Yanling Deng, Hua Hao, Qiao Zhu, Yang Liu, Kyle Steenland
Growing evidence suggests that long-term exposure to fine particulate matter (PM2.5) increases the risk of depression, yet the roles of specific chemical components and the vulnerability of individuals with pre-existing conditions remain poorly understood. In this nationwide cohort study, we investigated the individual and joint effects of long-term exposure to PM2.5 and its major components on late-life depression, and examined whether common comorbidities modify these associations. Using U.S. Medicare data from 2000 to 2018, we followed 23.7 million older adults (mean age at entry, 76.0 years). Annual concentrations of PM2.5 mass and six major components—elemental carbon, ammonium, nitrate, sulfate, soil dust, and organic carbon—were estimated using a high-resolution spatiotemporal exposure model. Incident depression was identified through Medicare claims. We applied Cox proportional hazards models and quantile g-computation to evaluate both single-component and mixture effects, with additional stratified analyses to assess effect modification by comorbidities. During follow-up, 5.5 million participants developed depression. In single-pollutant models, PM2.5 mass and several components—including sulfate, elemental carbon, soil dust, and ammonium—were significantly associated with increased depression risk. Notably, the combined mixture of PM2.5 components showed a substantially stronger association with
depression than PM2.5 mass alone. A one-quartile increase in the component mixture was associated with a 7% higher risk of depression (hazard ratio [HR], 1.07; 95% CI, 1.06–1.07), compared with a 2% increase in risk per interquartile-range increase in PM₂.₅ mass (HR, 1.02; 95% CI, 1.01–1.02). Quantile g-computation identified soil dust, sulfate, and elemental carbon as the dominant contributors to the mixture effect. Associations were particularly pronounced among individuals with cardiometabolic and neurologic comorbidities. In this large, population-based study of U.S. older adults, specific PM2.5 components—rather than overall particle mass alone—were strongly associated with depression risk, especially among those with pre-existing health conditions. These findings highlight the importance of component-specific air pollution regulation and targeted protection of vulnerable populations to reduce the mental health burden of air pollution.