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Although there is evidence on the effects of short-term ozone (O 3) exposures on children's respiratory health, few studies have reported results on the effects of long-term exposures. We report the effects of long-term exposure to O 3 on respiratory health outcomes in 10–11-year old children. We conducted a panel study in a sample of the general population of school children in two cities with high average O 3 concentrations, Athens and Thessaloniki, Greece. All 186 participating students were followed up intensively for 5 weeks spreading across a school year. Data was collected through questionnaires, weekly personal O 3 measurements, spirometry, FeNO and time-activity diaries. Long-term O 3 exposure was assessed using fixed site measurements and modeling, calibrated for personal exposures. The associations between measured lung function parameters and lung function growth over the study period, as well as FeNO and the occurrence of symptoms with long-term O 3 exposure were assessed through the application of multiple mixed effects 2-level regression models, adjusting for confounders and for short-term exposures. A 10 μg/m3 increase in calibrated long-term O 3 exposure, using measurements from fixed site monitors was associated with lower FVC and FEV 1 by 17 mL (95% Confidence Interval: 5–28) and 13 mL (3–21) respectively and small decreases in lung growth: 0.008% (0.002–0.014%) for FVC and 0.006% (0.000–0.012%) in FEV 1 over the study period. No association was observed with PEF, FeNO or the occurrence of symptoms. A similar pattern was observed when the exposure estimates from the dispersion models were employed. Our study provides evidence that long-term O 3 exposure is associated with reduced lung volumes and growth. • We investigated the effects of long-term exposure to ambient ozone on respiratory health outcomes in children. • This study shows adverse associations between long-term O 3 exposure and children's lung development, which may be of clinical relevance for a subset of the general population. • These early life impacts of long-term O 3 exposure may provide an explanation for premature mortality and increased morbidity in later life. [ABSTRACT FROM AUTHOR]