Impacts of Secondhand Smoke on the Onset and Progression of Multiple Sclerosis: A Systematic Review

Introduction: Unlike cigarette smoking, secondhand smoke (SHS) has not been as well described as an environmental risk for Multiple sclerosis (MS) nor as a risk factor for disease progression. We systematically reviewed the association between SHS and the risk of onset and/or progression of MS.Methods: We systematically screened MEDLINE/PUBMED, Science Direct, LILACs, and SCIELO searching for publications between January 1st , 2010, and July 5, 2021 with the following keywords: “multiple sclerosis and smoking”; “multiple sclerosis and passive smoking”; “multiple sclerosis and secondhand smoking”. An 11-year filter was applied from 2010 to 2021. Results: Fifteen articles were included in this review, which consisted of systematic reviews with meta-analysis (N = 2), systematic reviews (N = 2), and observational studies (N = 11). Both meta-analyses reported an impact of SHS on MS onset among secondhand smokers. One of the systematic reviews selected two observational studies showing the association between SHS and MS development, and one study that did not find a significant association between SHS and the risk of MS development. The other systematic review identified selected eight articles showing a relationship between SHS and MS. Seven observational studies reported higher odds of MS onset when associated with SHS. Four observational studies did not show a relationship between SHS and MS onset or progression.Discussion: Most articles showed a positive association between SHS exposure and the risk of developing MS. On the other hand, an association between SHS and a higher risk for MS progression could not be established.