Occurrence, Seasonal Variation, and Health Risks of PM2.5-bound Liquid Crystal Monomers (LCMs) in Beijing, China

Abstract

Liquid crystal monomers (LCMs) are potentially persistent, bioaccumulative, and toxic emerging pollutants. However, their occurrence in outdoor PM_2.5 and related human exposure risks remain unknown. In this study, 32 composite samples were analyzed, which were prepared from daily PM_2.5 samples collected throughout the year 2021−2022 in Beijing, China. In total, fifty-six of 78 LCMs were presented at a median concentration of 66.0 pg/m³ (range: 13.3 to 375.6 pg/m³), with fluorinated LCMs (FBAs) predominating and accounting for 90.7% of the total LCMs. This concentration surpasses that of halogenated persistent organic pollutants (e.g., polychlorinated dibenzo-p-dioxins/furans) in ambient PM_2.5. Higher concentrations of LCMs were found in spring and summer compared to autumn and winter, which could be explained by correlations of concentrations with temperature (p < 0.05). Trans,trans-3,4-Difluoro-4'-(4'-pentylbicyclohexyl-4-yl)biphenyl, trans,trans-3,4-Difluoro-4'-(4'-propylbicyclohexyl-4-yl)biphenyl, and trans,trans-3,4,5-Trifluoro-4'-(4'-propylbicyclohexyl-4-yl)biphenyl were identified for the first time as dominant compounds in ambient samples. Based on predicted biological toxicities, 48 LCMs were categorized as high priority due to their high potential for human absorption, including several compounds previously overlooked. The non-carcinogenic risks of LCMs through inhalation and dermal were negligible for children and adults. This study firstly established a priority list of LCMs in PM_2.5, highlighting the need for heightened awareness of their health risks.