Long-term exposure to wildfire smoke particulate matter and incident stroke: a US nationwide study

Wildfire smoke is a growing public health concern, particularly due to its high fine particulate matter (PM2.5) concentrations. While short-term exposure has been linked to adverse cardiovascular outcomes, long-term effects remain understudied. This study examined the association between long-term wildfire smoke PM2.5 exposure and incident stroke among older US adults.

A national open cohort study was conducted among Medicare Fee-for-Service beneficiaries aged ≥65 years from 2007 to 2018. Stroke cases were identified using validated Medicare claims. Wildfire smoke PM2.5 exposure was estimated at a 1 km2 resolution using a machine learning model distinguishing smoke-specific PM2.5 from other ambient sources. Stratified Cox proportional hazards models with generalized estimating equations were used to estimate hazard ratios (HRs) per 1 µg/m3 increase in wildfire smoke PM2.5, adjusting for non-smoke PM2.5, meteorological variables, socio-economic status, and healthcare capacity.

Among ∼25 million beneficiaries, ∼2.9 million incident stroke cases occurred. Long-term wildfire smoke PM2.5 exposure was significantly associated with increased stroke risk. Stronger associations were observed for longer exposure windows. For instance, per 1 µg/m3 increase in 3-year average wildfire smoke PM2.5 was associated with a 1.3% increase in stroke risk (HR: 1.013; 95% CI: 1.006, 1.020). Associations for wildfire smoke PM2.5 were generally stronger than for non-smoke PM2.5 per unit exposure.

This study provides national-scale evidence that chronic exposure to wildfire smoke PM2.5 increases stroke risk in older adults. These findings underscore the long-term health risks of wildfire smoke and support policies aimed at reducing exposure, especially among vulnerable populations.