Armstrong Laboratory Shield

Radiofrequency Radiation
Dosimetry Handbook

(Fourth Edition)


Chapter 1. Introduction

The radiofrequency portion of the electromagnetic spectrum extends over a wide range of frequencies, from about 10 kHz to 300 GHz. In the last two or three decades, the use of devices that emit radiofrequency radiation (RFR) has increased dramatically. Radiofrequency devices include, for example, radio and television transmitters, military and civilian radar systems, extensive communications systems (including satellite communications systems and a wide assortment of mobile radios), microwave ovens, industrial RF heat sealers, and various medical devices.
The proliferation of RF devices has been accompanied by increased concern about ensuring the safety of their use. Throughout the world many organizations, both government and nongovernment, have established RFR safety standards or guidelines for exposure. Because of different criteria, the USSR and some of the Eastern European countries have more stringent safety standards than most Western countries. The Soviet standards are based on central-nervous-system and behavioral responses attributed to RFR exposure in animals. In Western countries the standards are based primarily on the calculated thermal burden that would be produced in people exposed to RFR. In each case, better methods are needed to properly extrapolate or relate effects observed in animals to similar effects expected to be found in people. (The development of new RFR safety guidelines is discussed in Chapter 11.) Safety standards will be revised as more knowledge is obtained about RFR effects on the human body.
An essential element of the research in biological effects of RFR is dosimetry--the determination of energy absorbed by an object exposed to the electromagnetic (EM) fields composing RFR. Since the energy absorbed is directly related to the internal EM fields (that is, the EM fields inside the object, not the EM fields incident upon the object), dosimetry is also interpreted to mean the determination of internal EM fields. The internal and incident EM fields can be quite different, depending on the size and shape of the object, its electrical properties, its orientation with respect to the incident EM fields, and the frequency of the incident fields. Because any biological effects will be related directly to the internal fields, any cause-and-effect relationship must be formulated in terms of these fields, not the incident fields. However, direct measurement of the incident fields is easier and more practical than of the internal fields, especially in people, so we use dosimetry to relate the internal fields (which cause the effect) to the incident fields (which are more easily measured). As used here, the term "internal fields" is to be broadly interpreted as fields that interact directly with the biological system and include, for example, the fields that, in perception of 60-Hz EM fields, move hair on the skin as well as fields that act on nerves well inside the body. In general, the presence of the body causes the internal fields to be different from the incident fields (the fields without the body present).
Dosimetry is important in experiments designed to discover biological effects produced by RFR and in relating those effects to RFR exposure of people. First, we need dosimetry to determine which internal fields in animals cause a given biological effect. Then we need dosimetry to determine which incident fields would produce similar internal fields in people, and therefore a similar biological effect. Dosimetry is needed whether the effects are produced by low-level internal fields or the higher level fields that cause body temperature to rise.
In small-animal experiments dosimetry is especially important because size greatly affects energy absorption. For example, at 2450 MHz the average absorption per unit mass in a medium rat could be about 10 times that in an average man for the same incident fields. Thus at 2450 MHz at the same average energy absorption per unit mass, a hypothetical biological effect that occurred in the rat should not be expected to occur in man unless the incident fields for the man were much higher than those irradiating the rat. Similarly, an effect observed in one animal in some given incident fields may not be observed in a different-size animal in the same incident fields, only because the internal fields could be quite different in the two animals. Another possibility is that the physiological response to the internal fields of the two species could be quite different. For example, different species often respond differently to the added heat burden of applied EM fields. The dosimetric data are presented here in terms of the specific absorption rate (SAR) in watts per kilogram. Adoption of the term "SAR" was suggested by the National Council on Radiation Protection and Measurement and has been generally accepted by the engineering and scientific community. The terms "dose rate" and "density of absorbed power" (often called absorbed-power density), which commonly appear in the engineering literature, are equivalent to SAR. Each of these terms refers to the amount of energy absorbed per unit time per unit volume, or per unit time per unit mass. In this document we give the SAR in watts per kilogram by assuming that the average tissue density is 1 g/cm3. The total power absorbed in comparison with the body surface is also of interest. In many animals heat is dissipated through the surface by evaporation or radiative heat transfer; thus power density in watts per square meter of body surface area may indicate the animal's ability to dissipate electromagnetic power. This is not a rigorous indicator of hazard for animals, however, as many other heat-dissipation mechanisms specific to species are also important, as well as environmental temperature and humidity effects.
The rigorous analysis of a realistically shaped inhomogeneous model for humans or experimental animals would be an enormous theoretical task. Because of the difficulty of solving Maxwell's equations, which form the basis of analysis, a variety of special models and techniques have been used, each valid only in a limited range of frequency or other parameter. Early analyses were based on plane-layered, cylindrical, and spherical models. The calculated dosimetric data presented in this handbook are based primarily on a combination of cylindrical, ellipsoidal, spheroidal, and block models of people and experimental animals. Although these models are relatively crude representations of the size and shape of the human body, experimental results show that calculations of the average SAR agree reasonably well with measured values. Calculations of the local distribution of the SAR, however, are much more difficult and are still in early stages of development.



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Last modified: June 14, 1997
© October 1986, USAF School of Aerospace Medicine, Aerospace Medical Division (AFSC), Brooks Air Force Base, TX 78235-5301

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