Derivation of occupational exposure limits: Differences in methods and protection levels

Frameworks for deriving occupational exposure limits (OELs) and OEL-analogue values (such as derived-no-effect levels [DNELs]) in various regulatory areas in the EU and at national level in Germany were analysed. Reasons for differences between frameworks and possible means of improving transparency and harmonisation were identified. Differences between assessment factors used for deriving exposure limits proved to be one important reason for diverging numerical values. Distributions for exposure time, interspecies and intraspecies extrapolation were combined by probabilistic methods and compared with default values of assessment factors used in the various OEL frameworks in order to investigate protection levels. In a subchronic inhalation study showing local effects in the respiratory tract, the probability that assessment factors were sufficiently high to protect 99% and 95% of the target population (workers) from adverse effects varied considerably from 9% to 71% and 17% to 87%, respectively, between the frameworks. All steps of the derivation process, including the uncertainty associated with the point of departure (POD), were further analysed with two examples of full probabilistic assessments. It is proposed that benchmark modelling should be the method of choice for deriving PODs and that all OEL frameworks should provide detailed guidance documents and clearly define their protection goals by stating the proportion of the exposed population the OEL aims to cover and the probability with which they intend to provide protection from adverse effects. Harmonisation can be achieved by agreeing on the way to perform the methodological steps for deriving OELs and on common protection goals.