BackgroundSecond-hand smoke exposure has been associated with poor mental health. However, among Japanese adults, little is known about the association between second-hand smoking and depressive symptoms. We examined this association in a cross-sectional study among a Japanese general adult population sample.MethodsJapanese adults were recruited from the Japan Multi-Institutional Collaborative Cohort Study in the Okazaki area between 2012 and 2017. Second-hand smoke exposure and smoking status were assessed using a self-administered questionnaire. Based on their frequency of exposure to second-hand smoke, non-smokers and smokers were categorized as "almost never," "sometimes," and "almost every day." Depressive symptoms were defined by a Kessler 6 score ≥ 5 points. We performed a multivariable Poisson regression analysis to obtain adjusted prevalence ratios (PRs) and 95% confidence intervals (CIs) for depressive symptoms.ResultsOverall, 5,121 participants (4,547 non-smokers and 574 smokers) were included whose mean age was 63.6 years (SD = 10.3 years) for non-smokers and 59.33 years (SD = 10.2 years) for smokers. The association between second-hand smoking and depressive symptoms was significant among non-smokers, but not among smokers. Among non-smokers, PRs (95% CIs) compared with "almost never" were 1.25 (1.09 - 1.42) for "sometimes," 1.41 (1.09 - 1.84) for "almost every day" (p for trend < 0.001), and among smokers, 1.30 (0.82 - 2.06) for "sometimes," 1.44 (0.90 - 2.33) for "almost every day" (p for trend = 0.144).ConclusionsSecond-hand smoking and depressive symptoms were associated among non-smokers. Our findings indicate the importance of tobacco smoke control for mental health.