Source apportionment and health-risk assessment of PM2.5-bound elements in indoor/outdoor residential buildings in Chinese megacities

Particulate matter with a diameter of 2.5 μm or smaller (PM_2.5) in indoor environments originates from both indoor and outdoor sources, influencing associated human health risks through different compositions. This study simultaneously collected and analyzed indoor and outdoor PM_2.5 samples in three major Chinese megacities—Beijing, Shanghai, and Shenzhen—to characterize PM_2.5 sources and assess their health impacts. A total of seven distinct sources, both indoor and outdoor, were identified for PM_2.5: indoor activities, metal smelting, industrial activities, soil dust, vehicle emissions, coal combustion, and fuel oil combustion. Indoor activities accounted for approximately 20 % of the residential indoor PM_2.5, with the remainder predominantly due to outdoor PM_2.5 infiltration. The contributions of indoor activities to noncarcinogenic and carcinogenic risks ranged from 3.6 % to 28.5 %, whereas outdoor PM_2.5 sources posing greater health risks. The cumulative noncarcinogenic risks for adults in Beijing, Shanghai, and Shenzhen were 0.99, 1.15, and 0.72, respectively, slightly higher than those for children. The cumulative carcinogenic risks for adults were approximately five times those for children, with values of 6.90 × 10⁻⁵, 6.34 × 10⁻⁵, and 6.83 × 10⁻⁵, respectively, all surpassing the acceptable limit. Noncarcinogenic risks were predominantly attributed to Ni, Co, and Mn, contributing over 85 % to the total risk, while Cr was the primary contributor (>89 %) to carcinogenic risks. Indoor environmental exposure accounting for over 80 % of noncarcinogenic and carcinogenic risks for adults, and exceeding 90 % for children. This study provides significant insights into the effective control of PM_2.5 pollution and the reduction of health risks from a source perspective.