Recent epidemiological studies investigating the modifying effect of air temperature in ozone-mortality associations lack consensus as how to adjust for nonlinear temperature effect in addition to including an interaction term.Methods
We evaluated the influence of temperature confounding control on temperature-stratified ozone-mortality risks in a time-series setting in eight European cities and 86 U.S. cities, respectively. To investigate potential residual confounding, we additionally incorporated next day’s ozone in models with differing temperature control.Results
Using only a categorical variable for temperature or only controlling nonlinear effect of low temperatures yielded highly significant ozone effects at high temperatures, but also significant residual confounding in both regions. Adjustment for a nonlinear effect of temperature, especially high temperatures, substantially reduced ozone effects at high temperatures and residual confounding. For example, when using a distributed lag nonliner temperature term, risk estimates at low, medium, and high temperatures were 0.23% (95% CI: −0.09% to 0.55%), 0.23% (95% CI: −0.06% to 0.53%), and 0.36% (95% CI: 0.04% to 0.68%) in European cities, and 0.11% (95% CI: −0.31% to 0.54%), 0.17% (95% CI: −0.07% to 0.41%), and 0.59% (95% CI: 0.32% to 0.85%) in U.S. cities.Conclusion
Inadequate control for confounding by air temperature leads to residual confounding and an overestimation of the temperature modifying effect in studies of ozone-related mortality.