Characterization of the concentration-response curve for ambient ozone and acute respiratory morbidity in 5 US cities

Although short-term exposure to ambient ozone (O₃) can cause poor respiratory health outcomes, the shape of the concentration-response (C-R) between O₃ and respiratory morbidity has not been widely investigated. We estimated the effect of daily O₃ on emergency department (ED) visits for selected respiratory outcomes in 5 US cities under various model assumptions and assessed model fit. Population-weighted average 8-h maximum O₃ concentrations were estimated in each city. Individual-level data on ED visits were obtained from hospitals or hospital associations. Poisson log-linear models were used to estimate city-specific associations between the daily number of respiratory ED visits and 3-day moving average O₃ levels controlling for long-term trends and meteorology. Linear, linear-threshold, quadratic, cubic, categorical, and cubic spline O₃ C-R models were considered. Using linear C-R models, O₃ was significantly and positively associated with respiratory ED visits in each city with rate ratios of 1.02-1.07 per 25 ppb. Models suggested that O₃-ED C-R shapes were linear until O₃ concentrations of roughly 60 ppb at which point risk continued to increase linearly in some cities for certain outcomes while risk flattened in others. Assessing C-R shape is necessary to identify the most appropriate form of the exposure for each given study setting.