Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder which is caused by the interplay of genetic and environmental risk factors such as second-hand smoke (SHS). The association between postnatal exposure to SHS and ADHD risk in children was still inconclusive. We performed a systematic review and meta-analysis to explore the definite association. We searched for relevant studies from PubMed, Embase, Ovid, and Web of Science databases up to January 2020. We used random effect models to calculate pooled odds ratio (OR) with 95% confidence interval (CI). Subgroup analyses and sensitive analyses were also performed to solve the heterogeneity. According to our inclusion criteria, 9 studies including 6 cross-sectional studies, 2 cohort studies, and 1 case-control study were included in the final analysis. Postnatal exposure to SHS increased the risk of ADHD in children (OR: 1.60; 95% CI: 1.37-1.87). Children who exposed to SHS were found a slight risk for conduct problems (OR: 1.33, 95% CI: 1.00-1.77). Among the studies which used cotinine as a biomarker for SHS exposure, a lower pooled OR (OR = 1.16, 95%CI = 1.01, 1.33) was observed between cotinine and ADHD in children. Our meta-analysis results suggested that SHS exposure may be a risk factor for ADHD. We also found that SHS exposure may be associated with some adverse behavioral outcomes. More prospective studies should be conducted to confirm the relationship between SHS exposure and ADHD in children.
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