Health risks and sources of PM_2.5-bound heavy metals in Southwest China: evidence from a 9-year monitoring program

Numerous studies have demonstrated that PM_2.5-bound heavy metals pose non-negligible inhalation carcinogenic risks to the population. As one of the largest megacities in southwest China, Chengdu's industrial activity combined with its geographic features may present ongoing health challenges. By monitoring heavy metals and inorganic ion in atmospheric PM_2.5 in Chengdu City for nine years, the inhalation health risks of 12 metal elements were analyzed by applying a year-weighted health risk assessment model. Afterwards, the sources of heavy metals and ions were resolved by using a positive matrix factorization (PMF) model. Further, 34 socio-economic development indicators in Chengdu were collected and analyzed by Spearman correlation and partial correlation to support the source resolution results. The health risks from different emission sources were further quantified. The median hazard index HI (interquartile range) for all metals measured was 0.26 (0.19, 0.31), indicating a negligible non-carcinogenic risk. The order of male's carcinogenic risk was As (1.90 × 10^-6 (1.40 × 10^-6, 2.43 × 10^-6)) > Cr (4.43 × 10^-7 (3.21 × 10^-7, 6.21 × 10^-7)) > Cd (1.41 × 10^-7 (9.68 × 10^-8, 2.09 × 10^-7)) > Pb (1.17 × 10^-7 (8.59 × 10^-8, 1.53 × 10^-7)) > Ni (1.81 × 10^-8 (1.27 × 10^-8, 2.46 × 10^-8)) > Be (5.01 × 10^-9 (5.01 × 10^-9, 5.10 × 10^-9)), and similar patterns for female and child. PMF identified 5 possible sources of heavy metals, including the chemical industry, motor vehicles, soil dust, coal combustion, and the nonferrous metals industry. These sources were systematically supported and explained through trend and correlation analysis with Chengdu's socio-economic indicators. The non-ferrous metal industry and the chemical industry had the highest population cancer risk levels of 4.71 × 10^-6 and 2.24 × 10^-6, respectively. As (71.8%) and Cr (17.3%) contributed the most to the total carcinogenic risk and are the metal species requiring more stringent regulation. Heavy metal pollution from Chengdu's fast-growing non-ferrous metal industry, chemical batteries, and new energy vehicle industry requires more concern. Moreover, further policies related to cleaner production, energy conservation, and recycling are needed to address pollution from these sectors.