The various mathematical models of the dose‐response relationship are briefly reviewed and it is concluded that the Weibull and gamma‐multihit models are at present the best ones. They permit the computation of a ‘virtually safe dose’, for which the carcinogenic risk to man is reasonably negligible, but because of the unknown mechanism of the background response, the models should be used in a mixed form. A modified Crouch and Wilson approach is proposed to evaluate the risk from an actual environmental dose. This takes into account the various uncertainties in risk computations, and results in the application of a safety factor when extrapolating from animals to man.