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Mark Hamer, Ph.D., of University College London, and colleagues studied 5,560 non-smoking adults (average age 49.8) and 2,595 smokers (average age 44.8) who did not have a history of mental illness and participated in the Scottish Health Survey in 1998 or 2003. Participants were assessed with a questionnaire about psychological distress, and admissions to psychiatric hospitals were tracked over six years of follow-up.
Exposure to secondhand smoke among non-smokers was assessed using saliva levels of cotinine -- the main product formed when nicotine is broken down by the body -- "a reliable and valid circulating biochemical marker of nicotine exposure," the authors write. A total of 14.5 percent of the participants reported psychological distress. Non-smokers with a high exposure to secondhand smoke (cotinine levels between 0.70 and 15 micrograms per liter) had higher odds of psychological distress when compared with those who had no detectable cotinine.
Over the six-year follow-up, 41 individuals were newly admitted to psychiatric hospitals. Smokers and non-smokers with high exposure to secondhand smoke were both more likely than non-smokers with low levels of secondhand smoke exposure to be hospitalized for depression, schizophrenia, delirium or other psychiatric conditions.
Animal data have suggested that tobacco may induce a negative mood, and some human studies have also identified a potential association between smoking and depression. "Taken together, therefore, our data are consistent with other emerging evidence to suggest a causal role of nicotine exposure in mental health," the authors write.