Ozone and Survival in Four Cohorts with Potentially Predisposing Diseases

Abstract

Time series studies have reported associations between ozone and daily deaths. Only one cohort study has reported the effect of long-term exposures on deaths, and little is known about effects of chronic ozone exposure on survival in susceptible populations. We investigated whether ozone was associated with survival in four cohorts of persons with specific diseases in 105 United States cities, treating ozone as a time varying exposure. We used Medicare data (1985–2006), and constructed cohorts of persons hospitalized with chronic conditions that might predispose to ozone effects: chronic obstructive pulmonary disease, diabetes, congestive heart failure, and myocardial infarction. Yearly warm-season average ozone was merged to the individual follow-up in each city. We applied Cox proportional hazard model for each cohort within each city, adjusting for individual risk factors, temperature, and city-specific long-term trends. We found significant associations with a hazard ratio for mortality of 1.06 (95% confidence interval [CI], 1.03–1.08) per 5-ppb increase in summer average ozone for persons with congestive heart failure; of 1.09 (95% CI, 1.06–1.12) with myocardial infarction; of 1.07 (95% CI, 1.04–1.09) with chronic obstructive pulmonary disease; and of 1.07 (95% CI, 1.05–1.10) for diabetics.We also found that the effect varied by region, but that this was mostly explained by mean temperature, which is likely a surrogate of air conditioning use, and hence exposure. This is the first study that follows persons with specific chronic conditions, and shows that long-term ozone exposure is associated with increased risk of death in these groups.