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

A scoping review of evaluations of and recommendations for default uncertainty factors in human health risk assessment

Uncertainty factors (UFs) are used to account for uncertainties and variability when setting exposure limits or guidance values. Starting from a proposal of a single UF of 100 to extrapolate from an animal NOAEL to a human acceptable exposure, the aspects of uncertainty and number of UFs have diversified and today there are several risk assessment guidelines that contain schemes of default UFs of varying complexity. In the present work, we scoped the scientific literature on default UFs to map developments regarding recommendations and evaluations of these. We identified 91 publications making recommendations for one or several UFs and 55 publications evaluating UFs without making explicit recommendations about numerical values; these were published between 1954 and 2021. The 2000s was the decade with the largest number of publications, interspecies differences and intraspecies variability being the most frequent topics. The academic sector has been the most active (76 out of 146 publications). Authors from the private sector more often presented UF recommendations, but differences between sectors regarding size of recommendations were not statistically significant. The empirical underpinning of the reviewed recommendations ranges from four to 462 chemicals, that is, relatively low numbers compared with the range of chemicals these default UFs are expected to cover. The recommended UFs have remained remarkably constant, with merely a slight decrease over time. Although chemical specific UFs are preferable, the widespread use of default UFs warrants further attention regarding their empirical and normative basis.

An interdisciplinary framework for derivation of occupational exposure limits

Protecting the health and safety of workers in industrial operations is a top priority. One of the resources used in industry to ensure worker safety is the occupational exposure limit (OEL). OELs are derived from the assessment and interpretation of empirical data from animal and/or human studies. There are various guidelines for the derivation and implementation of OELs globally, with a range of stakeholders (including regulatory bodies, governmental agencies, expert groups and others). The purpose of this manuscript is to supplement existing guidance with learnings from a multidisciplinary team approach within an industry setting. The framework we present is similar in construct to other risk assessment frameworks and includes: (1) problem formulation, (2) literature review, (3) weight of evidence considerations, (4) point of departure selection/derivation, (5) application of assessment factors, and the final step, (6) derivation of the OEL. Within each step are descriptions and examples to consider when incorporating data from various disciplines such as toxicology, epidemiology, and exposure science. This manuscript describes a technical framework by which available data relevant for occupational exposures is compiled, analyzed, and utilized to inform safety threshold derivation applicable to OELs.

Distributions for time, inter‐ and intraspecies extrapolation for deriving occupational exposure limits

This work aimed at improving the empirical database of time (i.e., exposure duration), interspecies and intraspecies extrapolation when deriving occupational exposure limits (OELs). For each extrapolation step, a distribution was derived, which can be used to model the associated uncertainties. For time and interspecies extrapolation, distributions of ratios of dose descriptors were derived from studies of different length or species. National Toxicology Program (NTP) study data were manually assessed, and data from REACH (Registration, Evaluation and Authorisation of Chemicals) registration dossiers were evaluated semi‐automatically. Intraspecies extrapolation was investigated by compiling published studies on human toxicokinetic and toxicodynamic variability. A new database was established for toxicokinetic differences in interindividual susceptibility, including many inhalation studies. Using NTP data produced more reliable results than using REACH data. The geometric mean (GM) for time extrapolation subacute/chronic agreed with previous evaluations (GM = 4.11), whereas the GM for subchronic/chronic extrapolation was slightly higher (GM = 2.93) than the GMs found by others. No significant differences were observed between systemically and locally acting substances. Observed interspecies differences confirmed the suitability of allometric scaling, with the derived distribution describing remaining uncertainty. Distributions of intraspecies variability at the 1% and 5% incidence level had medians of 7.25 and 3.56, respectively. When compared with assessment factors (AFs) currently used in the EU, probabilities that these AFs are protective enough span a wide range from 10% to 95%, depending on the extrapolation step. These results help to select AFs in a transparent and informed way and, by allowing to compare protection levels achieved, to harmonise methods for deriving OELs.

This work aimed at improving the empirical database of time (i.e., exposure duration), interspecies and intraspecies extrapolation. Distributions were derived, which can be used to model the associated uncertainties. When compared with assessment factors (AFs) currently used in the EU, probabilities that these AFs are protective enough span a wide range from 10% to 95%, depending on the extrapolation step. These results help to select AFs in a transparent and informed way and to harmonise methods for deriving OELs.