Short-term exposure to nitrogen dioxide and ischemic stroke incidence in Shenzhen, China: Modification effects by season and temperature

Background and objectives: China has experienced a serious public health burden because of the increased incidence of ischemic stroke. Evidence describing the association between short-term exposure to nitrogen dioxide (NO2) and ischemic stroke morbidity is limited, and few studies have focused on the effects of season and temperature. This study aimed to evaluate the acute effects of NO2 on ischemic stroke incidence in Shenzhen, a southeastern city of China, considering the modified effects of season and temperature. Methods: A time-stratified case-crossover study was conducted between 2003 and 2014 among 98,482 ischemic stroke hospitalizations. Conditional quasi-Poisson regression was used to estimate the percentage changes in ischemic stroke admissions in relation to each 10 mu g/m(3) increment in NO2. Results: NO2 was positively associated with ischemic stroke onset over the full year, as well as in the cold season (November through April) and on cold days (ambient temperature <= median temperature), with significant single day effects within 3 days after the exposure, and significant cumulative effects within the delayed five days. The maximum percentage changes were obtained at lag0-5, with 1.81% (95% confidence interval (CI) was 0.86-2.76%) over the full year, 2.75% (1.48-4.03%) in the cold season, and 3.04% (1.74-4.35%) on cold days. Additionally, the effects of exposure were found to be greater in males and people with higher education, and were lasting longer in subgroups of older individuals. Conclusions: Our findings provide evidence that reductions in NO2 levels might decrease ischemic stroke morbidity, and enhance the understanding of ischemic stroke occurrence associated with NO2 modified by season and temperature.