absorbed by the tissues. Anything introduced into the microwave radiation field tends to distort the field pattern in a generally unpredictable manner. The field strength as measured by a fine probe at a particular location is quite changed when the human subject replaces the probe, with a standing wave in space on the illuminated side of the man and pronounced shadows extending to a considerable distance on the opposite side. These perturbations occur at the external tissue-air interface; similar effects can be envisaged at internal tissue-air and soft tissue-bone interfaces. The problems of environmental monitoring, personal dosimetry, and indeed depthdosimetry, are immediately apparent. BIOLOGICAL EFFECTS: THERMAL Exposure of various species of animals to whole body microwave radiation at levels of 100 mW/cm2 or more is characterized by a temperature rise which is a function of the thermal regulatory process and active adaptation of the animal. The end result is either reversible or irreversible change depending on the conditions of the irradiation and the physiological state of the animal. In areas of the body in which relatively little blood circulates, the temperature may rise more rapidly than in vascular parts of the body. Thus, the lens of the eye may be more susceptible to thermal damage, and cataractogenesis from microwave radiation has been produced experimentally many times, well reviewed by Mitchell (5) and Carpenter et al. (6). The threshold for irreversible changes from a single long exposure (up to 1 h) depends on frequency. For 2-10 GHz, the lowest reported threshold in published studies is about 150 ± 20 mW/cm2. Cumulative effects of repeated exposures result in cataracts only when the exposure power density is at least 2/3 of the single exposure value. However, an absolute power density threshold cannot be considered as established for there are no data available on very long-term continuous exposure at low power densities. Human studies by Zaret et al. (7) add little to our knowledge of microwave radiation cataractogenesis. None of the cataracts reported can be attributed conclusively to microwave radiation although the possibility of an association cannot be questioned. The dose levels with which clinically significant cataracts have been associated have been generally high and point to a threshold of at least 100 mW/cm2. Retrospective studies of microwave workers have provided only a finding of clinically insignificant opacities, possibly representing an ageing effect—so the vast majority of those occupationally exposed do not develop cataract. BIOLOGICAL EFFECTS: NONTHERMAL The Eastern European countries have paid considerable attention to the so-called nonthermal effects of microwave radiation. It is claimed that many biological effects occur at levels of incident power of less than 10 mW/cm2. Effects such as headaches, sleeplessness, irritability, fatigue, memory loss, cardiovascular changes, and disturbances in circadian rhythm are described (8). There has been considerable controversy and scepticism in the West about the health hazard of prolonged low intensity exposures and the validity of the observations. There are several reasons for this. The complaints of the occupationally exposed workers are primarily amorphous and neurasthenic, and nonspecific in that
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