Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
EXECUTIVE SUMMARY HISTORICAL PERSPECTIVE "PAVE PAWS" is the name used by the U.S. Air Force for a fixed-base solid-state radar system, including a radar at Otis Air Force Base, on Cape Cod, Massachusetts. Its primary purpose is to detect and determine attack characteristics of sea-launched ballistic missiles that might penetrate the PAVE PAWS field of view. As a secondary function, PAVE PAWS will support the Air Force Spacetrack System by providing surveillance and tracking of earth satellites and identification of other space objects. An identical radar with similar functions will be constructed at Beale Air Force Base, California. The PAVE PAWS project was conceived as a result of a memorandum issued on November 6, 1972, by the Joint Chiefs of Staff. The Raytheon Company was selected as the primary contractor on April 12, 1976. The project manager for the Air Force Systems Command is the Electronic Systems Division, Hanscom Air Force Base, Bedford, Massachusetts. On March 17, 1976, the Air Force issued an environmental assessment of PAVE PAWS. This document was prepared in August 1975 and later revised. For purposes of this assessment, a power density or incident intensity of 10 milliwatts per square centimeter (mW/cm ) as averaged over any 6-min period was established as a guideline for limiting occupational exposure. The guideline was provided by the U.S. Air Force School of Aerospace Medicine, Brooks Air Force Base, San Antonio, Texas, on the basis of existing Occupational Safety and Health Administration guidelines. Guidelines were not specified for nonoccupational exposure. The Electromagnetic Compatibility Analysis Center (ECAC) contracted with the Illinois Institute of Technology Research Institute for a report on the PAVE PAWS project. This report was issued in May 1976 and updated in July 1978. Its objective was to determine the impact of the proposed PAVE PAWS radar system on the electromagnetic environment at and near Otis Air Force Base. The ECAC, a Department of Defense (DOD) facility, was established to provide advice and assistance on electromagnetic compatibility to the Secretary of Defense, the Joint Chiefs of Staff, the military depart- ments, and other DOD components. On December 22, 1977, the Environmental Protection Agency (EPA) pre- pared an environmental impact analysis of the PAVE PAWS system at the request of Representative Gerry E. Studds, 12th Congressional District, Massachusetts. -1-
The Cape Cod Environmental Coalition, Inc., a citizens group, under- took court action in the U.S. District Court against various Air Force officials on March 3, 1978 (amended April 12, 1978). The main issue was that the Otis Air Force Base PAVE PAWS project was in violation of the National Environmental Policy Act of 1969 (42 U.S.C. 4321 et seq.), because an environmental impact statement (EIS) had not been prepared. On April 11, 1978, Air Force officials announced that they would complete an EIS for the Otis Air Force Base PAVE PAWS system; Stanford Research International was to provide the EIS under contract with the Air Force. The Air Force asked the National Academy of Sciences to perform two studies relevant to the EIS. One would focus on the safety measures in- herent in the engineering design of the radar system and would be prepared by a panel of the National Research Council's Assembly of Engineering. The second, on the extent of human exposure, the subject of this report, was performed by a panel administered by the National Research Council's Assembly of Life Sciences. An Air Force survey team measured the microwave radiation at four locations in front of the south face of the PAVE PAWS radar on August 26, 1978. On October 20-21, 1978, the team measured the microwave radiation at 21 locations when both faces of the PAVE PAWS radar were operating under normal conditions. In all cases, the tests were witnessed by designated independent observers from the surrounding communities. On October 31, 1978, the Cape Cod Environmental Coalition, Inc., and the U.S. Air Force entered into a stipulation agreement, temporarily suspending further litigation brought by some Cape Cod residents against the Air Force in connection with the Otis Air Force Base PAVE PAWS system. Under the agreement, the Air Force is allowed to continue with construction of the facility while it completes an environmental study of the facility. The Coalition and the Air Force have agreed that, in the event of further litigation, the issues to be litigated will be limited to allegations related to the substantive sufficiency of the environmental study. Such allegations must be made by the Coalition during the 45-day public-comment period to be scheduled by the Air Force. CHARACTERISTICS OF PAVE PAWS EXPOSURE CONDITIONS Chapter 1 of this report describes the operating characteristics of PAVE PAWS as they are related to possible exposure effects on humans, as well as the safety measures used to control unwarranted exposure. Radar systems like PAVE PAWS typically radiate beams of energy at microwave frequencies in short pulses of high peak power and scan such that the beam direction is constantly altered, in contrast with FM and TV broad- cast stations, which radiate energy diffusely at a fairly constant power. -2-
