This chapter was written by John C. Mitchell, Chief, Radiation Physics Branch, Radiation Sciences Division, USAF School of Aerospace Medicine, Brooks Air Force Base, Texas 78235-5301.
The development and application of devices that emit radiofrequency radiation have significantly increased the quality of life throughout the world. Yet the beneficial aspects of RF/microwave technology have been somewhat overshadowed in recent years by the public's fear of potential adverse effects. This fear, in turn, has led to increased RFR research (resulting in a much better understanding of the interaction of RFR fields and biological systems) and to new RFR safety guidelines. The new exposure standards are based on what is known about the frequency-dependent nature of RFR energy deposition in biological systems and about any biological effects. In general, the new guidelines provide an added margin of safety over those previously used.
Inherent health risks from RFR exposures are directly linked to absorption and distribution of RFR energy in the body, and the absorption and distribution are strongly dependent on body size and orientation and on frequency and polarization of the incident radiation, as indicated by the data in Chapters 6 and 8. Both theoretical and experimental dosimetric data show that RFR absorption approaches maximum when the long axis of the body is both parallel to the E-field vector and equal to four-tenths of the wavelength of the incident RFR field. Thus a 70-kg, 1.75-m human exposed to a planewave RFR field in free-space with the E-field aligned with the long axis of the body, would absorb the most energy at a frequency of about 70 MHz (Chapter 6). If the person were standing in contact with a conducting ground plane (producing a change in the apparent long-axis length), the frequency for maximum RFR absorption would be about 35 MHz. This frequency-dependent behavior is illustrated in Figure 11.1 for several human sizes (using prolate spheroidal models having body masses of 10, 32, and 70 kg) . The average whole-body specific absorption rate in watts per kilogram is plotted as a function of radiation frequency in megahertz for an incident average power density of 1 mW/cm².
For more than 20 years the United States and most of the free world used a single field-intensity value to maintain the safety of personnel exposed to RFR. A power density of 10 mW/cm2, time averaged over any 6-min period, was applied as an acceptable exposure level and generally was thought to include a safety factor of 10. During the past 5-10 years it has become well accepted that the RFR absorption and distribution in humans are strongly dependent on the frequency of the incident radiation, as shown in Figure 11.1. When ANSI revised its safety standard in 1982, it incorporated this frequency-dependency concept and used SAR as a common denominator for biological effects. The ANSI standard (1 Sep 1982) allows average incident-power densities from 1 to 100 mW/cm² depending on the radiation frequency. It limits the average whole-body absorption to 0.4 W/kg or less and the spatial peak SAR to 8 W/kg as averaged over any 1 g of tissue.
Figure 11.2 illustrates how the ANSI standard was derived. The relative power-absorption curves illustrated in Figure 11.1 were used to establish the shape of the ANSI curve. It was normalized to 0.4 W/kg because the ANSI committee, after reviewing the biological effects data base, believed the threshold for adverse biological effects to be greater than 4 W/kg. Thus the 0.4 W/kg was selected to include a safety factor of 10. Table 11.1 shows the ANSI RFR Protection Guides in terms of the mean squared E- and H-field strengths and of the equivalent planewave free-space power density as a function of frequency.
At the low end of the frequency coverage (<3 MHz), ANSI placed an arbitrary cap at 100 mW/cm² rather than extending the frequency-dependent boundary. If the permissible exposure levels were allowed to increase without limit, other phenomena such as corona discharge and electric shock effects could become problems. On the other hand, there was concern that, at the lower frequencies, the 100 mW/cm² cap would be too conservative and might place unnecessary operational constraints on some RFR operations.
In May 1983, ACGIH published new threshold limit values (TLVs) for RF microwave radiation (ACGIH, 1984). Like the ANSI standard, the ACGIH TLVs limit human absorption to an SAR of 0.4 W/kg or less, averaged over any 6-min period. Unlike the ANSI standard, the TLVs cover the added frequency range from 10 to 300 kHz and from 100 to 300 GHz. Because the TLVs are to be applied in occupational settings, they are based on the assumption that no children (small humans) will be in the workplace. This assumption allows an average incident-power density of 10 mW/cm² at frequencies greater than 1 Ghz, while maintaining the same 0.4-W/kg whole-body absorption limit. This can be seen from Figure 11.2 if the absorption curve for the 10-kg human is removed. The arbitrary 100-mW/cm² cap applied in the frequency range from 10 kHz to 3 MHz appears safe on the basis of whole-body SAR; however, RFR intensities of 100 mW/cm² can result in shocks or burns under certain conditions. The 100-mW/cm² limit should not restrict many operations but serves as a reminder that at this level potentially significant shock and burn problems may occur. The ACGIH TLV provides procedures to minimize these problems and reduce operational constraints while maintaining personnel safety. The ACGIH TLVs (Table 11.2) are established as safety guidelines for the workplace. They are intended for use in the practice of industrial hygiene and should be interpreted and applied only by a person trained in this discipline.
