The various modulation schemes used with cellular wireless communications systems [code
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