Objectives: To estimate the age-specific effects of 8-hour maximum ozone levels on pneumonia in children and adolescents.

Methods: We performed quasi-Poisson regression analyses at 0-4, 5-9, 10-14, and 15-19 years of age using nationwide time-series data from the Republic of Korea (2011-2015). We constructed distributed lag linear models employing a generalized difference-in-differences method and controlling for other air pollutants.

Results: A 10.0-ppb increase in 8-hour maximum ozone levels was associated with a higher risk of hospital admissions due to pneumonia at 0-4 (relative risk [RR] = 1.02, 95% confidence interval [CI]: 1.01, 1.03) and 5-9 years of age (RR = 1.06, 95% CI: 1.04, 1.08) but not at 10-14 (RR = 1.01, 95% CI: 0.98, 1.04) or 15-19 years of age (RR = 1.01, 95% CI: 0.97, 1.06). The association between ozone and hospital admissions due to pneumonia was stronger was stronger in cool seasons (from November to April) than in warm seasons (from May to October) but was similar between boys and girls.

Conclusion: Short-term exposure to ozone was associated with a higher risk of pneumonia at 0-4 and 5-9 years of age but not at 10-14 or 15-19 years of age. Our findings can help identify vulnerable periods, determine the target populations for public health interventions, and establish air pollution standards.