Concentration-response of short-term ozone exposure and hospital admissions for asthma in Texas

Background: Short-term exposure to ozone has been associated with asthma hospital admissions (HA) and emergency department (ED) visits, but the shape of the concentration-response (C-R) curve is unclear. Methods: We conducted a time series analysis of asthma HAs and ambient ozone concentrations in six metropolitan areas in Texas from 2001 to 2013. Using generalized linear regression models, we estimated the effect of daily 8-hour maximum ozone concentrations on asthma HAs for all ages combined, and for those aged 5-14, 15-64, and 65+ years. We fit penalized regression splines to evaluate the shape of the C-R curves. Results: Using a log-linear model, estimated risk per 10 ppb increase in average daily 8-hour maximum ozone concentrations was highest for children (relative risk [RR] = 1.047, 95% confidence interval [CI]: 1.025-1.069), lower for younger adults (RR = 1.018, 95% CI: 1.005-1.032), and null for older adults (RR = 1.002, 95% CI: 0.981-1.023). However, penalized spline models demonstrated significant nonlinear C-R relationships for all ages combined, children, and younger adults, indicating the existence of thresholds. We did not observe an increased risk of asthma HAs until average daily 8-hour maximum ozone concentrations exceeded approximately 40 ppb. Conclusion: Ozone and asthma HAs are significantly associated with each other; susceptibility to ozone is age dependent, with children at highest risk. C-R relationships between average daily 8-hour maximum ozone concentrations and asthma HAs are significantly curvilinear for all ages combined, children, and younger adults. These nonlinear relationships, as well as the lack of relationship between average daily 8-hour maximum and peak ozone concentrations, have important implications for assessing risks to human health in regulatory settings.