Causal association between ambient ozone concentration and mortality in Seoul, Korea

Background: The linearity of concentration-response (C-R) curve between ambient ozone (O-3) concentration and mortality has been controversial. The aim of the present analysis was to examine the C-R curve between O-3 concentration and mortality with a causal framework approach. Methods: We extracted data of hourly meteorology, hourly O-3 concentration and daily non-accidental mortality in Seoul from 2001 to 2009. We divided the dataset into two, odd-number (training set) and even-number years (testing set). Using the training set, we constructed a prediction model from hourly O-3 concentration with support vector regression estimating the daily variations of mean O-3 concentration caused by sun irradiation, wind speed and direction, controlling temperature, barometric pressure and temporal trend. With this model we predicted variance of daily O-3 from the testing set, thus creating an instrumental variable. We analyzed the association between the instrumental variable and daily mortality. We also analyzed the association according to the quartiles of daily mean O-3 concentration to examine the linearity of the association. Results: The instrumental variable was significantly and negatively associated with daily mortality in the linear model. In the stratified analysis, the negative slope was observed in the lowest quartile and the negative slope of the association diminished as the quartile increased, and the slope became positive over the 3rd quartile (O-3 > 23.3 ppb). The interaction between quartiles and instrumental variable was significant (P = 0.0108). Conclusion: We observed unequal effect of exposure to ambient O-3 concentration on mortality according to the different ranges of daily mean O-3 concentration with a causal framework approach.