Beyond Mass Concentration: The Critical Role of Chemical and Biological Compositions and Sources in PM2.5-induced Toxicity in Two Chinese Megacities

Studies have established evidence between the chemical composition of PM_2.5 and its toxic effects, yet the toxicological contributions of biological components like endotoxins remain understudied. To address this gap, we developed an integrated assessment combining in vitro mixture-toxicity assays and source apportionment modeling. This approach quantifies the contributions of endotoxins, trace metals, and polycyclic aromatic hydrocarbons (PAHs) to PM_2.5-induced intracellular oxidative stress and identifies source-specific toxicity patterns across six sites in Nanjing and Guangzhou, China. Our analysis revealed significant spatial gradients in PM_2.5-induced toxicity correlated with anthropogenic activities. In Nanjing, suburban-industrial PM_2.5 exhibited double the toxicity of rural samples, while urban and semi-rural industrial sites in Guangzhou showed double the toxicity of the suburban area. These patterns suggest that reduced exposure to anthropogenic emissions corresponds to lower PM_2.5-induced toxicity, highlighting anthropogenic contributions as key toxic drivers. Although endotoxins, trace metals, and PAHs constituted approximately 2.23% of PM_2.5 mass, they accounted for 35.9–56.9% of total reactive oxygen species (ROS) generation, with trace metals as the dominant contributor (31.6–46.7%), followed by endotoxins (4.24–12.2%) and PAHs (0.0218–0.135%). Source apportionment revealed dominant regional toxic drivers: fugitive dust (23.2–24.6%) and combustion (19.0–20.5%) in Nanjing; while vehicle (19.8%), industrial (18.4%), combustion (18.5%) and biological (20.0%) emissions in Guangzhou. Notably, biological emissions contributed 10.3–20.0% of total PM_2.5-induced toxicity. These findings suggest that PM_2.5-induced toxicity depends more on bioactive constituents than total mass concentration, emphasizing the need for toxicity-oriented air quality regulations, especially regarding bioaerosols, to supplement mass-based air quality standards.