Carbonaceous Components Underlying Wildfire Smoke PM2.5-Associated Mortality in Older U.S. Adults

Wildland fire activity has increased in the U.S., elevating population exposure to smoke-derived fine particulate matter (PM2.5). Emerging studies suggest higher toxicity per unit mass for PM2.5 from wildfire smoke. However, source-resolved evidence on long-term mortality effects of major carbonaceous components in smoke PM2.5 remains scarce. We linked annual, high-resolution estimates of smoke and non-smoke PM2.5 mass and carbonaceous matter to 77,470,752 Medicare beneficiaries during 2002-2019 and fit stratified Cox models. Long-term exposure to smoke PM2.5 mass and its carbonaceous fraction were associated with higher all-cause mortality risk per unit mass than their non-smoke counterparts. Per 1 μg/m3 increase, hazard ratios were 1.032 (95% CI: 1.027-1.037) for smoke PM2.5 mass and 1.087 (1.076-1.098) for smoke carbonaceous components. On the interquartile-range scale, the association was larger for non-smoke PM2.5 than for smoke PM2.5, whereas smoke carbon remained more strongly associated with mortality. Associations were stronger among Black and other racial groups and Medicaid-eligible beneficiaries. Substantial spatial heterogeneity was observed across empirical smoke regimes and geographic regions. The associations correspond to 22,420 annual deaths attributable to smoke PM2.5, with the carbonaceous fraction accounting for most of this burden. These findings support strengthening wildfire smoke health-impact assessment and mitigation as climate change increases population exposure to wildfire smoke.