The stages of norm-setting for harmful substances in the air of the working environment in the USSR and on the international scale are expounded. Principal terms and concepts are mentioned, such as maximum admissible concentration, orientative safe level of effect, harmful substance, working environment, threshold of harmful effect, adaptation and compensation, medium lethal dose and concentration, zone of acute and chronic effect, as well as the classification of toxicity and dangerousness, conditions of experiment for the substantiation of MAC and some other data discussed by the member countries of CMEA at the 1st Conference on the problem of "hygienic norm-setting of chemical factors of the industrial environment. Unification of the methods of determining MAC in the air of the working environment and of the methods of control".