Ambient Air Pollution and the Risk of Acute Ischemic Stroke

Background: The link between daily changes in level of ambient fine particulate matter (PM) air pollution (PM <2.5 mu m in diameter [PM2.5]) and cardiovascular morbidity and mortality is well established. Whether PM2.5 levels below current US National Ambient Air Quality Standards also increase the risk of ischemic stroke remains uncertain. Methods: We reviewed the medical records of 1705 Boston area patients hospitalized with neurologist-confirmed ischemic stroke and abstracted data on the time of symptom onset and clinical characteristics. The PM2.5 concentrations were measured at a central monitoring station. We used the time-stratified case-crossover study design to assess the association between the risk of ischemic stroke onset and PM2.5 levels in the hours and days preceding each event. We examined whether the association with PM2.5 levels differed by presumed ischemic stroke pathophysiologic mechanism and patient characteristics. Results: The estimated odds ratio (OR) of ischemic stroke onset was 1.34 (95% CI, 1.13-1.58) (P<.001) following a 24-hour period classified as moderate (PM2.5 15-40 mu g/m(3)) by the US Environmental Protection Agency's (EPA) Air Quality Index compared with a 24-hour period classified as good (<= 15 mu g/m(3)). Considering PM2.5 levels as a continuous variable, we found the estimated odds ratio of ischemic stroke onset to be 1.11 (95% CI, 1.03-1.20) (P=.006) per interquartile range increase in PM2.5 levels (6.4 mu g/m(3)). The increase in risk was greatest within 12 to 14 hours of exposure to PM2.5 and was most strongly associated with markers of traffic-related pollution. Conclusion: These results suggest that exposure to PM2.5 levels considered generally safe by the US EPA increase the risk of ischemic stroke onset within hours of exposure.