title: Outdoor fine particles and nonfatal strokes: systematic review and meta-analysis.
abstract: BACKGROUND
  Epidemiologic studies find that long- and short-term exposure to fine particles (PM2.5) is associated with adverse cardiovascular outcomes, including ischemic and hemorrhagic strokes. However, few systematic reviews or meta-analyses have synthesized these results.
  
  
  METHODS
  We reviewed epidemiologic studies that estimated the risks of nonfatal strokes attributable to ambient PM2.5. To pool risks among studies we used a random-effects model and 2 Bayesian approaches. The first Bayesian approach assumes a normal prior that allows risks to be zero, positive or negative. The second assumes a gamma prior, where risks can only be positive. This second approach is proposed when the number of studies pooled is small, and there is toxicological or clinical literature to support a causal relation.
  
  
  RESULTS
  We identified 20 studies suitable for quantitative meta-analysis. Evidence for publication bias is limited. The frequentist meta-analysis produced pooled risk ratios of 1.06 (95% confidence interval = 1.00-1.13) and 1.007 (1.003-1.010) for long- and short-term effects, respectively. The Bayesian meta-analysis found a posterior mean risk ratio of 1.08 (95% posterior interval = 0.96-1.26) and 1.008 (1.003-1.013) from a normal prior, and of 1.05 (1.02-1.10) and 1.008 (1.004-1.013) from a gamma prior, for long- and short-term effects, respectively, per 10 μg/m PM2.5.
  
  
  CONCLUSIONS
  Sufficient evidence exists to develop a concentration-response relation for short- and long-term exposures to PM2.5 and stroke incidence. Long-term exposures to PM2.5 result in a higher risk ratio than short-term exposures, regardless of the pooling method. The evidence for short-term PM2.5-related ischemic stroke is especially strong.
authors.affiliation: From the aEnvironmental Health Science and Research Bureau, Health Canada, Ottawa, ON, Canada; bDepartment of Mathematics and Statistics, Queen's University, Kingston, ON, Canada; cHealth and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, NC; and dHealth Effects Institute, Boston, MA.
authors.forename: Hwashin H
authors.initials: HH
authors.lastname: Shin
authors.forename: Neal
authors.initials: N
authors.lastname: Fann
authors.forename: Richard T
authors.initials: RT
authors.lastname: Burnett
authors.forename: Aaron
authors.initials: A
authors.lastname: Cohen
authors.forename: Bryan J
authors.initials: BJ
authors.lastname: Hubbell
mesh_terms: D000393:Air Pollutants / Q000633:toxicity*; D001499:Bayes Theorem; D004781:Environmental Exposure / Q000009:adverse effects*; D006801:Humans; D052638:Particulate Matter / Q000633:toxicity*; D012307:Risk Factors; D020521:Stroke / Q000139:chemically induced* / Q000453:epidemiology*
chemical_list: D000393:Air Pollutants; D052638:Particulate Matter