Use of human data for the classification of skin sensitization hazard and potency
Since the 1940s, human predictive patch tests (HPPTs) have been used to study chemicals that can cause skin sensitization, which occurs when the immune system overreacts to certain substances after skin exposure. These tests involve applying a chemical to healthy volunteers to observe reactions. Using a previously compiled database of 2,277 HPPT results for 1,366 unique chemicals (Strickland et al. 2024), scientists from CPSC, FDA, NICEATM, and the German Federal Institute of Risk Assessment conducted an analysis (Herzler et al. 2024) focused on how HPPT data can help classify chemicals as skin sensitizers under the United Nations Globally Harmonized System for Classification and Labelling of Chemicals. They suggested that the standard dose per skin area metric could be refined to include improved descriptors, such as the dose per skin area that causes sensitization in one person or 5% of tested individuals. These new measures could provide more accurate assessments of the risks posed by chemicals.
While HPPT data alone are not always enough for clear classifications, their use in combination with data from other sources, such as the local lymph node assay, improves assessment accuracy. The study also introduced standardized workflows for analyzing individual HPPT results, combining multiple results, and incorporating them into a broader weight-of-evidence approach with local lymph node assay data. Overall, this research demonstrates that while HPPT data have limitations, their use remains a valuable tool for chemical classification when combined with other methods. Integration of these data into modern approaches enhances reliability and reproducibility, paving the way for safer chemical regulations and improved skin sensitization assessments.