Secondhand smoke exposure and oral cancer risk:A systematic review and meta-analysis

Objectives Inhalation of secondhand smoke (SHS) causes several diseases, including lung cancer. Tobacco smoking is a known cause of oral cancer; however, it has not been established whether SHS also causes oral cancer. The aim of this study was to evaluate the potential association between SHS exposure and the risk of oral cancer. Methods A systematic review and meta-analysis study (following the PRISMA guidelines) was developed to examine the studies reporting on the associations of SHS and the risk of oral cancer, employing a search strategy on electronic databases (PubMed, Web of Science, Scopus, Cochrane Library, Open Grey, and ProQuest databases for dissertations) until 10 May 2020. Meta-analyses and sensitivity analyses were performed using random-effect models. The protocol was registered in PROSPERO (CRD42020189970). Results Following the application of eligibility criteria, five studies were included, comprising a total of 1179 cases and 5798 controls, with 3452 individuals exposed and 3525 individuals not exposed to SHS. An overall OR of 1.51 (95% CI 1.2o to 1.91, p=0.0004) for oral cancer was observed, without significant heterogeneity (I 2 =0%, p=0.41). The duration of exposure of more than 10 or 15 years increased the risk of oral cancer (OR 2.07, 95% CI 1.54 to 2.79, p<0.00001), compared with non-exposed individuals, without significant heterogeneity (I 2 =0%, p=0.76). Conclusions This systematic review and meta-analysis supports a causal association between SHS exposure and oral cancer. Our results could provide guidance to public health professionals, researchers, and policymakers to further support effective SHS exposure prevention programs worldwide.