The associations of secondhand smoke exposure with neurodevelopmental disorders and critical time window identification: A systematic review and meta-analysis

Background: Secondhand smoke (SHS) exposure was harmful for brain development. However, the association between SHS exposure and NDDs diagnosis were unclear. Objectives: To evaluate associations between SHS exposure and NDDs diagnosis, identify critical time windows, and summarize the strength of evidence. Methods: To investigate the associations of SHS exposure and the development of NDDs, we searched Ovid, EMBASE, Web of Science, Cochrane Library, and PubMed for all the relevant studies up to 31 March 2023. The risk estimates and standardized mean differences (SMD) for the individuals with any NDDs who were exposed to SHS exposure compared with those unexposed or low -exposed. Results: The results showed that a total of 31,098 citations were identified, of which 54 studies were included. We identified significant associations between SHS exposure and the risks of NDDs including specific types of NDDs like attention deficit hyperactivity disorder (ADHD) and learning disabilities (LD) despite the observed hetero- geneity for NDDs and ADHD. We also observed a significant association between cotinine exposure and ADHD. However, inconsistent ratings between the two quality -of -evidence methods for all the meta -analyses indicated the current evidence of the associations and the potential exposure window remained inconclusive. Discussion: Our findings suggested that SHS exposure was associated with a higher risk of developing ADHD and LD, with inconclusive quality -of -evidence. In addition, period -specific associations remained unclear based on current evidence.