Occurrence of p-phenylenediamine antioxidants in human urine

General populations are widely exposed to various p-phenylenediamine antioxidants (PPDs). N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), a typical p-phenylenediamine antioxidant, has been detected in human urine samples. However, the occurrence of other widely used PPDs in human urine is still unclear. This study comprehensively characterized the occurrence of 9 PPDs in human urine from 151 Chinese adults. Our results showed that all target PPDs were detected in human urine samples, with the total concentrations of PPDs ranging from 0.41 to 38 ng/mL. PPDs in human urine was dominated by 6PPD (mean 1.2 ng/mL, range < LOD − 3.8 ng/mL), followed by N-phenyl-N′-cyclohexyl-p-phenylenediamine (CPPD; 0.85 ng/mL,