Disproportionately Higher Contribution of Endotoxin to PM

The contribution of microbial components and their sources to the bioactivity of airborne fine particulate matter (PM2.5) remains unclear. This study validated the concentration-additive mixture effects of ambient PM2.5 fractions on in vitro interleukin-8 (IL-8) induction, enabling quantification of the role of endotoxin from Gram-negative bacteria in IL-8 induction by PM2.5 from contrasting environments. Endotoxin consistently contributed to PM2.5-induced IL-8 at both the coastal site (0.1-10%) and the urban site (0.1-17%), far exceeding its mass fraction in PM2.5 (<0.0001%). Among reported components, endotoxin demonstrated the highest toxicity-to-mass contribution ratio (10,000:1-100,000:1) due to its high potency. This suggests that reducing PM2.5 toxicity may not necessarily require a proportional reduction in its mass. The sources of Gram-negative bacteria shifted from predominantly natural origins at the coastal site (natural-to-anthropogenic ratio of 1.6:1) to increasing anthropogenic contributions at the urban site (natural-to-anthropogenic ratio of 0.7:1). These anthropogenic sources, including the built environment, sewage treatment, and humans, fall outside conventional pollution-intensive categories. These findings highlight the importance of identifying high-potency, low-concentration components and their sources. Targeting such components is critical for cost-effective PM2.5 abatement strategies and achieving significant public health benefits, especially in regions with low-to-moderate PM2.5 levels.