Explorations of tire and road wear microplastics in road dust PM_2.5 at eight megacities in China

Tire and road wear microplastics (TRWMPs) in road dust are a key source of atmospheric particulate matter and have an adverse impact on human health and the environment. In this study, samples of particulate matter with a diameter of 2.5 μm or less (PM2.5) in road dust were collected from eight megacities in China to determine the TRWMP content, including that of natural rubber (NR), styrene–butadiene rubber (SBR), and butadiene rubber (BR). The total abundance of TRWMPs was the highest in Lanzhou (174.7 ± 17.0 μg g−1), followed by Xi'an (169.3 ± 23.8 μg g−1), Beijing (107.5 ± 7.5 μg g−1), Changchun (102.2 ± 8.4 μg g−1), Chengdu (101.6 ± 12.9 μg g−1), Guangzhou (98.8 ± 6.5 μg g−1), Wuhan (96.0 ± 5.3 μg g−1), and Shanghai (86.1 ± 30.1 μg g−1). A considerably higher TRWMP fraction in road dust PM2.5 was observed in the northern cities than in the southern cities and is attributable to the higher frictional resistance of roads subjected to less precipitation. The abundance of TRWMPs in the southern cities was dependent on road type, but this dependence was not observed in the northern cities. In the south, road dust PM2.5 on main roads contained more TRWMPs than that on branch roads. Correlation analysis indicated that TRWMPs were associated with tire, road, and break wear. In relation to intracellular oxidative stress factors, higher correlations were observed between TRWMPs and lactate dehydrogenase (r = 0.83) than between TRWMPs and reactive oxygen species (r = 0.59), possibly because TRWMPs destroy the integrity of the cell membrane, with NR exhibiting a higher cytotoxicity than SBR or BR. This study provides evidence that TRWMPs have an adverse impact on human health by inducing cellular oxidative stress. Therefore, further research on TRWMPs in respirable road dust is required.