This is first meta-analysis to evaluate cancer risk associated with secondhand smoking across all cancers. A literature search was conducted for articles published before June 2014 on Pubmed, SCOPUS, Cochrane library, and CINAHL, and 40 articles on secondhand smoke and the prevalence of cancer among never smokers were selected for final analysis as per the inclusion criteria. Of the 40 articles, 27 were case-control studies and 13 were prospective cohort studies. With respect to overall cancer risk, odds ratio (OR) involving never smokers with significant exposure to secondhand smoke compared to never smokers without such exposure was 1.163 (95%CI 1.058⁻1.279). Subgroup meta-analyses by study design showed significant positive associations for both case-control studies and prospective cohort studies (OR 1.165, 95%CI 1.029⁻1.320; and OR 1.160, 95%CI 1.002⁻1.343, respectively). The association was stronger in the case of females (OR 1.253, 95%CI 1.142⁻1.374), lung cancer (OR 1.245, 95%CI 1.026⁻1.511), and breast cancer (OR 1.235, 95%CI 1.102⁻1.385). Secondhand smoking may increase the overall risk of cancer for never smokers, particularly lung and breast cancer, and especially in women. Strict implementation of smoking cessation programs should be encouraged, not only to reduce active smoking but also to limit exposure to secondhand smoke.
Keywords: cancer; meta-analysis; neoplasm; passive smoking; secondhand smoking; tobacco smoke pollution.