Long-term air pollution exposure and incident asthma risk in Chinese middle-aged and older adults: a national prospective cohort study with mechanistic insights

Ambient air pollution is a major environmental determinant of respiratory health. Evidence linking long‑term particulate matter exposure to incident asthma remains limited among middle‑aged and older adults in China. We used data from the China Health and Retirement Longitudinal Study (CHARLS), enrolling 10,672 adults aged ≥ 45 years who were free of asthma at baseline. Time-varying annual average concentrations of PM1, PM2.5, PM10, O₃, and NO₂ in 2010 were assigned to participants based on residential geocodes with a high-resolution (1 km × 1 km) air pollution dataset. Incident asthma was identified through follow-up surveys conducted in 2013, 2015, 2018, and 2020. Conventional Cox proportional hazards model and time-varying Cox model were applied to estimate hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for demographic, socioeconomic, lifestyle , baseline health status , and spatial confounders. Restricted cubic spline models were used to explore potential non-linear exposure–response relationships. Complementary transcriptomic analyses were performed to assess biological pathways potentially involved in particulate matter–related asthma risk. During follow-up, higher long-term exposure to PM1 and PM2.5 was associated with an increased risk of incident asthma in fully adjusted time-varying models [PM1: HR = 1.199 (95% CI: 1.047–1.373); PM2.5: HR = 1.112 (95% CI: 1.029–1.202)]. Linear exposure–response relationships were observed for PM1 and PM2.5, with asthma risk increasing as concentrations rose. Subgroup analyses showed generally consistent associations across age, education, residence, socioeconomic status, and smoking status. Transcriptomic analyses suggested that PM2.5 exposure was associated with alterations in gene expression, with enrichment of pathways related to inflammation, immune response, and cytokine. Long-term exposure to ambient particulate matter, particularly PM1 and PM2.5, was associated with an elevated risk of asthma among middle-aged and older adults in China. The observed linear exposure–response patterns and supportive biological evidence highlight the potential importance of further reducing particulate matter concentrations to protect respiratory health in aging populations.