The question of what constitutes a hazardous level of something—for example, radiation or a chemical that is carcinogenic or mutagenic—is not easily answered. In part this is because years may pass before the consequences become identifiable. Even for materials with a more immediate effect, establishment of toxic levels may be problematic if the effects are not highly specific. Moreover, individuals vary in response, and the same responses may be due to alternate causes. Data on hazardous exposure levels could be obtained from epidemiological studies on large groups of people, but this would require the exposure of people to conditions known to be harmful. Experimental studies must use other species than humans, with the accompanying problems of species specificity because of differing metabolisms, for example. (With our current knowledge, computer modeling or extrapolation from tests on simple organisms is not sufficient, although such exercises may be helpful.) To make the scale of such experiments reasonable, usually small numbers of individuals are tested at high levels of dose, and results extrapolated to lower levels to get an estimate of risk for low levels of exposure. Past practice has generally been to use a linear extrapolation, implicitly assuming that there is zero effect only

at zero concentration or exposure. Hence we had in the United States the Delaney clause prohibiting any detectable level of a carcinogenic additive in any food. There is abundant evidence, however, that organisms are often able to avoid or repair damage from substances up to a limit; that is, a threshold exists for many harmful effects, though that limit may be difficult to establish.

Even when hazardous levels can be identified, these are statistical; not all members of the exposed population will react the same way. Regulations designed to protect the public from exposure to harmful levels of substances involve some level of balance between cost and risk, although this practice may not be acknowledged explicitly at the time. Intelligent decisions about what this level should be involve many considerations, are often controversial, and are beyond the scope of this book.

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