Background and Purpose— The association between short-term elevations in ambient air particles and increased cardiovascular morbidity and mortality is well documented. Ambient particles may similarly increase the risk of stroke.

Methods— We evaluated the association between daily levels of respirable particulate matter (aerodynamic diameter ≤10 μm, PM₁₀) and hospital admission for ischemic and hemorrhagic stroke among Medicare recipients (age ≥65 years) in 9 US cities using a 2-stage hierarchical model. In the first stage, we applied the time-stratified case-crossover design to estimate the effect of PM₁₀ in each city. We used a 3-day unconstrained, distributed lag model to simultaneously estimate the effect of PM₁₀ 0 to 2 days before the admission day and controlled for meteorological covariates in all of the models. In the second stage, we used random-effects metaanalytic techniques to combine the city-specific effect estimates.

Results— Ischemic (n=155 503) and hemorrhagic (19 314) stroke admissions were examined separately. For ischemic stroke, an interquartile range increase in PM₁₀ was associated with a 1.03% (95% CI, 0.04% to 2.04%) increase in admissions on the same day only. Similar results were observed with CO, NO₂, and SO₂. For hemorrhagic stroke, no association was observed with any pollutant 0 to 2 days before admission.

Conclusions— These results suggest that elevations in ambient particles may transiently increase the risk of ischemic, but not hemorrhagic, stroke. Studies with more accurate assessment of timing of stroke onset are necessary to confirm or refute these findings.