Wildfire-Sourced PM2.5 and Emergency Department Visits for Migraine and Other Primary Headache.

Importance: There is limited research examining the association between wildfire smoke, an increasingly frequent exposure, and headache-related emergency department (ED) visits, despite headaches being a leading cause of years lived with disability globally. Objective: To examine the association between wildfire-sourced fine particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5) and ED visit for migraine and other primary headache syndromes (MOPHS) compared with nonwildfire-sourced PM2.5, and to assess the variation across sociodemographic factors. Design, Setting, and Participants: This case-crossover study used conditional logistic regression to examine ED visits during wildfire seasons (May 1 to October 31) from 2010 to 2023 in Alberta and Ontario provinces in Canada. Case days were matched to referent days by day of week, month, year, and forward sortation area (first 3 postal code characters). Seven-day cumulative lags were evaluated. All ED visits recorded in the National Ambulatory Care Reporting System with a primary diagnosis of MOPHS were included. Statistical analyses were conducted from April to December 2025. Exposure: Total PM2.5 concentrations and wildfire day (WFD). Daily mean wildfire-sourced PM2.5 was estimated using the Canadian Optimized Statistical Smoke Exposure Model. WFDs were defined by smoke plume presence and total PM2.5 concentration exceeding a predefined threshold based on the mean plus 1.5 SD of PM2.5 on days without a smoke plume. Wildfire-sourced PM2.5 was defined as the difference between total PM2.5 concentration on a WFD and expected background PM2.5 concentration on non-WFDs. Main Outcomes and Measures: ED visits for migraine (International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Canada code G43) and other primary headache syndromes (code G44). Results: Among the 997 701 ED visits for 622 753 patients (mean [SD] age, 24.6 [16.8] years; 760 795 females [76.3%]) for MOPHS identified during the study period, 86.2% of visits were for migraines and 13.8% were for other primary headache syndromes. Wildfire-sourced PM2.5 was associated with a 6.07% (95% CI, 5.75%-6.39%) increase in ED visits. Associations appeared attenuated in the least materially and socially deprived quantile (for both provinces combined, the percentage increase in MOPHS was smallest for the Material and Social Deprivation Index quintile 1: 3.61% [95% CI, 1.93%-5.63%] and largest for quintile 5: 10.42% [95% CI, 7.41%-13.49%]). In contrast, total PM2.5 concentration on non-WFDs was not significantly associated with ED visits (0.21%; 95% CI, -0.20% to 0.62%). Conclusions and Relevance: In this case-crossover study of ED visits in Alberta and Ontario, acute exposure to wildfire-sourced PM2.5 was associated with increased ED visit for MOPHS; associations were greater in magnitude for wildfire-sourced PM2.5 than nonwildfire-sourced PM2.5. These findings support the need for further research into wildfire smoke and severe headache-related outcomes.