0191-9067/81/010103-06$02.00/0 1981 SUNSAT Energy Council MICROWAVE RADIATION: BIOLOGICAL EFFECTS AND EXPOSURE STANDARDS I. R. LINDSAY Royal Air Force Institute of Naval Medicine Alverstoke Hants PO12 2DL, United Kingdom Abstract — The thermal effects of microwave radiation are well recognised and are discussed with particular reference to cataractogenesis. None of the human cataracts reported can be attributed conclusively to microwave radiation, although the possibility of an association cannot be questioned. Postulated nonthermal effects comprise an asthenic syndrome. and for the most part the disturbances lie within clinical norms and tolerances, and are reversible. World opinion on safe exposure levels for microwave radiation is varied, and this had led to national standards disparate by three to four orders of magnitude. The US and UK exposure standard of 10 mW/cm2 was determined over two decades ago; the possibility of a change to a more restrictive level, in line with other countries, in the near future is examined. It is concluded that such a change, without scientific rationale, is not justified. Some biological implications of the microwave radiation from the solar power satellite are considered in terms of precautions to be taken by personnel working in the vicinity of the rectenna, effects on cardiac pacemakers, and any potential effects on birds. INTRODUCTION Microwave radiation falls in the band of frequencies from 30 MHz to 300 GHz (1000 MHz= 1 GHz), overlapping, at the ends, the radio and infrared portions of the electromagnetic spectrum. When microwave radiation impinges on tissue, any one or any combination of three interactions may occur; it may be reflected, it may be absorbed, or it may pass completely through. The actual effect depends on the frequency of the radiation, the orientation, and composition and thickness of the body of tissue. The depth of penetration decreases with increasing frequency, with absorption primarily in the skin in man for frequencies greater than 3 GHz. When microwave radiation is absorbed the energy is transformed into increased kinetic energy of the absorbing molecules which, by increased oscillation and collision with adjacent molecules, produces a general heating of the entire medium. A complete description of microwave radiation interaction with tissue is not possible in this review and papers by Schwan (1) and Johnson and Guy (2) are recommended for further reading. DOSIMETRY Both Michaelson (3) and Cleary (4) emphasize that one of the chief difficulties in working with microwave radiation is to determine how much of the incident energy is
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