in the rectenna area, and then protective clothing would be required for any entry. The UK does not manufacture a protective suit, but there is an American suit made of material giving a 45 dB attenuation. What if an anomalously high power density was produced by chance at the centre of the beam? It is generally agreed that for thermal effects, both whole body exposure and local to the eye, there is a threshold power density of 100 mW/cm2 over a period up to one hour. An anomalously high power density of up to 100 mW/cm2 in the area of the rectenna therefore would appear to be nonhazardous to the microwave radiation worker without protective clothing—and at 2.45 GHz the sensation of heating would warn him of his exposure long before the hour was up. Because of arcing problems there is a practical limitation of 200 mW/cm2 exposure when wearing the American suit, so perhaps this is the level that should be quoted for the protected worker. Certainly working in a suit at an exposure of 200 mW/cm2, a permissible level of 1 mW/cm2 to the body would not be exceeded. What if the beam moves from its nominal position to inhabited areas; how long could a civilian population be exposed? Under the current standard of 10 mW/cm2, an answer of 10 min from start of exposure to completion of evacuation or switch off is suggested. This is derived from the formula, i.e., at 23 mW/cm2 it is the allowable occupational time of 11.3 min in any hour rounded off; this gives the considerable safety factor of a once only emergency exposure of the public at a level that is permissible for repeated occupational exposure. This can be defended, in the aftermath, on the basis of experience of microwave radiation worker exposure. The implications of a more restrictive occupational standard are obvious. PACEMAKERS The interaction of microwave electromagnetic radiation fields with cardiac pacemakers represents an unique indirect biological effect (11). This results primarily from the fact that current pacemaker interference thresholds begin as low as 10 V/m. The E field level of a cw 10 mW/cm2 power density is about 200 V/m, but it can be much higher for a low duty cycle pulsed source having the same 10 mW/cm2 average power density. Microwave radiation emitters, such as surveillance radars, can cause many pacemakers to miss single beats as the radar beam scans past, an effect most likely unnoticed. Even more serious interference may not be identified because, most often, interaction times are short in that the source of interference is moved or turned off, or the user moves from the particular area of the effect. It is a recognised problem—and microwave radiation screening is built into pacemakers. Reports on testing of pacemakers concentrate on pulsed microwave effects. With regard to the solar power satellite 2.45 GHz cw, guidelines can be found by extrapolating from microwave ovens, 2.45 GHz cw. These equipments are required to meet a leakage test of not more than 5 mW/cm2 at 5 cm or more from any surface; personnel with pacemakers are advised to stay outwith a radius of 25 m from the oven. BIRDS The use of radar as a tool in ornithological research is treated in Eastwood’s book Radar Ornithology (12). The validity of radar research on birds depends almost
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