Health risks and source apportionment assessment of heavy metals in PM 2.5 : A one year observation study in Chengdu

Heavy metal (HMs) pollution in PM _2.5 and its health risks have attracted increasing attention. In this study, nine PM _2.5 -bound HMs (Cr, Mn, Co, Cu, Zn, Cd, Ba, Pb, Fe) were continuously measured in Chengdu during 2023 and their concentrations, sources, seasonal variations and health risks were systematically evaluated. Cr and Cd frequently exceeded guideline values, with annual mean Cr reaching 134.8 times the National Ambient Air Quality Standards of China and 13.5 times the WHO guideline, while Cd was 3.1 times. Monthly concentrations exhibited a U-shaped pattern, peaking in January–May and November–December. Overall, non-carcinogenic risks for all population groups were below the safety threshold, whereas carcinogenic risks were of concern, especially for Cr; in winter, the incremental lifetime cancer risk of Cr for adults exceeded 1.0×10 ^− 4 . Source apportionment showed that mixed industrial-crustal sources (43% ~ 64%) dominated HMs in all seasons and were the largest contributor (86% ~ 94%) to the hazard index (HI). The contributors to total carcinogenic risk (CR) varied seasonally: in spring and autumn, traffic and mixed industrial-crustal sources were the primary contributors; in summer, mixed industrial-crustal sources were dominant; and in winter, traffic emissions were leader, accounting for 72% of CR. A fireworks episode during the Spring Festival sharply increased Cu, Cr, Ba and Pb, with short-term exposures posed risks comparable to or exceeding seasonal levels, contributing 46% of HI and 55% of CR. This study reveals discrepancies between source apportionment by mass and by health risk, and highlights overlooked health risks from fireworks.