The United States has not established Federal guidelines for RFR exposures. The ANSI and ACGIH voluntary guidelines coupled with those used by the individual services of the Department of Defense and some State and local standards represent the range of RFR safety guidelines applied in the United States in the past few years (U.S. AFOSH Standard 161-9, 1984; Coymnonwealth of Massachusetts, 1983; Johns Hopkins Applied Physics Laboratory Health and Safety Bulletin, 1984).
On 8 July 1983 the Executive Council of the International Radiation Protection Association (IRPA) approved interim guidelines on limits of exposure to radiofrequency electromagnetic fields in the frequency range from 100 kHz to 300 GHz (Interim Guidelines, 1984). The International Nonionizing Radiation Committee of IRPA included participants from France, Netherlands, Poland, Denmark, the Federal Republic of Germany, Great Britain, Australia, and the United States of America. Environmental Health Criteria 16 (1981) "Radiofrequency and Microwaves," serves as the primary scientific rationale for the development of the IRPA RFR guidelines. These guidelines apply to RFR exposure of both occupational workers and the general public. The basic limits of exposure for frequencies greater than 10 MHz are expressed in wholebody averaged SAR. For practical purposes, derived limits of exposure are also expressed in average incident-power density. See Table 11.3. The derived limits are extremely conservative in the frequency range 10-30 MHz. This approach, to state the exposure limit in terms of whole-body SAR, represents a departure from current practices; i.e., most new standards express the permissible exposure levels in average incident-power density even though they are based on limiting the whole-body SAR. For occupational workers the IRPA exposure limit for frequencies greater than 10 MHz is 0.4 W/kg when averaged over any 6 min and over the whole body or 4 W/kg when averaged over any 6 min and any 1 g of tissue. For the general public, the IRPA exposure limit is 5 times lower; i.e., 0.08 W/kg when averaged over any 6 min and over the whole body or 0.8 W/kg when averaged over any 6 min and any 1 g of tissue.
Figure 11.3 presents a comparison of the ANSI, ACGIH, and IRPA RFR safety standards plotted as average incident-power density versus frequency. These standards are based on the same assumption, that 4 W/kg is a reasonable threshold for adverse biological effects. Differences in the permissible incident-power densities as a function of frequency result from the degree of conservatism applied in each instance.
Most biological-effects studies used to establish the current RFR safety guidelines have used small laboratory animals (mice and rats) and 2450-MHz radiation sources. In such studies the RFR energy is deposited throughout the animal's body and includes "hot spots" of RFR absorption that can be 10 to 20 times higher than the average. The bioeffect thresholds were established using worst-case data. The most likely human exposures are much less traumatic because the RFR energy is not generally deposited throughout the body (see Section 5.1). To illustrate this fact, Table 11.4 gives some rough approximations of the penetration depth in biological tissue and the percentage of total body mass (in humans) that might be exposed as a function of radiation frequency. Depth of penetration is defined as the distance at which the power absorption is (0.135) of the surface value. The results in Table 11.4 are for a cylindrical model of man. Similar information is given in Figure 3.36 for a planar model.
For example, a standard man exposed to a 1-GHz field might receive a unilateral exposure, penetrating to a depth of ~4 cm, resulting in ~21% of the total-body mass receiving RFR energy. For a 10-GHz exposure the RFR energy might penetrate only ~0.5 cm and result in less than ~3% of the total-body mass receiving RFR energy. In fact, real-world exposure situations are much more complicated because the RFR energy is deposited in a very nonuniform manner that results in hot spots difficult to predict. These so-called hot spots do not relate to temperature excursions but to the fact that the SAR at different locations in the body can vary by an order of magnitude. Nevertheless, the data in Table 11.4 illustrate that in most exposure situations the RFR energy is deposited unilaterally in a relatively small volume of the body; in many exposure situations an appreciable fraction of the body is not subjected to any significant energy deposition. With regards to potential bioeffects of RFR exposures, this is considered an added safety factor for exposures at or below the safety guidelines. The body's thermoregulatory system must still handle the total energy deposited, but such heat loads are minimal (generally less than one-half of the basal metabolic rate) for permissible exposure levels.
