The Association between Secondhand Smoke Exposure and Childhood Asthma: A Systematic Review and Meta-Analysis

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Background: Secondhand smoke (SHS) exposure can trigger asthma exacerbations in children. Different studies have linked increased asthma symptoms and even deaths in children with SHS, but the risk has not been quantified uniformly across studies. We aimed to investigate the role of second-hand smoke (SHS) exposure as a risk factor of asthma among children. Methods We performed a systematic review in PubMed, Scopus, and Google Scholar from June 1975 to November 16, 2018. Effect estimates were pooled using random-effects models. Publication and small study biases were reduced using funnel plots. Findings: Of the 24,319 studies, we identified 8 eligible studies (5 cross-sectional, 2 cohort, and 1 case-control), with a total of 24,573 children included in the meta-analysis. Positive and significant exposure-response relationship between SHS exposure and childhood asthma was observed among the total cohort of the children included (OR 1.15, 95% CI 1.08-1.22, P<0.01). Interestingly, we found that in children aged 6 years old or older, the SHS exposure has more impact on the occurrence of childhood asthma than those aged less than 6-year-old (OR 1.30, 95%CI 1.11-1.51, P<0.05). was no evidence of significant publication or small study bias (Egger test, P = 0.02). Interpretation: The results show a moderate positive association between secondhand smoking exposure and childhood asthma. Thus, these results lend support to continued efforts to reduce childhood exposure to secondhand smoke. Funding Statement: The authors stated: There was no funding source for this study. Declaration of Interests: Authors declare they have nothing to disclose. Ethics Approval Statement: The reporting of this systematic review based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statements to identify studies related to SHS exposure and asthma and we referred to PRISMA checklists one by one during the research process.