| Summary: | Numerous studies have demonstrated that exposure to ambient ozone (O<sub>3</sub>) could have adverse effects on children’s respiratory health. However, previous studies mainly focused on asthma and wheezing. Evidence for allergic rhinitis and bronchitic symptoms (e.g., persistent cough and phlegm) associated with O<sub>3</sub> is limited, and results from existing studies are inconsistent. This study included a total of 59,754 children from the seven northeastern cities study (SNEC), who were aged 2 to 17 years and from 94 kindergarten, elementary and middle schools. Information on doctor-diagnosed allergic rhinitis (AR), persistent cough, and persistent phlegm was collected during 2012–2013 using a standardized questionnaire developed by the American Thoracic Society (ATS). Information for potential confounders was also collected via questionnaire. Individuals’ exposure to ambient ozone (O<sub>3</sub>) during the four years before the investigation was estimated using a satellite-based random forest model. A higher level of O<sub>3</sub> was significantly associated with increased risk of AR and bronchitic symptoms. After controlling for potential confounders, the OR (95% CI) were 1.13 (1.07–1.18), 1.10 (1.06–1.16), and 1.12 (1.05–1.20) for AR, persistent cough, and persistent phlegm, respectively, associated with each interquartile range (IQR) rise in O<sub>3</sub> concentration. Interaction analyses showed stronger adverse effects of O<sub>3</sub> on AR in children aged 7–17 years than those aged 2–6 years, while the adverse association of O<sub>3</sub> with cough was more prominent in females and children aged 7–12 years than in males and children aged 2–6 and 13–17 years. This study showed that long-term exposure to ambient O<sub>3</sub> was significantly associated with higher risk of AR and bronchitic symptoms in children, and the association varies across age and gender. Our findings contribute additional evidence for the importance of controlling O<sub>3</sub> pollution and protecting children from O<sub>3</sub> exposure.
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