The PAVE PAWS radar system pulses ultrahigh-frequency (420-450 mega- hertz, or MHz) nonionizing radiation focused in a plane 3-85° above the horizon with a scanning sector of 120°. The peak radiated power of the radar is 580 kilowatts (kW), and the time-averaged power of transmitted energy is approximately 145 kW. The time-averaged power of the PAVE PAWS radar is thus approximately 3 times the average power transmitted by a large TV broadcast station and somewhat more than the average power of a large FM broadcast station. Because the beam is directed above the horizon, people at ground level may be exposed to "sidelobes" of the main beam, but not to the main beam itself. Sidelobes are secondary beams of much lower intensity than the main beam. Inasmuch as there can be many sidelobes of the main beam, potential exposures of members of the public would be to a time-varying field at a substantially lower power than that of the main beam. Microwave exposure of humans could be greater in the event of failure of the radar system to operate in the mode described above. For example, increased exposure could result if a well-formed beam of the radar were directed at an angle less than 3° above the horizon, if a beam were poorly formed and had more intense sidelobes, if excessive power were transmitted, or if the beam remained in a fixed position, rather than scanning. The safety measures in the engineering design of the radar to prevent such occurrences are described in Chapter 1. EXTENT OF RADIATION AND POPULATION EXPOSED In December 1977, power densities that could result from PAVE PAWS operation were calculated by the EPA in the preparation of an environ- mental impact analysis. In 1978, the Air Force measured power densities at various points within, at, and up to 5 miles (8 km) beyond the "exclusion" fence, which is 1,000 ft (305 m) from the radar; and the National Bureau of Standards reviewed data from the Air Force's measurements and reviewed the measurement techniques (as described in Chapter 1). In general, it was found that, at the exclusion fence, microwave power densities averaged about 5 microwatts per square centimeter (yW/cm ), with decreasing densities at increasing distances from the radar. At the loca- tion where members of the public would most likely be exposedâ3,450 ft (1,050 m) from the radar on Highway Route 6âthe measured intensity was 0.06yW/cm . Exposure would, in general, be for various periods, depending on whether those exposed were stationary or mobile. (Time-averaged field intensity is used here and throughout the report, unless otherwise stated; see glossary.) Data from the latest (1970) U.S. Bureau of the Census Master Enumeration District List with Coordinates permitted estimation of the number of people living in areas 5-10 miles (8-16 km) away from the radar; in 1970, 1,239 persons lived within 5 miles of the PAVE PAWS site; 11,235 within 10 miles; and 63,289 within 20 miles (32 km), with higher populations during the -3-
vacation season. Although the data cannot be confidently used for distances of less than 5 miles from PAVE PAWS, they indicate that there is little or no habitation within a mile of the radar site. The Air Force measurements indicated that power density does not exceed 0.1 yW/cm beyond about a mile from the radar site. GENERAL ENVIRONMENTAL EXPOSURE The population of the United States is routinely exposed to radio- frequency (RF) radiation, including microwaves whose frequency is approx- imately 30 MHz to 300 gigahertz (GHz). Measurements made by the EPA indicate that such exposure is due mainly to energy transmitted by AM and FM radio and vhf and uhf TV stations. Only a small part of the total radiation environment is attributable to such sources as military and civilian radar, satellite communication systems, telephone and television communication, and microwave ovens. On the basis of EPA studies in 12 large U.S. cities, it has been estimated that less than 1% of the population in these cities is exposed to RF radiation intensities of over 1 yW/cm continuously. Power densities near FM antennas and on the upper floors of tall buildings may, however, be hundreds of microwatts per square centimeter. The EPA defines a high-power source as one in which the power density of the main beam is 100 yW/cm at a distance of 100 m from the antenna. Broadcast transmitters for FM radio and TV are in this category, as are radar and communication systems, such as satellite-communication earth terminals. Although the sources of RF radiation may operate at high power output, the probability of exposure to such high-intensity fields is low, because, with the exception of broadcast transmitters, high-power sources, such as radar, use directional antennas to achieve high effective power of radiated energy. The resulting beam is unlikely to irradiate humans for sustained periods, because the beam is normally so directed as to avoid human exposure and many of the sources are remote and surrounded by ex- clusion areas that further reduce the probability of exposure to high- intensity fields. The duration of exposure to radiation from such sources is limited, in some cases, because of the nonstationary scanning mode of the beam. In general, exposure of humans to radiation from most high-power sources is estimated to be at less than 50 yW/cm (time-averaged) at or near ground level more than 0.5 mile (0.8 km) from the source. Instantaneous peak intensities can be orders of magnitude higher for pulses-modulated radar. -4-
