title: Short-term changes in ambient temperature and risk of ischemic stroke.
abstract: BACKGROUND
  Despite consistent evidence of a higher short-term risk of cardiovascular mortality associated with ambient temperature, there have been discrepant findings on the association between temperature and ischemic stroke. Moreover, few studies have considered potential confounding by ambient fine particulate matter air pollution <2.5 μm in diameter (PM2.5) and none have examined the impact of temperature changes on stroke in the subsequent hours rather than days. The aim of this study was to evaluate whether changes in temperature trigger an ischemic stroke in the following hours and days and whether humid days are particularly harmful.
  
  
  METHODS
  We reviewed the medical records of 1,705 patients residing in the metropolitan region of Boston, Mass., USA, who were hospitalized with neurologist-confirmed ischemic stroke, and we abstracted data on the time of symptom onset and clinical characteristics. We obtained hourly meteorological data from the National Weather Service station and hourly PM2.5 data from the Harvard ambient monitoring station. We used the time-stratified case-crossover design to assess the association between ischemic stroke and apparent temperature averaged over 1-7 days prior to stroke onset adjusting for PM2.5. We assessed whether differences in apparent temperature trigger a stroke within shorter time periods by examining the association between stroke onset and apparent temperature levels averaged in 2-hour increments prior to stroke onset (0-2 h through 36-38 h). We tested whether the association varied by health characteristics or by PM2.5, ozone or relative humidity.
  
  
  RESULTS
  The incidence rate ratio of ischemic stroke was 1.09 (95% confidence interval 1.01-1.18) following a 5°C decrement in average apparent temperature over the 2 days preceding symptom onset. The higher risk associated with cooler temperatures peaked in the first 14-34 h. There was no statistically significant difference in the association between temperature and ischemic stroke across seasons. The risk of ischemic stroke was not meaningfully different across subgroups of patients defined by health characteristics. The association between ischemic stroke and ambient temperature was stronger on days with higher levels of relative humidity.
  
  
  CONCLUSIONS
  Lower temperatures are associated with a higher risk of ischemic stroke onset in both warm and cool seasons, and the risk is higher on days with higher levels of relative humidity. Based on this study and the body of literature on ambient temperature and cardiovascular events, identifying methods for mitigating cardiovascular risk may be warranted.
authors.affiliation: Cardiovascular Epidemiology Research Unit, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass., USA ; Department of Epidemiology, Harvard School of Public Health, Boston, Mass., USA.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Mostofsky
authors.affiliation: Cardiovascular Epidemiology Research Unit, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass., USA.
authors.forename: Elissa H
authors.initials: EH
authors.lastname: Wilker
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, Boston, Mass., USA ; Department of Environmental Health, Harvard School of Public Health, Boston, Mass., USA.
authors.forename: Joel
authors.initials: J
authors.lastname: Schwartz
authors.affiliation: Department of Environmental Health, Harvard School of Public Health, Boston, Mass., USA.
authors.forename: Antonella
authors.initials: A
authors.lastname: Zanobetti
authors.affiliation: Department of Environmental Health, Harvard School of Public Health, Boston, Mass., USA ; Department of Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Boston, Mass., USA.
authors.forename: Diane R
authors.initials: DR
authors.lastname: Gold
authors.affiliation: Department of Epidemiology, School of Public Health, Brown University, Providence, R.I., USA.
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Wellenius
authors.affiliation: Cardiovascular Epidemiology Research Unit, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass., USA ; Department of Epidemiology, Harvard School of Public Health, Boston, Mass., USA.
authors.forename: Murray A
authors.initials: MA
authors.lastname: Mittleman
keywords: Environment; Epidemiology; Ischemic stroke; Particulate matter