Association between second-hand smoke exposure and depression and its moderation by sex: Findings from a nation-wide population survey in Germany.

Background: Findings on the association between exposure to second-hand smoke (SHS) and depression are ambiguous. Similarly, results concerning sex-specific associations are contradictory. Using large and representative survey data from Germany, we examined whether SHS exposure is associated with depressive symptoms/major depression (MDD) and whether this association differs by sex.

Methods: We pooled data from three waves of the German Health Update telephone surveys from 2009-2012 on 28,670 never-smokers. Information on SHS exposure was based on self-reports. The outcome was a reported diagnosis of a depressive episode/MDD in the last 12 months before the survey. Logistic regression with a sex-by-SHS interaction term adjusted for socioeconomic and health-related confounders was conducted. Interaction effects were evaluated based on average marginal effects (AME).

Results: No significant association between SHS exposure and depressive episodes/MDD could be observed (adjusted OR=1.01; 95%-CI=0.86, 1.15). Male never-smokers exposed to SHS were at a significantly lower likelihood of depression than unexposed men (AME= -0.009; 95%-CI= -0.018, -0.0001). No significant differences were observed for women (AME= 0.007; 95%-CI= -0.003, 0.017). Limitations: The association between SHS exposure and depression is prone to residual confounding. Furthermore, all information were cross-sectional and based on self-report.

Conclusions: No association between SHS exposure and depression was observed in the combined sample. Among men, the association was counterintuitive. Never-smokers exposed to SHS are presumably more frequently involved in social interactions than those not exposed which may be protective for the development of depressive symptoms. Additional research accounting for residual confounding is necessary to allow further insights into the relationship between SHS exposure and depression.