Global Occurrence and Ecological Risk Assessment of 6PPD and 6PPD-Quinone: Derivation of PNECs Using Species Sensitivity Distribution

Tire antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and its transformation product 6PPD-quinone (6PPD-Q) have recently been recognized as emerging contaminants of global concern due to their widespread occurrence and high toxicity to aquatic organisms. Although the rapidly accumulating monitoring and toxicological data suggesting substantial ecological risks, a quantitative and cross-species assessment framework remains lacking. Here, we compiled 77 reported concentration records of 6PPD and 6PPD-Q from 21 studies worldwide and assembled 991 toxicity endpoints across 34 species to derive Predicted No-Effect Concentrations (PNECs) by species sensitivity distributions (SSDs) and probabilistic ecological risk estimates. Both compounds were ubiquitously detected in surface waters, with 6PPD-Q frequently exceeding the concentration of its parent compound. Salmonids exhibited exceptional sensitivity to 6PPD-Q, with lethal thresholds in the nanogram-per-liter range, whereas tolerance varied markedly among non-salmonid taxa. Model-averaged SSDs yielded mortality-based hazardous concentrations for 5% of species (HC5) of 24.3 µg/L for 6PPD and 0.0559 µg/L for 6PPD-Q, corresponding to PNECs of 4.87 and 0.0112 µg/L, respectively. Probabilistic risk characterization indicated negligible global risk for 6PPD, whereas 6PPD-Q exhibited elevated risk potential, with mortality-based overall risk probabilities reaching 11.4%. Risk levels followed the pattern of surface runoff > river waters > wastewater effluent, and were higher in North America and Europe than in Asia. Regional differences in species sensitivity and environmental exposure contributed to substantial uncertainties, underscoring the need for localized PNEC derivation and expanded toxicity datasets, particularly for transformation products. This study provides the first integrated global SSD-based benchmarks for 6PPD and 6PPD-Q, offering a quantitative foundation for monitoring, regulation, and ecological protection of tire-derived contaminants.