Effect modification of the short-term effects of air pollution on morbidity by season: A systematic review and meta-analysis.

Studies of the health effects of air pollution have traditionally controlled for ambient temperature as a confounder, and vice versa. However, season might be an important factor contributing to adverse health effects of air pollution. Given the current inconsistencies in results of previous studies on the effect modification of air pollution on morbidity by season, a systematic review and meta-analysis was conducted to synthesize the current evidence on effects of season on air pollution and morbidity. The electronic databases including PubMed, Web of Science, Embase, CNKI, and Wanfang were used to identify papers published up to the 30st of November in 2019. We identified 4284 articles, after screening, eighty papers met the inclusion criteria. Significant effect modification of CO, O3, SO2 and NO2 on morbidity by season was observed, with corresponding ratio of relative risk of 1.0009 (95% CI: 1.0001-1.0018), 1.0080 (95% CI: 1.0021-1.0138), 0.9828 (95% CI: 0.9697-0.9962) and 0.9896 (95% CI: 0.9824-0.9968), respectively. Season significantly modified the effect of CO on pneumonia, the effect of SO2 on cardiovascular disease, the effect of PM10 on stroke, and the effect of O3 on stroke, asthma and pneumonia. The effect modifications of air pollution by season were similar among males and females, while the effect estimates seem to be higher among children under 18 years old and the elderly aged 75 or over. Further research is needed to better understand the mechanisms underlying the seasonal variance of the effect of air pollutants on morbidity.