Levels in the Environment
The artificial EMF environment of the U.S. is a superimposition of contributions from many sources having diverse operating characteristics. They include high- and low-power emitters that can be omnidirectional or directional, and that can operate continuously or intermittently. At high frequencies, the general EMF background consists predominantly of the AM radio band (0.535-1.604 MHz) and the FM and TV band (54-806 MHz). About half the U.S. population is exposed to these sources at levels above 0.005 µW/cm2 at any given moment, and about I% is exposed above 1 µW/cm2 (Fig. 10.1)(1). The actual number of people exposed above 1 µW/cm2 in any given day, week, or month is considerably greater because of population mobility.
Fig. 10.1 Population of some U.S. cities exposed to radio and TV signals above (the level considered safe in the U.S.S.R.). Nationwide, the total population exposed above 1 µW/cm2 at any given time is about 2 million (1).
EMFs emanating from the electrical power system (60 Hz in the U.S., 50 Hz in Europe and the U.S.S.R.) constitute most of the artificial low-frequency electromagnetic background. They are extremely pervasive; except for remote areas such as forests, it is difficult to find places where the electric and magnetic fields are less than 0.1v/m and 100 µgauss respectively. But even these fields are several orders of magnitude greater than the naturally present 60-Hz fields. The average man-made back-ground electric field is probably in the order of 1 v/m (2), and the average background magnetic field is about 800-900 µgauss (3).
EMFs much greater than the background are found in the vicinity of specific sources. The power density at various distances from a typical 50,000-watt AM radio station is shown in table 10.2 (4); within a radius of about 3,280 feet, the level does not decrease below 1 µW/cm2. FM radio stations vary considerably in strength and antenna design, but it has been estimated that 193 of 2750 such stations in the U.S. could have levels exceeding 1000 µW/cm2 within 200 feet of the antenna (5). In large urban areas, the elevation necessary for transmission of radio and TV signals is sometimes attained by mounting the antenna atop a tall building. This produces high EMF levels in nearby buildings (Table 10.3) (12). When radio and TV antennas are grouped, they produce relatively intense EMF levels over broad areas. Mount Wilson, California, for example, has 27 radio and TV antennas serving the Los Angeles area (Fig. 10.2). This produces EMFs of 720-1200 µW/cm2 in the backyard of the post office on Mount Wilson, and 120-840 µW/cm2 inside the post office (6). The Sentinel Heights area south of Syracuse, New York, contains about a dozen transmitters and they result in essentially ambient levels of about 1 µW/cm2 throughout an area of several square miles (7).
Table 10.2. POWER DENSITY AT VARIOUS DISTANCES FROM A 50,000 WATT AM RADIO STATION
Table 10.3. EMF IN TYPlCAL TALL BUlLDlNGS
Fig. 10.2. The antenna farm at Mt. Wilson, California. The complex consists of 27 antennas that radiate approximately 10 MW, thereby producing ground-level power densities of up to 28,000 µW/cm2. (Reproduced by permission, from ref. 6.)
The average contribution of high-power radars to the urban EMF environment is low because their beams are directed away from population centers. But, because of stray radiation, exposure levels near airports and military bases can be in the range of 10-100 µW/cm2 at distances up to one-half mile (8). Airborne radar makes a further contribution to the airport EMF environment.
Microwave-relay antennas, located at intervals of about 20 miles, are used for long-distance telephone service and for private communications. A 10-foot diameter antenna positioned 100 feet above the ground produces ground-level EMFs of approximately 0.03-7.5 µW/cm2 within 376 feet of the tower (9). There are several thousand microwave-relay towers in the U.S., each with two or more antennas.
Mobile communications equipment and hand-held walkie-talkies are relatively low-power sources, but they account for significant exposure levels because the radiating antenna is ordinarily close to the user. Figure 10.3 depicts the power densities in the head area that arise from a typical walkie-talkie (10). Figure 10.4 gives the power densities inside and outside a truck with a roof-mounted antenna (10).
Fig. 10.3. Power density (µW/cm2) in the area of the head of a Motorola HT-220 walkie-talkie operating at 165.45 MHz with an output of 1.8 W (10). The measurements were made in the near field where the plane wave relation between the electric and magnetic fields does not strictly apply: the listed values are an upper limit for the actual power densities. The same comment applies to Figure 10.4.
Fig. 10.4. Power density (µW/cm2) inside and outside a truck arising from a roof-mounted 100w transmitter operating at 41.31 MHz (10).
The 60-Hz electric and magnetic fields associated with typical household appliances are listed in tables 10.4 and 10.5 respectively (13). There are about 500,000 miles of high-voltage power lines in the U.S., and they produce fields that depend principally on the line's voltage, current, and geometry. The ground-level electric field at various distances from typical high-voltage power lines is shown in Figure 10.5. Ground-level magnetic fields from high-voltage power lines are generally in the range 0.1-1 gauss within 150 feet of the line.
Table 10.4. POWER-FREQUENCY ELECTRIC FIELDS OF HOUSEHOLD APPLIANCES MEASURED AT A DISTANCE OF ONE FOOT
Table 10.5. POWER-FREQUENCY MAGNETIC FIELDS OF HOUSEHOLD APPLIANCES
Low-frequency environmental EMFs are also produced by many other man-made sources. Weapons- and theft-detection systems, for example, produce magnetic fields of 1-2 gauss, 100-10,000 Hz. But not all manmade EMFs are produced by design: it has recently been found, for example, that the starting and stopping of trains in the Bay Area Rapid Transit System in California produced low-frequency EMFs throughout the entire San Francisco Bay Area (11).
Fig. 10.5. Ground-level electric fields of typical high-voltage power lines. a, 115 kv; b, 230 kv; c, 345 kv; d, 500 kv; e, 765 kv.