Chronic exposure to high levels of fine particulate matter (PM_2.5) from wildfire smoke increases the risk of developing stroke in U.S. adults, according to a national open cohort study published Jan. 27 in EHJ.

Among approximately 25 million Medicare Fee-for-Service beneficiaries (88% White, 54% women), Hua Hao, MPH, et al., used validated Medicare claims from 2007 to 2018 to identify cases of incident stroke. Of note, wildfire smoke PM_2.5 exposure was estimated at a 1 km² resolution using a machine learning model distinguishing smoke-specific PM_2.5 from other ambient sources.

Results showed there were about 2.9 million cases of incident stroke. Researchers found a significant association between long-term wildfire exposure and increased risk of stroke. Moreover, there was a dose-response relationship, with longer windows of exposure increasing stroke risk. There was a 1.3% increase in stroke risk for each 1 μg/m³ increase in three-year average wildfire smoke PM_2.5 exposure. Associations for wildfire smoke PM_2.5 were stronger than for non-smoke PM_2.5 per unit of exposure.

In what they believe is the first study focusing on long-term effects of wildfire smoke PM_2.5 on incident stroke, the authors note that "... the adverse effects of wildfire smoke PM_2.5 on stroke risk became stronger with longer exposure windows and higher smoke-day thresholds – reflecting both increasing cumulative concentrations and more frequent or intense smoke days over time." They write the findings highlight wildfire smoke as a "distinct environment risk factor for stroke in older adults."

"Along with other recent studies, these findings highlight wildfire smoke as a chronic risk factor for cardiovascular health," write Siqi Zhang and Kai Chen in an accompanying editorial comment. "Further research is needed to include diverse populations and regions, enhance our understanding of the underlying mechanisms, and evaluate the effectiveness of protective measures."