Photochemical ultrafine particles pose potential inflammatory risk despite low PM2.5 mass.

Background

Air quality improvements have reduced PM2.5 mass, but particle surface area, which primarily contributes to inflammatory responses, may not follow the same trend, potentially creating health-relevant exposure.

Methods

To investigate this potential decoupling, we analyzed three years of observations in Fuzhou, China. Potential inflammatory responses were estimated from the lung-deposited surface area (LDSA) exceeding PM2.5-based expectations, using an animal-derived dose-response relationship that relates particle surface area to pulmonary inflammation.

Results

We identified that 6.6% of PM2.5 attainment hours (<35 μg m-3) exhibited elevated LDSA, with surface area exposure 7.5-fold greater than expected from the LDSA/PM2.5 ratio during other attainment hours, and 82% of potential inflammatory risk was undetected by mass-based assessment. During these periods, despite median PM2.5 mass concentrations of only 3.0 μg m-3 (well below air quality standards), toxicological dose-response assessment predicts potentially clinically relevant inflammatory responses (27% lung neutrophil infiltration). Machine learning reveals that photochemical oxidation (elevated O3 and secondary organic carbon importance) contributes to these periods by producing abundant 50-100 nm particles during summer-autumn afternoons under warm, high-radiation conditions.

Conclusions

These photochemical ultrafine particle events represent a mechanistically distinct pollution regime where mass-based frameworks may underestimate potential inflammatory effects, a gap that may widen as emission controls reduce primary sources and climate warming enhances photochemistry.