The Concept of “Practical Thresholds” in the Derivation of Occupational Exposure Limits for Carcinogens by the Scientific Committee on Occupational Exposure Limits (SCOEL) of the European Union

Abstract

In Europe, there has been a scientific discussion on possible thresholds in chemical carcinogens since the late 1990s. Based on this discussion, the Scientific Committee on Occupational Exposure Limits (SCOEL) of the European Union has discussed a number of chemical carcinogens and has issued recommendations. For some carcinogens, health-based Occupational Exposure Limits (OELs) were recommended, while quantitative assessments of carcinogenic risks were performed for others. For purposes of setting OELs the following groups of carcinogens were adopted: (A) Non-threshold genotoxic carcinogens; for low-dose assessment of risk, the linear non-threshold (LNT) model appears appropriate. For these chemicals, the 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, and the ALARA principle may be applied as well. (C) Genotoxic carcinogens with a practical threshold is supported by studies on mechanisms and/or toxicokinetics; health-based exposure limits may be based on an established no-observed adverse effect level (NOAEL). (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 tumor 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. In order to highlight the most important differentiation between Groups B and C, the basic reasoning is given for the six compounds formaldehyde, vinyl acetate, acrylonitrile, acrylamide, trichloroethylene and methylene chloride.