Exposure assessment for air-to-skin uptake of traditional and emerging organophosphate flame retardants

Human exposure to organophosphate flame retardants (OPFRs) via direct air-to-skin uptake that may be comparable to or even exceed inhalation. Previous studies have mainly focused on traditional OPFRs (tOPFRs) without sufficient attentions to emerging OPFRs (eOPFRs), such as isopropylated or tert-butylated triaryl phosphate ester isomers (ITPs or TBPPs). This study aims to further explore the transport of both OPFRs through the skin, specifically the stratum corneum (SC). In addition to tOPFRs, eOPFRs were frequently detected in the forehead and the back. The composition of ITPs and TBPPs was predominated by 2-isopropylphenyl diphenyl phosphate and bis(4-tert-butylphenyl) phenyl phosphate, respectively. Removal of the OPFRs with consecutive tape strips demonstrated that OPFRs diffusion through the skin exhibited declining concentration patterns with depth. A diffusion model was applied to the surface skin to simulate the OPFR concentration profiles and was compared with measured concentrations. We found that tris(1-chloroethyl) phosphate, tri-ethyl phosphate, and tris(2-chlorothyl) phosphate-with higher skin diffusion coefficients-could penetrate deeper into skin than other targeted OPFRs, observations confirmed by the model results as well, indicating that OPFRs have the potential for absorption into the bloodstream.