Environmental exposure risk of typical rubber antioxidant 6PPD and its oxidation product 6PPD-Q

Environmental distribution, transformation pathways, and scenario-specific non-dietary intake of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and 6PPD-quinone (6PPD-Q) are reviewed. Both compounds occur in water, air, dust, and soil, with distributions governed by traffic emissions, hydrological connectivity, particle properties, and environmental conditions. Runoff-affected waters are major hotspots for 6PPD and 6PPD-Q contamination and potential ecological risk. 6PPD transformation modifies compound-specific mobility, persistence, and hazard but does not consistently increase toxicity across species and endpoints. Unlike previous reviews focused primarily on environmental occurrence, transformation, and toxicity, this review provides a comparative screening-level assessment of non-dietary exposure to 6PPD and 6PPD-Q across representative microenvironments, considering inhalation, dermal contact, and incidental dust ingestion. Results showed that estimated intakes were primarily governed by source proximity and contaminant loading. Where all three pathways were evaluated, incidental dust ingestion and dermal contact generally contributed more than inhalation. Across different exposure scenarios, the highest upper-bound ΣEDI for adults was estimated at 3.751 ng/(kg bw·day) for 6PPD in parking-related environments, whereas the corresponding maximum for 6PPD-Q was 1.995 ng/(kg bw·day) at production sites, indicating compound-specific priority exposure scenarios. These screening-level estimates represent potential external exposure rather than absorbed dose or direct health risk. Nevertheless, the elevated exposure potential in source-proximal microenvironments warrants further investigation of possible health implications. Future assessments should integrate dietary and non-dietary pathways with biomonitoring, toxicokinetic modeling, mixture analysis, and probabilistic approaches.