they can be produced by stresses other than microwave radiation. For the most part, the disturbances lie within clinical norms and tolerances, and are reversible—in Western eyes a reversible effect is not significant (9). This latter statement may require qualification as the repeated causation of a reversible effect may well be regarded as significant in the future. With an increasing dialogue between East and West on microwave radiation experiment design and dosimetry, there is a greater appreciation of the postulated behavioural effects. No doubt there will be further investigation of the effects of low intensity exposure, but for the moment there is no scientific evidence to require a tightening of present exposure standards. STANDARDS The purpose of protection standards is to give guidance on safe exposure levels, and in specifying these it is necessary that the degree of risk shall be acceptable scientifically and socially. As is now widely appreciated, world opinion on safe exposure levels for microwave radiation ranges over three to four orders of magnitude, and different countries are obviously applying very different criteria—the standard dilemma. The US and Western Europe, generally, have adopted 10 mW/cm2 for both public and occupational continuous exposure, with higher levels permissible occupationally by various formulae for short-term or intermittent exposures. This is based on physiological considerations, notably the heat balance of man. The most restrictive standards are those of the Soviet Union which allow only 0.01 mW/cm2 for continuous occupational exposure up to 8 h a day and 1 gW/cm2 for long term public exposure. These standards have arisen from consideration both of animal experiments and of clinical symptoms developed by workers—the asthenic syndrome already described. A few countries have protection standards between these extremes. The Polish standard, for instance, permits unlimited public exposure to 0.01 mW/cm2 and a continuous occupational exposure to 0.2 mW/cm2. Baranski and Czerski (10) have distinguished between three basic principles which may be applied in determining safe limits for protection standards. In summary these are 1. The principle of “zero” interaction; no demonstrable effects. 2. The principle of maximum comfort; discernible effects but no change in functional efficiency. 3. The principle of the limit of physiological compensation; exposure results in some stress, but only within the limits of normal physiological compensation. These authors also state another distinct requirement, which must be considered separately and is that the exposure must not induce any genetic effects. They believe that their Polish occupational standard lies between the second and third principles. It is clear from statements by national standards writers that the Soviet Union subscribes to the first principle and the United States to the third. This goes some way in explanation of the large differences between national standards. Previously Canada has agreed to the Western standard of 10 mW/cm2. The newly introduced standard requires - for occupational exposure, 10 MHz to less than 1 GHz, a limit of 1 mW/cm2 - for occupational exposure, 1 GHz to 300 GHz, a limit of 5 mW/cm2 - for the general public, 10 MHz to 300 GHz, a limit of 1 mW/cm2.
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