Health risk assessment of heavy metal emissions from on-road vehicles in a metropolitan area in southeastern China

Heavy metals (HMs) in PM_2.5 have been extensively studied for their toxicity and carcinogenic risk. In this study, the toxicity and spatial health risks of PM_2.5-HMs from on-road vehicles were investigated in the Xiamen-Zhangzhou-Quanzhou (XZQ) metropolitan area in southeast China in 2021. The results show that the emissions of PM_2.5 and PM_2.5-HMs were 3219.54 and 89.48 t, with non-exhaust emissions contributing 62.1% and 87.6%, respectively. However, there were differences in the contribution of different sources to different HMs. The spatial distribution of PM_2.5 was characterized by high levels in the urban centers with high traffic flow and population. The daily PM_2.5 concentration could reach up to 6.85 μg/m³ in the area with heavy traffic. Wind speed and direction had a significant effect on the daily PM_2.5 concentration, while hourly concentrations were more influenced by variations in vehicle activity. The annual average concentration of Cr(VI) (0.15 ng/m³, estimated as 12% of total Cr) in the hotspot grid was six times the limit (0.025 ng/m³) of China’s air quality standard. Other toxic metals such as As, Cd and Pb were well below their guidelines. The health risk assessment showed that there was no threat of non-carcinogenic risks, but the carcinogenic risks in some urban centers exceeded the safety level of 10^–6. More than 90% of the carcinogenic risk came from Cr(VI), which mainly came from brake wear (55.7%) and diesel exhaust (32.5%). This study provides a scientific basis for the development of more effective pollution control strategies and public health policies.