Regional heterogeneity and the role of social vulnerability in air pollution-related mortality from chronic lower respiratory diseases: a nationwide analysis of the United States (2010–2020)

Fine particulate matter (PM2.5) is a recognized risk factor for chronic lower respiratory diseases (CLRD) mortality. While Environmental Justice frameworks posit that socioeconomically disadvantaged populations bear disproportionate health burdens, nationwide studies using pooled estimates may mask critical geographic and seasonal heterogeneities. This study aimed to evaluate these spatiotemporal variations across the contiguous US and examine effect modification by the Social Vulnerability Index (SVI). We analyzed monthly time-series data from 3,108 counties (2010–2020). CLRD mortality data (ICD-10: J40-J47) were obtained from CDC WONDER, and monthly average concentrations of PM2.5 and ozone were aggregated from daily measurements derived from the US EPA Air Quality System using inverse distance weighting (IDW). Generalized Linear Models (GLM) with a negative binomial distribution were employed, adjusting for ozone, temperature, seasonality, and long-term trends. Analyses were stratified by Census Region and season. We introduced an interaction term (PM2.5×SVI) to test for effect modification and used restricted cubic splines (RCS) to visualize exposure-response associations. Sensitivity analyses, including removing ozone adjustment, altering temperature controls, and excluding 2020 data to rule out COVID-19 anomalies, were conducted to verify robustness. Nationwide, the association was borderline (RR = 1.02, 95% CI: 1.00-1.04, P = 0.075), but regional disparities were striking. The Midwest exhibited exceptionally high risk (RR = 2.61, 95% CI: 2.45–2.79, P 0.05), and results remained robust across sensitivity models. The impact of PM2.5 on CLRD mortality exhibits substantial regional heterogeneity. The Midwest identified as a highly susceptible hotspot, while the South’s negative association likely reflects exposure misclassification (e.g., air conditioning use). Our findings support the “Universal Susceptibility” hypothesis in the Midwest, suggesting that regional pollution haze poses a consistent threat across socioeconomic spectrums, unaffected by community vulnerability levels. Policy interventions should prioritize region-specific emission controls, particularly in the Midwest.