Cigarette smoke inhalation and risk of lung cancer: a case-control study in a large Japanese population

Several studies have shown that cigarette smoke inhalation is associated with an increased risk of lung cancer (LC) in European populations. The aim of our study was to clarify the relationship between cigarette smoke inhalation and the risk of LC in a Japanese population. We carried out a large case-control study of cigarette smoking and the risk of LC in Japan. Cases were newly diagnosed patients with histologically confirmed LC (n=653). Controls (n=1281) included hospital controls (n=453) and community-based controls (n=828). Odds ratios (OR) and 95% confidence intervals (CI) were derived from unconditional logistic regression analysis, adjusted for basic confounding variables including age, sex, drinking status, fruit and vegetable intake, family history of LC, occupation, and years of education. Compared with never smokers, ORs for ever smokers who do not inhale cigarette smoke (noninhalation) and ever smokers who inhale cigarette smoke (inhalation) were 1.72 (95% CI: 1.15-2.59) and 3.28 (95% CI: 2.38-4.53), respectively, when adjusted for basic confounding variables. When the analysis was restricted to ever smokers, the OR adjusted for basic confounding factors and pack-year of the risk of LC in the inhalation group was significantly higher than that in the noninhalation group. OR for the inhalation group compared with the noninhalation group was 1.52 (95% CI: 1.06-2.18, P=0.021). A similar pattern was observed in subcategory analyses for adenocarcinoma, squamous cell carcinoma and small cell carcinoma, and other histological types, although without statistical significance. Our case-control study showed that inhalation of cigarette smoke is a significant risk for LC independent from pack-years in a Japanese population.