eling facilitates a detailed exposure assessment but may also be more prone to misclassification errors. The time series and case crossover approaches yielded similar effect estimates. Abstract Background: Short-term exposure to high levels of air pollution can increase stroke risk. In this study we investigated the short-term effects of air pollution on hospital admissions for stroke in a setting where pollutant levels are rather low. We also addressed methodological issues in evaluating the

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... ort-term effects of air pollution. Methods: Daily admissions of ischemic (n = 11,267) and hemorrhagic (n = 1,681) stroke were obtained from a Swedish quality register for stroke, Riks-Stroke. We used two types of exposure data: (1) daily measured background levels of ozone, temperature and particles with a diameter ! 10 m (PM 10 ) and (2) modeled levels of a mixture of NO and NO 2 (NO x ) at the residential address of each individual. Results: We estimated a 13% (95% confidence interval, 4-22%) increased risk for hospital admissions for ischemic stroke for levels of PM 10 above 30 g/m 3 compared to ! 15 g/m 3 , whereas temperature above 16 ° C decreased the risk. No consistent associations were found for hemorrhagic stroke or for ischemic stroke and ozone or NO x . Conclusion: Particulate air pollution and temperature seemed to be associated with ischemic stroke hospital admissions. Individual exposure mod-