Background: Mounting evidence suggests that short-term exposure to ozone increases the risk of asthma exacerbations. However, ozone exposures have been assessed using ambient ozone concentrations averaged over different time periods in different studies. Objective:To evaluate the risks for asthma exacerbations related to ambient ozone measured as 1-hour or 8-hour daily maximum and 24-hour average concentrations. Methods: Based on a literature search in PubMed, EMBASE and Web of Science, we identified all time-series studies as of December 4th, 2018 and included 47 eligible studies in our analyses. Asthma exacerbation is defined as the risk for emergency room visits or hospital admissions. Pooled relative risks (RRs) and 95% confidence intervals (95%CIs) for a 10 μg/m3 increase in daily ozone concentration were estimated using random effect models. Subgroup analyses and sensitivity analyses were also performed to examine the risks for different seasons, regions and age groups and for the robustness of our main findings. Results: Significant and similar associations were found for O3-1 h max (RR,1.012; 95%CI, 1.005–1.019) and O38 h max (RR, 1.011; 95%CI, 1.007–1.014), while marginal effect was identified for O3-24 h average (RR, 1.005; 95% CI, 0.996–1.014). No significant publication bias but high heterogeneities were observed. During the warm season, ozone was significantly associated with asthma exacerbation. O3-1 h max had the highest RR of 1.014 (95%CI, 1.005–1.024), followed by O3-8 h max (RR, 1.012; 95%CI, 1.009–1.016), while marginal association was identified for O3-24 h avg (RR, 1.008; 95%CI, 0.998–1.017). During the cold season, null associations were identified for all the three averaging times. Variations were also observed in region and age. Conclusion: Ozone exposure measured as 1-hour or 8-hour daily max were more consistently associated with asthma exacerbations than 24-hour average exposure during the warm season.