Health risk of PM2.5-bound heavy metals in a megacity in South China: Comparison between before and after the outbreak of COVID-19.

The COVID-19 pandemic led to three years (2020-2022) of human activity restrictions in China, significantly impacting urban air quality, yet long-term health risks from PM2.5-bound heavy metals remain unclear. This study observed PM2.5 and 21 associated metals in Shenzhen over six years (2017-2022), aiming to identify sources, assess their health impacts, and explore COVID-19 effects on health risks. The total carcinogenic risk (CR) of 5 metals decreased by 23 % during the pandemic (2020-2022) compared to pre-pandemic levels (2017-2019), similar to the trends of PM2.5 and total metals concentrations. In contrast, the non-carcinogenic risk (NCR) of 11 metals increased by 20 %, primarily due to the rise in manganese (Mn) concentration caused by vehicle emissions, but NCR still below the risk threshold. Vehicle emissions were the main contributor to NCR (68 %) and CR (55 %). Machine learning revealed that the source emissions reduction is the primary factor contributing to the decline in the CR of metals bound to PM2.5, except for the increased health risk associated with vehicles. During the pandemic, the slowed growth rate of CR of vehicle emissions and the significant decline in CR of fugitive dust demonstrated the effectiveness of pandemic-induced traffic and production restrictions in reducing anthropogenic emissions and mitigating PM2.5-bound heavy metal health risks in Shenzhen. However, total CR of heavy metals in 2022 exceeded the acceptable risk threshold (1 × 10-6), emphasizing the ongoing challenge of regulating PM2.5 and the need to prioritize vehicle emission management.