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 short-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 microm (PM(10)) and (2) modeled levels of a mixture of NO and NO2 (NOx) 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 microg/m(3) compared to < 15 microg/m(3) , whereas temperature above 16 degrees C decreased the risk. No consistent associations were found for hemorrhagic stroke or for ischemic stroke and ozone or NOx .
Conclusion: Particulate air pollution and temperature seemed to be associated with ischemic stroke hospital admissions. Individual exposure modeling 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.