Approaches to the formulation of standards for carcinogenic substances in the environment.

After having agreed that standards are necessary for carcinogens that cannot be completely eliminated from the environment, two exchange groups in the U.S.S.R.-U.S. Cooperative present their different approaches to the problem. The Russian groups has recommended a benzypyrene standard of 0.1 microgram/100m3 of atmospheric air over populated regions and gives its experimental basis and theoretical rationale in the first part of this joint paper. Lifetime experiments in adult rats over a wide range of dose levels permit the determination of a largest ineffective dose level with respect the theoretical time of first tumor as well as incidence. The standard is set by extrapolation based on body weight and uses a safety factor of 10 to account for the additional susceptibility in embryogenesis and childhood. The U. S. group presents a mathematical model of time-to-tumor occurrence which permits the prediction of population incidence and life span shortening from time-to-tumor data in animals or man. It assumes the distribution of mortality-corrected time to tumor is lognormal with the nth power of time inversely proportional to dose and with dose independence of the variability of the logarithm of time to tumor. The prediction is made by combining this distribution, fitted to the data, with population mortality tables. Both groups emphasize that substantial research efforts are necessary to improve the scientific basis for setting standards.