Risk Evaluation of Environmental Carcinogens

Quantitative risk evaluation of environmental carcinogens is required not only for their regulation but also for primary cancer prevention. Risk evaluation of carcinogens is carried out through identification of carcinogenicity, exposure assessment, dose-response assessment, and risk characterization. Exposure assessment is made by measuring the amounts of carcinogens in the environment and their human intake is estimated. Dose-response assessment is based upon whole-life carcinogenesis experiments in animals with multiple doses of carcinogens. By downward extrapolation of the dose-response curve obtained, a virtually safe dose (VSD), i.e., a dose which yields a cancer incidence of 10^-6 , is calculated and this value is compared with the actual human intake of the carcinogen concerned. When the human intake is around or less than the VSD, that environmental carcinogen is regarded safe to humans because humans as well as experimental animals develop cancer in almost 100% of cases during their whole life and the contribution of a chemical to the occurrence of cancer at an incidence of less than one out of one million is regarded as meaningless. These procedures for risk evaluation of carcinogenic food additives such as butylated hydroxyanisole and saccharin sodium showed that their human intake is one hundredth or much less than the respective VSD values, and this may be the case with most environmental carcinogens except some carcinogenic medical drugs. Human cancer development seems much more influenced by life-style factors, such as a high-fat diet or cigarette smoking, and recognition of this fact is crucial for primary cancer prevention.