The relation between physical and chemical constants and toxicometric indicators has been used as the basis of mathematical simulation both of separate indices of toxicity and preliminary safety exposure levels of the compounds of phenylurea series, thus it becomes possible to apply the derived mathematical links for rapid hygienic norm-setting of new substances of the above group. Multiple regression equations have been derived for calculating LD50, Limac, Limch and MACs of phenylurea in the work zone air. The proposed approach to MAC substantiation provides the opportunity to predict MACs for similar chemical compounds in the work zone air with sufficient accuracy.