The influence and impact of human relevance in (CLP) classification for human health hazard endpoints

Relevance to human health is the importance of factors affecting the well-being and health outcomes of individuals or populations. In toxicology, it concerns whether, and how, hazardous effects seen in test animals may also affect humans. This paper illustrates how human relevance is used in classifying cancer and reproductive toxicity hazards under the EU CLP Regulation, noting that the Regulation does not clearly define the concept. Different hazard endpoints use varying terminology and evidentiary standards, and ECHA's CLP Guidance does not always match the Regulation's wording. Different assessors can also reach different conclusions from the same data. These factors create uncertainty about how human relevance should be interpreted and applied. To improve consistency with the CLP Regulation's aim of protecting workers and consumers, a clear definition and more practical interpretation of human relevance are needed. Human relevance is not binary and is inherently subjective, so both qualitative and quantitative approaches should be combined. The threshold for human relevance should not be so low or vague that classification for human health would only rarely be considered unnecessary.