Widespread p-Phenylenediamine Derivatives in Indoor and Outdoor Dust: Occurrence, Fate, and Exposure

p-Phenylenediamines (PPDs) are emerging pollutants in the environment due to their extensive application in rubber-related products, yet their potential indoor exposure has been largely overlooked. This study investigated the occurrence characteristics of PPDs and their oxidative products (PPD-Qs) in dust from indoor, i.e., auto repair workshops, hardware malls, and home furnishings markets, as well as outdoor environments, i.e., highway and motorway intersections, revealed the influencing factors for transformation, and assessed associated exposure risks. The presence of PPDs and PPD-Qs in specific indoor environments has been documented. Particularly high levels of these compounds were observed in dust from auto repair workshops (median: 1.68 × 103 ng/g (PPDs) and 421 ng/g (PPD-Qs). The indicator monomers for PPDs and PPD-Qs emitted from different sources were identified. Furthermore, through concentration analysis and theoretical calculations, the transformation from PPDs to PPD-Qs was revealed to be influenced by the total organic carbon and transition metals present in the dust with Fe3+ and Cu2+ acting as the most effective catalysts. Finally, occupational populations in environments such as auto repair workshops were found to face extremely high exposure levels. This study emphasizes the need to recognize and address the risks associated with these compounds indoors.