A hazard classification system based on incorporation of REACH regulation thresholds in the USEtox method

REACH regulation classifies persistent, bioaccumulative, and toxic (PBT) substances based on regulatory thresholds established for selected physicochemical and toxicity properties. The underlying ideas behind this hazard classification system are: i) that is essentially dependent on the individual value of a restricted number of the substance properties, ii) the combinatory effect of the different substances properties is not relevant, iii) the regulatory thresholds established for the different substance properties are equivalent. These ideas have never been verified before, so the main aim of this paper is to do a critical appraisal of the REACH PBT classification. For that purpose, a conceptual framework is developed based on the use of the USEtox model to calculate characterization factors (CFs) associated with virtual substances (VS): hypothetical substances whose properties are based on the thresholds established to PBT substances. The analysis of the CFs calculated for the VS suggested that the essential ideas from the PBT classification are not verified, and important aspects that can affect the identification of hazardous substances are missing: a) the regulatory thresholds are not all equivalent; b) the combinatorial effect of the different physicochemical and toxicity properties is not negligible, so the hazard potential of a substance cannot be evaluated based on the value of individual properties; c) other properties not included in the PBT classification are as relevant as others currently included to evaluate the potential hazard on human health. A hazard classification system based on the use of the CFs of the virtual substances as a hazardous reference is proposed to help perform a preliminary screening, which can be integrated with other criteria to facilitate the identification of PBT chemicals.