In most human exposures only a part of the body is exposed at radiation intensities approaching the safety limits. For example, most radar systems propagate radiation beams that are confined to a few degrees in both lateral directions and depend on scanning to cover the surveillance volume. At intensities approaching the maximum permissible values, human exposures normally would occur only close to the source, where the beam size is relatively small. This situation also applies for exposures to leakage fields from microwave ovens and from a wide range of RFR-generation equipment. Such partial-body exposures at intensities that exceed the normal limits are often felt as a warming sensation, giving a person warning to terminate the exposure before it becomes more serious.
The radiation protection guides applied over the frequency range covering whole-body resonant conditions were selected to protect the human under the worst circumstances (ANSI, 1982; ACGIH, 1984). The guides are based on a person's being exposed for 6 min to a free-space planewave field at the radiation frequency equivalent to his or her resonant frequency (dependent on the person's height), with the E-field vector aligned with the long axis of the body, and at the maximum RFR intensity allowed by the guideline. These circumstances seldom, if ever, occur. For example, persons would rarely find themselves in a field having their resonant frequency at the maximum intensity allowed. Even with a measured level equal to the maximum, the isodose intensity contour will not likely be as large as the human. It is also unlikely that a human will maintain erect posture for 6 min at a time. Changes in posture (stooping, bending, squatting) significantly reduce RFR energy absorption. Figure 11.4 shows calculated relative power-absorption curves plotted for a 1.8-m, 70-kg human who has changed his or her effective height by squatting, sitting, and standing with arms both in normal position and raised above the head. As in Figure 11.1, these curves were developed using prolate spheroid models with a constant body mass of 70 kg.
In some jobs people might remain a fixed distance from an RFR source, but in most exposure situations the distance between the source and the person varies considerably--with movement often reducing the amount of RFR energy absorbed. Also, normal thermoregulatory response (blood flow) in living animals minimizes the temperature excursions predicted from static models (Krupp, 1983).
Advancements in understanding RFR interactions with living systems, based on dosimetry and biological effects research, are shared commonly throughout the world. Differences in RFR safety guidelines established by different governmental bodies depend largely on the degree of conservatism applied and philosophical approaches taken. These facts are well documented in current reviews of RFR-induced biological effects (ANSI, 1982; ACGIH, 1984; Environmental Health Criteria, 1981; Heynick, 1984; Biological Effects of Radiofrequency Radiation, 1984).
New research to assess the biological effects of RFR exposure has emerged rapidly and in considerable quantity over the past 5 years. Most reported effects are related to thermal insults from specific absorption rates greater than 4 W/kg. The new RFR safety guidelines and their rules of application provide greater safety than those used in the past; they also have a more credible scientific basis.
The whole-body (averaged) SAR used in developing new frequency-dependent RFR safety standards has been a significant improvement over what was previously used. The SAR relates well to most laboratory studies using small animals subjected to RFR fields at frequencies close to resonant conditions.
At frequencies greater than about 20 GHz and less than about 3 MHz however, whole-body SAR is not an adequate basis for the RFR safety guidelines. At frequencies greater than 20 GHz, RFR energy deposition in biologic tissue is very superficial (see Table 11.4 and Figure 3.36), and some form of localized SAR may serve as a better safety guideline than a whole-body SAR. New safety guidelines for the 20-300-GHz frequency range will likely emerge in the next few years.
Below about 3 MHz, the RFR absorption in biologic tissue decreases as a function of frequency squared below the resonant point. The actual RFR energy absorption becomes so small in the 10-kHz to 3-MHz frequency range that it can be considered safe at any practical value of incident-power density. However, valid questions remain concerning the potential for shocks and burns. These questions are being studied (as indicated in Chapter 9), and new safety guidelines can be expected in the future.
Go to References.
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