Introduction: Ambient air pollution (particulate matter (PM) 2.5, defined as particles <2.5 microns in diameter) has long been linked to increased stroke risk. However, few studies have described the specific sources or estimated the effects of the PM2.5 constituents responsible for stroke and stroke subtype risk.

Hypothesis: We hypothesize that chronic long-term exposure to a major source of air pollution, transportation and its related constituents, will be associated with elevated stroke risk.

Methods: In a prospective cohort study of 133,477 women enrolled in the California Teachers Study in 1995-96, we evaluated 110,120 women who resided in California from 2000-2018 (median follow-up=19 years) for residential exposure to primary aerosol concentrations of 8 PM2.5 sources (on- and off-road gasoline vehicles, on- and off-road diesel, biomass combustion, food cooking, aircraft, natural gas combustion), and 11 constituents (including copper, iron, manganese, nitrate, elemental carbon, organic compounds) generated from a 4-km grid chemical transport model. Stroke and stroke subtypes were identified during cohort follow-up with ICD-9 and ICD-10 codes from state hospitalization records (all stroke n=4,348; ischemic stroke n=3,596; hemorrhagic stroke n=752). Cox proportional hazards models were used to estimate hazard ratios (HRs) for associations between the pollutants (average exposure across follow-up) ) with overall stroke and stroke subtypes, adjusted for stroke risk factors.

Results: We observed positive associations between PM2.5 exposure from on-road gasoline vehicles with all stroke (HR=1.18, 95% CI=1.11-1.26), ischemic stroke (HR=1.16, 95% CI=1.09-1.26), and hemorrhagic stroke (HR=1.28, 95% CI=1.09-1.49). PM2.5 from off-road diesel was also associated with all stroke (HR=1.21, 95%CI=1.11-1.31), ischemic stroke (HR=1.19, 95% CI=1.09-1.30), and hemorrhagic stroke (HR=1.29, 95% CI=1.06=1.57). PM2.5 from biomass combustion was positively associated with ischemic stroke (HR=1.10, 95% CI=1.02-1.19), but not hemorrhagic stroke. Of PM2.5 constituents evaluated, organic compounds were associated with ischemic stroke (HR=1.14, 95% CI=1.00-1.31).

Conclusions: Two major PM2.5 sources, on-road gasoline transportation and off-road diesel, are associated with increased risk of all stroke and stroke subtypes among our female cohort. Long-term exposure to biomass combustion, which includes wildfires, and organic compounds from PM2.5 were linked to ischemic stroke only.

Key words:

air pollution, stroke, epidemiology, population science