Long-term Exposure to Ambient Ozone and Mortality in a population-based cohort of South Korea

Abstract

Background and aim: Compared to robust evidence from studies on short-term exposure, the health effects of long-term ozone exposure have been less studied, and the results are mixed. One of the potential sources of inconsistency is the difference in exposure metrics. This study aimed to investigate the association between long-term exposure to ambient ozone and mortality in South Korea, using different exposure metrics. We also examined sources of heterogeneity between previous studies. Methods: The study population comprised 179,806 subjects from the National Health Insurance Service-National Sample Cohort (2002-2015) residing in 7 major cities in South Korea. Several ozone exposure metrics (year-round 24-h, year-round 8-h, warm-season 24-h, and warm-season 8-h) were calculated for each district. Time‐varying Cox proportional hazards models were adopted to estimate the association between ozone and all-cause and cause-specific mortality. Random-effects meta-analysis and meta-regression analysis were performed to pool the effect estimates of previous studies and examined whether selected study characteristics can explain between-study heterogeneity. Results: The HRs per 10 ppb increment in year-round 24-h ozone for all-cause (HR, 1.18; 95% CI, 1.07-1.29) and circulatory (HR, 1.52; 95% CI, 1.25-1.84) mortality were higher than that of other exposure metrics. For respiratory mortality, year-round 8-h ozone showed the largest associations (HR per 10 ppb, 4.13; 95% CI, 1.04-1.96). The meta-analysis of 26 previous studies and the present study showed a positive association between long-term ozone exposure and all-cause mortality, with the largest HR from warm-season 8-h ozone (HR, 1.042; 95% CI, 1.015-1.071). The exposure metric was significantly associated with the effect estimates in the multivariable meta-regression model. Conclusions: In the population-based cohort of South Korea, we found positive associations between several long-term ozone exposure metrics and mortality. Different ozone exposure metrics could be an important source of heterogeneity between studies. Keywords: ozone, mortality, long-term exposure, cohort study, meta-analysis