Robert F. Cleveland, Jr.

I. BASIC CONCEPTS

Electromagnetic energy is present in a variety of forms that we encounter every day. The electromagnetic spectrum extends from extremely low-frequency (ELF) electromagnetic fields, such as those associated with electrical power systems up to, and including, X rays and high-energy γ rays. In between these extremes are found radiowaves, microwaves, infrared radiation, visible light, and ultraviolet radiation.

Electromagnetic radiation can be characterized as waves of electrical and magnetic energy radiating into space from a transmitting source. These waves are created by the acceleration of electrical charge. For example, the movement of charge in a transmitting radio antenna, i.e., the alternating current, generates electromagnetic waves that radiate away from the antenna and can be intercepted by a receiving antenna.

Electromagnetic waves travel at the “speed of light,” i.e., 300 million m/sec in a vacuum, and, as with all waves, they have characteristic frequencies and wavelengths. These quantities are related by the expression: frequency times wavelength = the speed of light. Therefore, since the speed of light is constant, waves with high frequencies have short wavelengths and waves with low frequencies have longer wavelengths. Electromagnetic energy, due to power line transmissions, is associated with low frequencies and long wavelengths, while, at the other end of the spectrum, the frequencies of γ rays and X rays are many orders of magnitude higher and the wavelengths correspondingly short.

The energy content of an electromagnetic wave increases as frequency increases, and the energy of a discrete electromagnetic quantity, the photon, is directly related to frequency by a constant quantity (Planck’s constant), i.e., energy = Planck’s constant times frequency. This leads to an important distinction between the various forms of electromagnetic energy. The less energetic forms, including ELF fields, radiowaves, microwaves, infrared radiation, and visible light are categorized as “nonionizing” since their energy content is not high enough to result in ionization of molecular structures. On the other hand, electromagnetic waves with higher frequencies, such as X rays and γ rays, have sufficient energy to cause ionization and are forms of ionizing radiation. Ionizing radiation can cause significant damage to the genetic material of living organisms. The mechanisms of interaction of ionizing radiation with biological systems have been well characterized and are significantly different than the known mechanisms of interaction of the various types of nonionizing radiation.

This chapter focuses on electromagnetic radiation with frequencies in the radiofrequency portion of the spectrum. Radiofrequency (RF) energy is used in a wide variety of ways in modern society, and new uses are increasingly being introduced. Important applications of RF energy include radio and television broadcasting, microwave communications for telephone systems, emergency two-way radio communications, such as police and fire radio, cellular radio, satellite communications systems, amateur radio, cordless telephones, and many other telecommunications uses. Applications other than telecommunications include microwave ovens; industrial heating; weather, marine, and air traffic control radar; military radar; police traffic radar; medical applications, including the treatment of cancer; and radio astronomy.

The RF region of the electromagnetic spectrum can be generally defined to include electromagnetic waves with frequencies in the range of 3 kHz to 300 GHz. One hertz (Hz) equals 1 cycle/sec. A kilohertz (kHz) is one thousand (103) Hz, a megahertz (MHz) is one million (106) Hz, and a gigahertz (GHz) is one billion (109) Hz. Figure 1 illustrates the electromagnetic spectrum and the position occupied by the radiofrequencies.

The RF spectrum is also sometimes characterized by frequency bands that are designated as HF (high frequency, 3–30 MHz), VHF (very high frequency, 30–300 MHz), UHF (ultrahigh frequency, 300–1000 MHz), and various microwave bands above 1000 MHz, e.g., L-band (1.0–2.0 GHz), S-band (2.0–4.0 GHz), C-band (4.0–8.0 GHz), and X-band (8.0–12.0 GHz). The term microwave is commonly associated with RF frequencies of several hundred MHz up to 1000 MHz (1 GHz) and higher.

At lower RF frequencies it is usually desirable to characterize an electromagnetic field in terms of its respective electric and magnetic components. Quantification of RF field components can be made in terms of field strength. Electric field strength (E) is usually expressed in terms of volts per meter (V/m), and magnetic field strength (H) in terms of amperes per meter (A/m). Some RF safety standards and RF measuring equipment define exposure limits in terms of the square of the field strength, i.e., volts squared per meter squared (V2/m2) for E2 and amperes per meter squared (A2/m2) for H2.

Another unit commonly used for describing the intensity of RF energy, particularly at VHF and microwave frequencies, is power density. Power density, also known as energy flux density, can be defined as the rate of energy transport into a small sphere divided by the cross-sectional area of that sphere, i.e., power per unit area. It is usually expressed in terms of milliwatts per square centimeter (mW/cm2) or watts per square meter (W/m2). It is most appropriate to refer to power density when the point of interest is far enough away from the RF emitter to be located in the “far field” zone of the radiator, i.e., where the distribution of electric and magnetic field strength is uniform and the RF field has the characteristics of a “plane wave.” Closer in to the RF source (the “near field”) the physical relationships between the electric and magnetic field components can be complex, and power density may not be a meaningful term for describing RF intensity. The distance to the far field will generally be less with respect to RF emitters operating at VHF and microwave frequencies than for those that transmit at lower frequencies. Therefore, power density is more commonly used to express intensity of RF radiation at higher frequencies but is not the best term to use in describing a field at low RF frequencies, e.g., in the HF band and below. However, sometimes the term “far-field equivalent” power density is used to express the intensity of an RF field when plane-wave conditions are not present. For a detailed discussion of RF units, properties, and measurements see National Council on Radiation Protection and Measurements (NCRP) (1981) and IEEE (1992d).

II. BIOLOGICAL CONSIDERATIONS AND SAFETY STANDARDS

When RF radiation intercepts an object, the constituent electromagnetic waves can be reflected, refracted (bent or scattered), absorbed by, or transmitted through the object. When the intercepted object is a biological organism, such as a human being, absorption of some or all of the energy can lead to effects on the organism that may or may not be harmful. Since the biological effects that may be produced by RF energy are described in detail elsewhere in this book, only a few of the more basic biophysical concepts and ideas relevant to RF energy are discussed here. Also, many other books, reports, and articles have been published in the last several years dealing with the issue of RF energy and biological effects (e.g., see Gandhi, 1980, 1982, 1987; Cleary, 1983; Elder and Cahill, 1984; Steneck, 1984; NCRP, 1986; Foster and Guy, 1986).

The amount of RF energy absorbed depends on several factors, including frequency (or wavelength), intensity of the RF radiation, and duration of exposure. A well-documented effect of RF radiation on living tissue is thermal heating that can occur when the rate of energy absorption is relatively high. The ability of high levels of RF power to cause heating of biological tissue is the principle by which microwave ovens cook food. The greater the specific absorption rate (SAR), the more likely that significant thermal heating, and possible tissue damage, may occur. SAR is a measure of the rate of energy absorption per unit mass, usually expressed in terms of watts per kilogram (W/kg) or milliwatts per gram (mW/g) (Durney, 1980; Durney et al., 1978; NCRP, 1981).

On a molecular level, the heating effect of RF energy is due to changes in rotational energy imparted to polar molecules, such as water in biological tissue, by RF and microwave photons. The increase in rotational kinetic energy of these molecules is then transferred through friction and collisions to other molecules in the tissue resulting in generalized heating. Biological effects that result from the heating of tissue are commonly referred to as thermal effects.

At relatively low levels of exposure to RF radiation, i.e., exposure to field intensities below those known to produce significant and measurable heating, the evidence for production of harmful biological effects is inconclusive. There have been reports in the scientific literature indicating the occurrence of biological...