Outdoor air pollution and the risk of asthma exacerbations in single lag0 and lag1 exposure patterns: A systematic review and meta-analysis

BackgroundAsthma exacerbations accelerate the disease progression, as well as increases the incidence of hospitalizations and deaths. There have been studies on the effects of outdoor air pollution and asthma exacerbations; however, evidence regarding single lag0 and lag1 exposure patterns is inconclusive. ObjectiveTo synthesize evidence regarding the relationship between outdoor air pollution and the asthma exacerbation risk in single lag0 and lag1 exposure patterns. MethodsWe performed a systematic literature search using PubMed, Embase, Cochrane Library, Web of Science, ClinicalTrials, China National Knowledge Internet, Chinese BioMedical, and Wanfang databases until August 1st, 2020. Additionally, we reviewed the reference lists of the relevant articles. Two authors independently evaluated the eligible articles and performed structured extraction of relevant information. Pooled relative risks (RRs) and 95% confidence intervals (CIs) of lag0 and lag1 exposure patterns were estimated using the random-effect models. ResultsEighty-four studies met the eligibility criteria and provided sufficient information for meta-analysis. Outdoor air pollutants were associated with significantly increased risks of asthma exacerbations in both single lag0 and lag1 exposure patterns [lag0: RR (95%CI) (pollutants), 1.057(1.011, 1.103) (air quality index; AQI), 1.007(1.005, 1.010) (PM2.5), 1.009(1.005, 1.012) (PM10), 1.010(1.006, 1.014) (NO2), 1.030(1.011, 1.048) (CO), 1.005(1.002, 1.009) (O3); lag1: RR (95%CI) (pollutants), 1.064(1.022, 1.106) (AQI), 1.005(1.002, 1.008) (PM2.5), 1.007(1.004, 1.011) (PM10), 1.008(1.004, 1.012) (NO2), 1.025(1.007, 1.042) (CO), 1.010(1.006, 1.013) (O3)], except SO2 [lag0: RR (95%CI), 1.004(1.000, 1.007); lag1: RR (95%CI), 1.003(0.999, 1.006)]. Subgroup analyses revealed stronger effects in children and asthma exacerbations associated with other events (including symptoms, lung function changes, and medication use as required). ConclusionThese findings demonstrate that outdoor air pollution significantly increases the asthma exacerbation risk in single lag0 and lag1 exposure patterns. PROSPERO registration numberCRD42020204097 (https://www.crd.york.ac.uk/PROSPERO). Strengths and limitations of this studyO_LIWe performed a systematic literature search of six databases (with no specified start date or language limitation). C_LIO_LISecondary references were included. C_LIO_LIPublication bias was assessed by applying Beggs and Eggers tests. C_LIO_LIThis study focused on the association between outdoor air pollution and the asthma exacerbation risk in single lag0 and lag1 exposure patterns. C_LIO_LIThere were few available studies regarding the AQI, other events, and death analyses. C_LI