Secondhand smoking and neurological disease: a meta-analysis of cohort studies

Background The results of studies on the relationship between secondhand smoking and neurological disease remain controversial. We conducted a meta-analysis to explore the association between secondhand smoking and risk of neurological disease. Methods Literature search was performed in PubMed, Embase and Web of Science through December 2019. We included cohort studies which examined the association between secondhand smoking and risk of neurological disease. According to the type of neurological disease, summary relative risks (RRs) with 95% confidence intervals (CIs) of stroke, cognitive impairment and Parkinson disease (PD) for secondhand smoking exposure were calculated. Results A total of 14 unique articles (n=697,185 participants) were eventually included in the analysis. When all studies were pooled, the RR of stroke for secondhand smoking exposure was 1.20 (95% CI: 1.04, 1.38). When studies included only non-smokers, the pooled RR of stroke was 1.20 (95% CI: 1.05, 1.37). Summary RRs of cognitive impairment and PD for secondhand smoking exposure were 1.43 (95% CI: 1.02, 2.00) and 0.83 (95% CI: 0.72, 0.95), respectively. Summary RRs did not change significantly when excluding one study at a time. Conclusions Secondhand smoking was found to be positively associated with risks of stroke and cognitive impairment, while inversely associated with risk of PD. Overall, our finding is a reminder of the need to step up smoking bans to prevent health damage, and also suggests that more researches on biological mechanisms whereby secondhand smoking affect PD may be beneficial to PD prevention and treatment.