Fig. 4.
Hazard ratios of stroke incidence associated with an IQR increase in annual PM2.5, annual NO2, and warm-season O3 concentration by study subgroups. All results were derived from the tri-pollutant models. The grey shades indicate the 95% CIs of the overall effect estimates. “Household income” denotes median household income at zip-code level, “Low household income” represents the subgroup with median household income lower than the population median while “High household income” represents the subgroup with median household income higher than the population median. “Education” denotes the proportion of the population > 65 years of age who had not graduated from high school at zip-code level, “Low education” represents the subgroup with the proportion of less educated people higher than the population proportion median while “High education” represents the subgroup with the proportion of less educated people lower than the population proportion median. IQR, interquartile range. HR, hazard ratio. CI, confidence interval.