BIOLOGIC CONSIDERATIONS Chapter 2 discusses the biologic characteristics related to exposure to RF radiation and outlines the physical factors pertinent to measurement of ambient fields. Calculations of a "probable worst-case" (predicted maximal) exposure to PAVE PAWS radiation are summarized. The physical factors include the orientation of the irradiated subject relative to the electric field vector; whether the irradiated subject is in free space or in an environment where reflection may increase the field intensity; whether the energy absorption is averaged over the entire body or over local areas, such as the head or particularly absorptive areas of the body or head; and whether the irradiated subject is free-standing, grounded, or near objects that can reflect or perturb the field. Calculations made in Chapter 2 indicate that the maximal rate of energy absorption at a "hot spot" in the head of an average adult in a building with unscreened windows, where reflections may increase the intensity of the field emanating from the PAVE PAWS radar, could be as high as 0.66 mW/g, for an incident free-space power density of 0.1 mW/cm . For purposes of comparison, normal average metabolic rates are near 3 mW/g for the whole body of an adult walking slowly and 11 mW/g in the brain. The remainder of Chapter 2 summarizes the state of knowledge concerning biologic effects of electromagnetic radiation. Highlights are presented in the following paragraphs. The frequencies used by the PAVE PAWS system (420-450 MHz) are in the microwave portion of the electromagnetic radiation (EMR) spectrum. It is difficult to assess the effects of exposure to such radiation, because of the lack of an adequate data base, particularly with respect to the effects of chronic or long-term exposure at low radiation intensities and because of the difficulty in determining the degree and manner in which the radiation is absorbed by biologic systems. Radiation from the PAVE PAWS system is nonionizing. It differs from ionizing radiation, such as x rays, in that it is characterized by much lower photon energies and cannot produce damage by molecular disruption in biologic matter at low field intensities. Nonionizing radiation at high intensities can produce excessive tissue-heating, whereas at low intensities it may induce reversible effects that are not known to be hazardous in biologic systems. (In this report, 1 mW/cm has been selected as the arbitrary boundary between highâintensity and low-intensity effects. The figure has no relationship to safe or unsafe exposure.) The available theoretical and experimental data relative to the power densities anticipated in areas of public access in the vicinity of PAVE PAWS suggest that effects in humans will be restricted to low-intensity effects, because flux densities of incident energy will apparently be below 1 mW/cm . -5-
The Panel wishes to emphasize that the low-intensity biologic effects described in this report refer to measurable biologic alterations. Physical and chemical agents may cause alterations that are not necessarily health hazards. For example, humans perceive light and sound waves (stimuli that result in measurable biologic effects) that are not health hazards unless the light is strong enough to damage the eye or the sound is loud enough to damage the ear. The detection and measurement of such effects may, in fact, lead to a better understanding of the mechanisms by which the effects occur. In this report, the Panel has attempted to differentiate between effects of scientific interest but undetermined clinical significance and effects that may represent a potential hazard to human health. In general, the primary reported effects of human exposure in occupa- tional settings in which low-intensity microwaves are present are psycho- logic alterationsâi.e., changes in mood or attitudeâthat suggest reversible alterations of the central nervous system. Thus, greater attention has been given to effects on the central nervous system and other nervous tissues, especially in the light of experimental evidence of power-density and modulation-frequency "windows" within which such effects appear more likely. Data from experiments on biologic systems indicate that exposure to low-intensity microwaves can have effects. But, on the basis of most of the available findings, the known or suspected effects are reversible and are not associated with increased human morbidity or mortality. Chapter 2 reviews in detail the data obtained from studies of the effects of microwave and RF radiation, primarily on experimental animals exposed to low-intensity fields, and discusses the human health effects of occupational exposure to low-intensity nonionizing electromagnetic fields. Assessment of data derived primarily from epidemiologic studies of occupationally exposed persons is difficult, because of the uncertainty in the degree or type of exposure. Particular attention is given to reported effects at exposures of 1 mW/cra and lower, inasmuch as PAVE PAWS may be expected to produce accessible radiation well below this. In conclusion, the PAVE PAWS radar may be anticipated to expose a limited number of members of the general public intermittently to low intensities of pulse-modulated microwave fields with maximal instantaneous intensities of 100 yW/cm or less and time-averaged intensities lower by two orders of magnitude. There are no known irreversible effects of such exposure on either morbidity or mortality in humans or other species. Thus, it is improbable that exposure will present any hazard to the public. In view of the known sensitivity of the mammalian central nervous system to electromagnetic fields, especially those modulated at brainwave frequencies, the possibility cannot be ruled out that exposure to PAVE PAWS radiation may have some effects on exposed people. Because these effects are still hypothetical, it is not feasible to assess their health implications. Such assessment will require additional research and surveillance and must be addressed in future evaluations of the potential exposure effects of PAVE PAWS and other high-power-output radar systems. -6-
