Nontarget Analysis Combined with TOP Assay Reveals a Significant Portion of Unknown PFAS Precursors in Firefighting Foams Currently Used in China

Firefighting foam is a significant source of per- and polyfluoroalkyl substances (PFAS) pollution, yet the PFAS profiles in foam formulations, particularly in China, remain unclear. Here, using target and nontarget analyses, we investigated 50 target PFAS in firefighting foams currently utilized in China, identified novel PFAS, and discovered new end products through a total oxidizable precursor (TOP) assay. We identified a total of 54 PFAS compounds (spanning 34 classes and containing seven novel PFAS) with total PFAS concentrations of 0.03-21.21 mM. Among seven novel PFAS, four PFAS met persistence, bioaccumulation, and toxicity criteria, and another PFAS had the highest ToxPi score among the identified 54 PFAS. Moreover, the predominant PFAS varied significantly in the studied foams and differed markedly from those used in other countries. After the TOP assay, nontarget analysis uncovered 1.1-55.5% more PFAS precursors and 8.25-55.5% more fluorine equivalents compared to traditional target analysis combined with TOP assay. Specifically, three double-bond perfluorinated alcohols were identified for the first time as end products of the TOP assay. This study provides crucial information for pollution control and risk assessment associated with PFAS in firefighting foam applications and emphasizes the importance of combining nontarget analysis with TOP assay in uncovering unknown PFAS precursors.