Assessing the Ecological and Health Risks Associated with Heavy Metals in PM2.5 Based on their Potential Bioavailability

The health impacts of heavy metals associated with PM2.5 have raised significant public concerns. However, the current approach to assessing these risks based on the total concentration may lead to an overestimation. In this study, PM2.5 samples were collected from the northern suburbs of Nanjing. The chemical speciation and seasonal variation characteristics of Cr, Co, Ni, Cu, As, Pb, and Cd were analyzed using the BCR sequential extraction method. Furthermore, an ecological risk assessment model and a health risk assessment model based on modification of bioavailable concentrations were employed to evaluate their environmental risks. The daily average concentration of PM2.5 was 104.02 ± 43.58 μg∙m−3, while the order of concentrations of heavy metals in PM2.5 was found to be as follows: Pb > Cr > Cu > Cd > Ni > As > Co, winter > autumn > summer > spring. Most of the heavy metals are primarily distributed in the diluted acid extractable fraction (F1), which exhibits high bioaccessibility. Moreover, during summer, a higher proportion of bioaccessible species is observed compared to winter. The evaluation results of modified ecological risk index demonstrated a reduction in the ecological risk grade of Ni from medium to low, and a decrease in the ecological risk grade of Cr from low to negligible. The non-cancer risks of Cu and Pb were negligible, while Cr, As, Co, Cd and Ni posed potential lifetime cancer risks for both children and adults. Furthermore, the results of ILCRmi indicated a transition in the carcinogenic risk assessment of Cr for adult male and famale, from being classified as significant to potential. Our study provided a reliable and accurate method to calculate the environment risks of heavy metals. The environmental risks of heavy metals (HMs) in PM2.5 are not only associated with their overall quantities, but also with their species and bioaccessibility. However, most previous studies did not consider the influence of chemical species of heavy metal which directly impact their migration, transformation, toxicity levels, and bioavailability within the environment. And the majority of ecological and health risk assessments pertaining to heavy metals carried by PM2.5 incorporate residual states, potentially leading to imprecise assessment outcomes. In this study, BCR sequential extraction procedure was applied to investigate the bioavailability concentration of heavy metals in PM2.5. Modifications were made to the ecological risk index model and EPA health risk exposure model based on concentrations of bioavailable forms. Assessments were conducted to evaluate both ecological risks and health risks for specific populations, while also comparing disparities between the two analysis outcomes. This study offers valuable insights for the scientific assessment of environmental risks associated with atmospheric heavy metals, thereby contributing to the advancement of environmental management strategies.