Impact of wildfire smoke waves on the association between long-term exposure to fine particulate matter and cardiovascular hospitalizations among older adults in the United States.

BACKGROUND: Fine particulate matter (PM2.5) is a risk factor for cardiovascular disease (CVD), even at concentrations below U.S. air quality standards. The frequency and severity of wildfires are increasing, causing episodic PM2.5 spikes. Our study examined whether associations between long-term exposure to PM2.5 and cardiovascular hospitalizations differed in the presence and absence of wildfire smoke waves. METHODS: We conducted a cohort study of 65.2 million U.S. Medicare beneficiaries aged ≥ 65 years across the contiguous U.S. during 2017-2022. Daily total and wildfire smoke PM2.5 concentrations were estimated at 1 km and 10 km spatial resolutions, respectively, using validated machine learning models that incorporated satellite, meteorological, emission, and land-use data. Exposure estimates were aggregated to beneficiaries' residential ZIP codes by population-weighted averaging. Wildfire smoke waves were defined as ≥ 2 consecutive days with daily smoke PM2.5 > 10 μg/m3. We applied quasi-Poisson regressions to estimate relative risks (RR) of first hospitalizations for overall CVD, ischemic heart disease, cerebrovascular disease, and hypertension in ZIP-years with and without smoke waves. RESULTS: In the full cohort, an increase in two-year average PM2.5 from 5 μg/m3 (WHO annual air quality guideline) to 9 μg/m3 (current U.S. annual PM2.5 standard) was associated with increased risks of overall CVD, cerebrovascular, and hypertension hospitalizations, with stronger associations in ZIP-years without smoke waves than in those with smoke waves (overall CVD: RR 1.09, 95% CI 1.09-1.10 vs. RR 1.04, 95% CI 1.03-1.04; ratio of RR 1.05, 95% CI 1.04-1.06). Similar patterns were observed among beneficiaries residing in ZIP codes with annual PM2.5 concentrations below 9 μg/m3 throughout the study period. Smoke wave-related differences in PM2.5-CVD associations were greater among males, urban residents, and residents in the western region compared with their counterparts. CONCLUSIONS: The association between long-term PM2.5 exposure and cardiovascular hospitalizations differed by wildfire smoke wave occurrence. These findings suggest that long-term average PM2.5 may not fully capture episodic high-intensity exposures and may underestimate the cardiovascular morbidity burden in smoke-affected areas, highlighting the need for risk assessments accounting for exposure patterns.