Chemical composition and potential health risks of tire and road wear microplastics from light-duty vehicles in an urban tunnel in China

Tire and road wear microplastics (TRWMPs) are one of the main non-exhaust pollutants of motor vehicles, which cause serious environmental and health issues. Here, TRWMPs in PM_2.5 samples were collected in a tunnel in urban Xi'an, northwest China, during four periods [I: 7:30–10:30, II: 11:00–14:00, III: 16:30–19:30, IV: 20:00–23:00 local standard time (LST)] in summer of 2019. The chemical components of rubbers, benzothiazoles, phthalates, and amines in TRWMPs were quantified, with a total concentration of 6522 ± 1455 ng m⁻³ (mean ± standard deviation). Phthalates were predominant in TRWMPs, accounting for 64.8% on average, followed by rubbers (33.2%) and benzothiazoles (1.19%). The diurnal variations of TRWMPs showed the highest concentration in Period III (evening rush hour) and the lowest concentration in Period I (morning rush hour), which were not exactly consistent with the variation of the number of light-duty vehicles passed through the tunnel. The result implied that the number of vehicles might not be the most important contributor to TRWMPs concentration, whereas meteorological variables (i.e., precipitation, and relative humidity), vehicle speed, vehicle class, and road cleaning also affected their abundances. The non-carcinogenic risk of TRWMPs in this study was within the international safety threshold, but their carcinogenic risk exceeded the threshold by 2.7–4.6 times, mostly dominated by bis(2-ethylhexyl)phthalate (DEHP). This study provides a new basis for the source apportionment of urban PM_2.5 in China. The high concentrations and high potential cancer risks of TRWMPs represent the requirement for more efficient measures to control light-duty vehicle emissions.