Low-Volatility Components from Gaseous Air onto Skin Dominate Human Exposure to Flame Retardants Indoors

To identify the primary pathway and source of human exposure to organophosphate and brominated flame retardants (OPFRs and BFRs), skin wipes (N = 336) and air samples (N = 60) were collected under controlled conditions from a residence in Xinxiang, China. In air, geometric mean concentrations of ∑₉OPFRs exceeded those of ∑₉BFRs by 2 orders of magnitude, with the air FR profile dominated by high-volatility compounds. On skin, however, ∑₉OPFRs levels were 1-4 times those of ∑₉BFRs. The skin FR profile was dominated by low-volatility compounds, with no significant gender differences. Furthermore, environmental interface contact contributed more significantly to dermal exposure for BFRs than for OPFRs. Principal component and correlation analyses indicated that flame retardants (FRs) on skin correlated more closely with gaseous air, but differed considerably from those in airborne particles. This indicates that gaseous air is the primary source of dermal exposure and that low-volatility FRs may transfer to skin lipids more readily. Finally, dermal uptake was identified as the major exposure pathway for both ∑₉OPFRs and ∑₉BFRs compared to inhalation and oral ingestion pathways, accounting for more than 99% of the total exposure. Notably, current air-to-skin transfer models considerably underestimated the actual dermal uptake of FRs.