Objectively Assessed Secondhand Smoke Exposure and Mental Health in Adults: Cross-sectional and Prospective Evidence from the Scottish Health Survey

Context: Secondhand smoke (SHS) exposure has been related to various somatic health outcomes, although very little is known about the association between SHS exposure and mental health.

Objective: To assess the associations between mental health and SHS exposure, which was objectively measured using the salivary cotinine level as a circulating biochemical marker.

Design, Setting, And Participants: In a cross-sectional and longitudinal study, a representative sample of 5560 nonsmoking adults (mean [SD] age, 49.8 [15.4] years; 45.5% men) and 2595 smokers (mean [SD] age, 44.8 [14.8] years; 50.2% men) without history of mental illness was drawn from the 1998 and 2003 Scottish Health Survey. A priori, study participants with cotinine values of 15.00 microg/L or higher (to convert to nanomoles per liter, multiply by 5.675) were assumed to be smokers and recategorized as such in all analyses.

Main Outcome Measures: A score greater than 3 on the 12-item General Health Questionnaire was used as an indicator of psychological distress. Incident psychiatric hospital admissions over 6 years of follow-up were also recorded.

Results: Psychological distress was apparent in 14.5% of the sample. In logistic regression analyses of the cross-sectional data, after adjustments for a range of covariates, high SHS exposure among nonsmokers (cotinine level >0.70 and <15.00 microg/L) was associated with higher odds of psychological distress (odds ratio = 1.49; 95% confidence interval, 1.13-1.97) in comparison with participants with cotinine levels below the limit of detection (< or = 0.05 microg/L). In prospective analyses, risk of a psychiatric hospital admission was related to high SHS exposure (multivariate adjusted hazard ratio = 2.84; 95% confidence interval, 1.07-7.59) and active smoking (multivariate adjusted hazard ratio = 3.74; 95% confidence interval, 1.55-8.98).

Conclusions: Exposure to SHS is associated with psychological distress and risk of future psychiatric illness in healthy adults. These concordant findings using 2 different research designs emphasize the importance of reducing SHS exposure at a population level not only for physical health but also for mental health.