Background: Air pollution is a major environmental hazard to human health and a leading cause of morbidity for asthma worldwide. Objectives: To assess the current evidence on short-term effects (from several hours to 7 days) of exposure to ozone (O 3 ), nitrogen dioxide (NO 2 ), and sulphur dioxide (SO 2 ) on asthma exacerbations, defined as emergency room visits (ERVs) and hospital admissions (HAs). Methods: We searched PubMed/MEDLINE, EMBASE and other electronic databases to retrieve studies that investigated the risk of asthma-related ERVs and HAs associated with short-term exposure to O 3 , NO 2 , or SO 2 . We evaluated the risks of bias (RoB) for individual studies and the certainty of evidence for each pollutant in the overall analysis. A subgroup analysis was performed, stratified by sex, age, and type of asthma exacerbation. We conducted sensitivity analysis by excluding the studies with high RoB and based on the E-value. Publication bias was examined with the Egger’s test and with funnel plots. Results: Our literature search retrieved 9,059 articles, and finally 67 studies were included, from which 48 studies included the data on children, 21 on adults, 14 on the elderly, and 31 on the general population. Forty-three studies included data on asthma ERVs, and 25 on asthma HAs. The pooled relative risk (RR) per 10 µg/m 3 in- crease of ambient concentrations was 1.008 (95%CI: 1.005, 1.011) for maximum 8-hour daily or average 24-hour O 3 , 1.014 (95%CI: 1.008, 1.020) for average 24-hour NO 2 , 1.010 (95%CI: 1.001, 1.020) for 24-hour SO 2 , 1.017 (95%CI: 0.973, 1.063) for maximum 1-hour daily O 3 , 0.999 (95%CI: 0.966, 1.033) for 1-hour NO 2 , and 1.003 (95%CI: 0.992, 1.014) for 1-hour SO 2 . Heterogeneity was observed in all pollutants except for 8-hour or 24-hour O 3 and 24-hour NO 2 . In general, we found no significant differences between subgroups that can explain this heterogeneity. Sensitivity analysis based on the RoB showed certain differences in NO 2 and SO 2 when considering the outcome or confounding domains, but the analysis using the E-value showed that no unmeasured confounders were expected. There was no major evidence of publication bias. Based on the adaptation of the Grading of Recommendations Assessment, Development and Evaluation, the certainty of evidence was high for 8-hour or 24-hour O 3 and 24-hour NO 2 , moderate for 24-hour SO 2 , 1-hour O 3 , and 1-hour SO 2 , and low for 1-hour NO 2 .