An overview of the problem of thresholds for chemical carcinogens

Toxicological evaluation in quantitative terms of substances that have been shown to be carcinogenic is very difficult, mainly because, for the great majority of environmental carcinogenic chemicals, the available data, both from experimental studies in animals and from epidemiological studies in humans, on dose-effect relationships, especially at low levels, are very limited. There is an urgent need for research in this area. At present, the most sensible approach is to try to determine no-effect levels by long-term investigation of the effects of low levels in animal species that are most similar to man in their metabolic potential and their sensitivity. Very low safety factors (2500 to 5000) should be applied to the values thus obtained, in order to be as far as possible on the safe side in establishing pragmatically acceptable limits. At present, since all chemical carcinogens pose a hazard, human exposure to them must be reduced to the feasible minimum.