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.