Short-term exposure to air pollution and incidence of stroke in the Women's Health Initiative

Background: Evidence of the association between daily variation in air pollution and risk of stroke is inconsistent, potentially due to the heterogeneity in stroke etiology.

Objectives: To estimate the associations between daily variation in ambient air pollution and risk of stroke and its subtypes among participants of the Women's Health Initiative, a large prospective cohort study in the United States.

Methods: We used national-scale, log-normal ordinary kriging models to estimate daily concentrations of fine particulate matter (PM_2.5), respirable particulate matter (PM₁₀), nitrogen dioxide (NO₂), nitrogen oxides (NOx), sulphur dioxide, and ozone at participant addresses. Stroke was adjudicated by trained neurologists and classified as ischemic or hemorrhagic. Ischemic strokes were further classified according to the Trial of ORG 10172 in Acute Stroke Treatment (TOAST) classification. We used a time-stratified case-crossover approach to estimate the odds ratio (OR) of the risk of stroke associated with an interquartile range (IQR) increase in concentrations of each air pollutant. We performed stratified analysis to examine whether associations varied across subgroups defined by age at stroke onset, US census region, smoking status, body mass index, and prior history of diabetes mellitus, hypertension, heart or circulation problems, or arterial fibrillation at enrollment.

Results: Among 5417 confirmed strokes between 1993 and 2012, 4300 (79.4%) were classified as ischemic and 924 (17.1%) as hemorrhagic. No association was observed between day-to-day variation in any pollutant and risk of total stroke, ischemic stroke, or specific etiologies of ischemic stroke. We observed a positive association between risk of hemorrhagic stroke and NO₂ and NO_x in the 3 days prior to stroke with OR of 1.24 (95% CI: 1.01, 1.52) and 1.18 (95% CI: 1.03, 1.34) per IQR increase, respectively. The observed associations with hemorrhagic stroke were more pronounced among non-obese participants.

Conclusions: In this large cohort of post-menopausal US women, daily NO₂ and NOx were associated with higher risk of hemorrhagic stroke, but ambient levels of four other air pollutants were not associated with higher risk of total stroke, ischemic stroke, or ischemic stroke subtypes.