Chemical carcinogens in the human environment; problems and quantitative aspects.

A short review on environmental carcinogenesis by chemicals is given. Problems related with the determination of carcinogenicity of chemicals are mentioned. Some critical factors in the monitoring of environmental carcinogens are discussed in relation to a quantification of human exposure to such compounds. An attempt at risk evaluation is made for polycyclic aromatic hydrocarbons, vinyl chloride, N-nitroso compounds and aflatoxins. It is concluded that there is a "threshold dose" for individual carcinogens that does not lead to tumor induction within the normal life span. The probability of multifactorial cause of human cancer by environmental carcinogens, however, does not allow the definition of "safe" levels of such compounds in view of syncarcinogenic and other enhancing and/or modifying effects.