The Meaning of Risk Assessment

For most chemicals, human risk assessment begins when a significant number of cancers appear in a bioassay in which animals are exposed to the chemical in question. The assumption is made that any substance capable of causing cancer in animals can also cause cancer in humans. However, this attitude, as far as risk assessment is concerned, may become modified by estimates of sensitivity to carcinogenesis for animals versus humans, by the significance of animal tumor types and, possibly, by the development of an operational definition of a threshold. Risk/benefit considerations are appropriate but rest on scanty data. For the present, models for the prediction of human cancer following exposure to known animal carcinogens must be conservative and based on a zero threshold.