Proposing stratum corneum as an alternative to sebum in dermal exposure assessment of organophosphate esters

As sebum offers transient exposure snapshots, we systematically characterized the performance of stratum corneum (SC) as a bioindicator of long-term panoramic exposure for assessing human dermal exposure to 20 organophosphate esters (OPEs) and developed an SC-based exposure assessment methodology under controlled conditions. During 12-h exposure, median ∑₂₀OPE concentrations of arm and back sebum (n = 15, collected by skin wiping) increased remarkably from nondetectable to 5280 ng m^-2 and 2280 ng m^-2, respectively, whereas SC samples (collected by tape stripping) remained stable [arm (1230-1410 ng m^-2) and back (1020-1240 ng m^-2)]. Similar results were also obtained after 28-d exposure (n = 8). After 6-h exposure, median ∑₂₀OPE across forehead, arm, calf, and back were 1420, 1420, 986, and 369 ng m^-2 (sebum, n = 13), and 1830, 1580, 1090, and 1550 ng m^-2 (SC), respectively, with higher sebum variation in different locations. A considerably uneven vertical gradient of OPEs in forehead sebum-SC (stratified into 5 layers by tape stripping, n = 12) revealed high concentrations in the outermost layer. Lipophilic OPEs (log K_OW > 4.7) accumulated in sebum, while hydrophilic OPEs penetrated the sebum and SC with high interlayer uniformity. The SC-based method circumvented the inconsistency arising from time, location, and wiping force in the sebum-based method, enabling a convenient, stable, representative, and accurate whole-body dermal exposure assessment.