Heterogeneous associations between short-term ambient ozone exposure and morbidities from infants to seniors: A nationwide case-crossover study in South Korea

• This study assessed the ozone (O3) risks of hospitalizations for major five diseases. • National Health Insurance data covering ∼99% of South Korean people was used. • The O3 risk estimates of mental disorders was observed across all adult ages (> 20y). • People aged 0–6y or ≥75y had a linkage to respiratory/genitourinary morbidities. • For endocrine/circulatory diseases, the association with O3 was weak. This study aimed to evaluate the nationwide age-disease-specific associations between short-term exposure to short-term ambient ozone and morbidity because existing studies were limited in assessing the associations between ambient ozone and various morbidities in young or middle-aged populations. Therefore, the study estimated the association between short-term exposure to ambient warm-season ozone (April–September) and hospitalizations via the emergency department (ED admissions) using the National Health Insurance Database, which includes around 99% of people residing in South Korea. The ED admission data covered five major cardiovascular, respiratory, genitourinary, endocrine, and mental disorders/diseases with 51 specific categories. A time-stratified case-crossover design and a machine learning-based ozone-prediction model (R 2 > 0.92) were used. In the total population, the relationship with ambient ozone was most pronounced in ED admissions for mental disorders (odds ratio [OR]: 1.007 with 95% CI: 1.004–1.011) compared to other causes of ED admission, and the higher relationship with ED admissions for mental disorders was generally observed in all adult ages (aged 20 years or older). The associations between ozone and ED admissions for genitourinary and respiratory diseases were observed in individuals aged 0–6 years and over 65–75 years. Excess ED admission costs due to non-compliance with the WHO guidelines (>100 µg/m 3 ) were prominent for respiratory diseases, genitourinary diseases, and mental disorders. The population-representative findings regarding heterogeneous excess risk and cost estimates of short-term ozone by combinations between ages and morbidities could contribute to establishing more targeted public health policies and precision social resource mobilizations against ambient ozone.