Strategy of the scientific committee on occupational exposure limits (SCOEL) in the derivation of occupational exposure limits for carcinogens and mutagens.

Setting standards, such as occupational exposure limits (OELs) for carcinogenic substances must consider modes of action. At the European Union level, the scientific committee on occupational exposure limits (SCOEL) has discussed a number of chemical carcinogens and has issued recommendations. For some carcinogens, health-based OELs were recommended, while quantitative assessments of carcinogenic risks were performed for others. For purposes of setting limits this led to the consideration of the following groups of carcinogens. (A) Non-threshold genotoxic carcinogens; for low-dose assessment of risk, the linear non-threshold (LNT) model appears appropriate. For these chemicals, regulations (risk management) may be based on the ALARA principle ("as low as reasonably achievable"), technical feasibility, and other socio-political considerations. (B) Genotoxic carcinogens, for which the existence of a threshold cannot be sufficiently supported at present. In these cases, the LNT model may be used as a default assumption, based on the scientific uncertainty. (C) Genotoxic carcinogens with a practical threshold, as supported by studies on mechanisms and/or toxicokinetics; health-based exposure limits may be based on an established NOAEL (no observed adverse effect level). (D) Non-genotoxic carcinogens and non-DNA-reactive carcinogens; for these compounds a true ("perfect") threshold is associated with a clearly founded NOAEL. The mechanisms shown by tumour promoters, spindle poisons, topoisomerase II poisons and hormones are typical examples of this category. Health-based OELs are derived for carcinogens of groups C and D, while a risk assessment is carried out for carcinogens of groups A and B. Substantial progress is currently being made in the incorporation of new types of mechanistic data into these regulatory procedures.