An association between second-hand smoke exposure and depressive symptoms has been reported; however, further research is needed for clarity.

This 20-year prospective cohort study aimed to longitudinally explore the relationships of smoking and second-hand smoke exposure with incident depressive symptoms in community-dwelling middle-aged and older adults.

Data of adults aged ≥40 years were collected from the National Institute for Longevity Sciences – Longitudinal Study of Aging database (third to ninth waves). Participants with baseline (third wave) depressive symptoms, missing data or no follow-up participation were excluded. Baseline data on current cigarette smoking and second-hand smoke exposure were collected. Depressive symptoms were defined as a Center for Epidemiologic Studies Depression Scale score ≥16. Generalised estimating equation models evaluated longitudinal relationships of smoking and second-hand smoke exposure with incident depressive symptoms.

The final analysis included 1697 participants (mean (s.d.) age, 58.7 (11.2) years; mean follow-up, 12.9 years). Depressive symptom incidence ranged from 8.0% (wave 4) to 6.5% (wave 9). Compared with non-current smokers, current smokers showed no significantly higher risk of incident depressive symptoms (multivariable-adjusted odds ratio (95% CI): 1.27 (0.96−1.68)). Subgroup analysis revealed higher risks in male current smokers (adjusted odds ratio (95% CI): 1.40 (1.00−1.94)) and current smokers aged ≥65 years (adjusted odds ratio (95% CI): 1.62 (1.00−2.63)). Current smokers exposed to second-hand smoke had a higher depressive symptom risk than unexposed non-smokers (odds ratio (95% CI): 1.50 (1.05−2.14)) and greater risk (odds ratio (95% CI): 1.39 (1.00−1.94)) than unexposed current smokers.

Smoking, combined with second-hand smoke exposure, is associated with future depressive symptoms in community-dwelling middle-aged and older adults.