Association Between Second-hand Smoke Exposure and Depressive Symptoms Among Japanese Adults: A Cross-sectional Study

Background: Second-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. Methods: Japanese 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. Results: Overall, 5,121 participants (4,547 non-smokers and 574 smokers) were included whose mean age was 63.6 (standard deviation [SD], 10.3) years for non-smokers and 59.33 (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 compared with "almost never" were 1.25 (95% CI, 1.09-1.42) for "sometimes" and 1.41 (95% CI, 1.09-1.84) for "almost every day" (P for trend <0.001); among smokers, PRs compared with "almost never" were 1.30 (95% CI, 0.82-2.06) for "sometimes" and 1.44 (95% CI, 0.90-2.33) for "almost every day" (P for trend = 0.144). Conclusions: Second-hand smoking and depressive symptoms were associated among non-smokers. Our findings indicate the importance of tobacco smoke control for mental health.