Grouping persistent and mobile substances to expedite assessments and avoid regrettable substitution

There is an interest by regulators and scientists working with chemical hazard assessment to transition from a one-substance-at-a-time approach to a substance-group approach based on based on structural similarity. Two main motivations for this are to expedite hazard assessments related to the large amounts of substances being introduced to the global chemical market, and avoiding regrettable substitution caused by drop-in substitution. The idea of substance grouping based on the relationship between chemical structure and hazard is not new. Many of the very first hazardous organic substances to be regulated as pollutants were groups sharing a similar structure, including DDT-derivatives, dioxins, PAHs, to name a few. This presentation will provide an overview of grouping hazardous substances based on chemical structure. In addition, we will also present our initial work on substance grouping in relation to persistent and mobile substances, as previously they have not received much attention. This work will be particularly relevant to identify groups substances with either a similar chemical structure, or common transformation products, that are considered persistent, mobile and toxic (PMT) or very persistent and very mobile (vPvM) (Arp and Hale, 2021). Tools are needed now to better assess and regulate them to protect drinking water sources. Grouping persistent and mobiles substances based on specific molecular substructures will support PMT/vPvM substance prioritization and directed risk assessment procedures.