Secondhand smoke enhances lung cancer risk in male smokers: An interaction

Introduction: Previous studies revealed that some indoor air pollutants and fine particle matter can interact with active smoking, enhancing lung cancer risk in smokers. Secondhand smoke (SHS), with remarkable differences from active smoking, contributes significantly to indoor air pollution and generates a considerable amount of fine particle matter, may cause a similar interaction with active smoking. Methods: Information on lifetime SHS along with active smoking and other confirmed or suspected risk factors for lung cancer was collected in this case-referent study. Odds ratios and the 95% confidence intervals (95% CIs) of smoking status in different levels of SHS were evaluated. Potential multiplicative and additive interactions were explored. Results: Compared with never-smokers without SHS, current smokers who were exposed to a high level of SHS demonstrated the highest odds ratio (15.13, 95% CI: 8.60, 26.65), almost doubles the effect in the current smokers without SHS. Significant additive interactions between current smoking and high level of SHS were observed for all lung cancers (synergy index = 1.80, 95% CI: 1.02, 3.24) and the squamous carcinoma subgroup. Conclusions: High level of SHS exposure greatly enhanced lung cancer risk among current smokers, consistent with an additive interaction; while this interaction was predominant for the squamous carcinoma. The results provide new evidence to the rationale of promoting global smoking cessation. Implications: Some indoor air pollutants can interact with active smoking, yielding a synergistic effect on inducing lung cancer. SHS, with noticeable differences from active smoking, is a major source of indoor air pollution. However, little has been known about the effect of SHS in smokers and whether there is a similar interaction between SHS and active smoking. In this study, we evaluated their separate and joint effects and indeed found a more than additive interaction between them. This finding suggests a potential problem of gathering smoking aggravating by venue restriction policies and re-advocates policy efforts on smoking cessation.