Long-Term Exposure to Ambient Ozone and Mortality in Korea: A 12-Year Nationwide Cohort Study

Abstract

Background/Aim: Although there is strong evidence that short-term exposure to ozone is associated with adverse health outcomes, the effects of long-term exposure to ozone have not been well studied. Investigating the health effects of long-term exposure to ozone is important in terms of climate change, disease burden and public health policy. In this study, we assessed the relationship between long-term exposure to ambient ozone and mortality in Korea, using a nationwide population-based cohort. Methods: Mortality data were obtained from National Sample Cohort (2002-2013) in Korea. Several ozone exposure matrices were calculated for each district in 7 major cities, and matched with the cohort data. The final study sample consisted of 212,315 individuals (2,047,103 person-year). Cox-proportional hazard regression model was used to estimate the effects of both fixed and time-dependent exposure of ozone on mortality. Results: Both fixed and time-dependent ozone exposures were associated with all-cause and cause-specific mortality. The association with respiratory mortality was relatively large. The hazard ratios for respiratory mortality in association with 1 ppb increase in ozone (24-hr annual mean) were: 1.027 (95% confidence interval (CI): 1.009, 1.045) for fixed baseline exposure; 1.038 (95% CI: 1.015, 1.061) and 1.028 (95% CI: 1.007, 1.051) for time-varying cumulative and yearly mean exposure, respectively. Inclusion of PM10 in the model did not considerably change the results. Conclusions: This study provides evidence on mortality effects of long-term exposure to ozone. The stronger association with respiratory mortality is consistent with previous studies, even though the effect sizes are relatively large. However, the paucity of independent studies regarding long-term ozone exposure and mortality, especially outside of the United States, limits the comparison of the present result with other studies.