LONDON – Passive smoking not only comes with well-known physical risks, but also seems to favour depression, a new study suggests.
The study, published in the journal Archives of General Psychiatry, examined the associations between mental health and second-hand smoke (SHS) exposure by measuring the biochemical marker cotinine, which is found in saliva and can be used to measure levels of exposure to tobacco smoke.
The study found that SHS exposure is associated with psychological distress and risk of future psychiatric illness in healthy adults.
A general population sample of 5,560 nonsmoking adults and 2,689 smokers without history of mental illness were drawn from the 1998 and 2003 Scottish Health Surveys. A score greater than 3 on the 12-item General Health Questionnaire was defined as an indicator of psychological distress.
Psychological distress was apparent in 14.5% of the sample. After adjustments for a range of potential confounding factors such as social status, the researchers found that high SHS exposure among nonsmokers (cotinine levels between 0.70 and 15 micrograms per litre) was associated with 50% higher odds of reporting psychological distress in comparison with participants with cotinine levels below the limit of detection.
As expected, active smokers were also more likely to report psychological distress. The risk of future psychiatric illness was related to high SHS exposure as well as to active smoking.
In previous studies, an association between depression and suicide risk had been demonstrated in smokers; however, confounding factors such as an increased prevalence of heavy alcohol consumption and low mental wellbeing among those who smoke made the interpretation of findings difficult.
The strong points of this prospective study are that it used an objective biomarker of nicotine exposure and included patients who do not smoke, concludes Dr Mark Hamer of the Department of Epidemiology and Public Health, University College London, UK.
After adjusting for psychological distress at baseline, nonsmokers exposed to passive smoke showed a “robust dose–response association between objectively assessed nicotine exposure and psychological distress, which was already apparent at low levels of SHS exposure.”
“Animal data have already indicated that tobacco can induce negative mood, suggesting that tobacco exposure may be a direct cause of psychiatric illness.”
As to the mechanisms of how passive smoking may affect the psyche, the authors write that contributing effects may be a dysfunction of the hypothalamic-pituitary-adrenal axis, low-grade inflammation associated with passive smoking and a heightened smoking-induced dopamine release in genetically predisposed individuals, which in itself has been associated with greater risk of depression and anxiety.
Hamer M. et al. Arch Gen Psychiatry. 2010 Jun 7. [Epub ahead of print]