The current discussion of the derivation of Occupational Exposure Limits for carcinogenic chemicals was the reason for putting together this summary of the main lines of development of concepts in this field. In the past, it was the accepted paradigm that there is no dose/concentration threshold for the effects of a compound which had clearly been identified as a carcinogen, and that the carcinogenic risk would only disappear at zero dose. Classical categorization concepts which distinguish between human, experimental and suspected carcinogens were based on this paradigm. In the meantime, much progress has been made in elucidating the multi-step process of chemical carcinogenesis and in characterizing modes of action for a number of important industrial chemicals. The resulting improved differentiation was the reason for the introduction of a new system of five categories of carcinogenic compounds by the German MAK Commission (DFG Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area) in the year 2000. With a very similar intention, the Scientific Committee on Occupational Exposure Limits (SCOEL) has presented a system classifying carcinogens into four groups. Both systems allow for the derivation of health-based Occupational Exposure Limits for carcinogens under certain defined conditions. This is closely associated with the question as to the conditions under which effect thresholds for carcinogens may be demonstrated or assumed and under which quantitative risk assessments should or could be performed for regulation purposes. The entire field is currently subject to intensive discussion and development at the international level. In this context, the situation now prevailing in Germany is discussed.
