Ambient fine particulate matter (PM2.5) is associated with increased mortality at concentrations below current regulatory standards. Studies of low-level exposure often rely on large administrative cohorts whose geographic and demographic composition may influence observed associations. In a prior analysis, we observed an association between long-term PM2.5 and all-cause mortality among Native American Medicare beneficiaries living in zip codes within the lowest decile of PM2.5 exposure. The present study, a case–control analysis of 1,713,399 low-PM2.5-exposed beneficiaries enrolled in traditional Medicare during 2015–2016, evaluated whether this association could be explained by geographic context, socioeconomic position (SEP), or baseline health status. We used principal components analysis to summarize area-level SEP indicators and beneficiary-level chronic disease diagnoses. In fully adjusted pooled models, PM2.5 was more strongly associated with mortality among Native American beneficiaries (odds ratio, OR = 1.12 per ug/m3; 95% CI 1.06–1.18) than among non-Native American beneficiaries (OR = 1.01 per ug/m3; 95% CI 1.001–1.02). Sequential adjustment among Native Americans showed that state-level geographic clustering accounted for most attenuation of the PM2.5 coefficient, with additional modest attenuation after adjustment for SEP and chronic disease patterns. These findings suggest that PM2.5–mortality associations observed in low-exposure populations may partly reflect geographic composition and underlying health differences within these large cohorts.
Hess et al. (Sat,) studied this question.