Low-Level PM2.5 Exposure and Mortality in the Medicare Cohort: The Role of Native American Beneficiaries

Fine particulate matter (PM_2.5) has been associated with mortality at low concentrations, with higher per-unit risk at lower exposure levels, and no threshold of effect. We examined characteristics of Medicare decedents living in zip codes at the lowest end of the U.S. PM_2.5 exposure distribution to determine whether there is a demographic, health or exposure profile of beneficiaries for whom even low PM_2.5 exposure is associated with increased mortality. The study included 2,773,647 decedent cases and 27,736,470 non-decedent controls, matched on decile of long-term PM_2.5 exposure from among 36 million Medicare fee-for-service beneficiaries enrolled 2015-2016. Outcomes of the study included all-cause and cause-specific mortality, stratified by decile and beneficiary characteristics. Increased PM_2.5-related mortality within the lowest exposure decile was found only among Native American beneficiaries, with odds ratios of 1.11 (95% CI, 1.01-1.21) and 1.21 (95% CI, 1.11-1.32) per 1 µg/m³ increase in PM_2.5, for those eligible and ineligible for Medicaid, respectively, and was driven by significant increases in selected kidney and cardiovascular outcomes, diabetes, and chronic obstructive pulmonary disease. These results may reflect particular sensitivity to PM_2.5; factors varying with PM_2.5 at the zip code level, including constituent exposures or social determinants of health; or inaccuracies in exposure estimates.