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NEW YORK (Reuters Health) - Other people's smoke is bad for your lungs and bad for your heart, and new research suggests it could be bad for your mental health, too.
Researchers found that non-smokers exposed to a lot of secondhand smoke were 50 percent more likely to suffer from psychological distress than those not exposed to other people's smoke.
And their risk of being admitted to a psychiatric hospital over the next six years was nearly tripled (it was almost quadrupled for smokers).
So-called "passive smoking" is very common, Dr. Mark Hamer of University College London in the UK and colleagues note in the Archives of General Psychiatry. One US study found evidence of secondhand smoke in 60 percent of non-smokers.
Studies measuring the nicotine byproduct cotinine have made it possible to precisely measure secondhand smoke exposure and its health effects, they add, but there is "very limited information" on how other people's smoke might affect mental health.
To investigate, Hamer and his colleagues studied 5,560 non-smoking adults and 2,595 smoking adults, none of whom had a history of mental illness. The study subjects answered questions about psychological distress and admissions to psychiatric hospitals were tracked for six years.
Exposure to secondhand smoke among non-smokers was determined using saliva levels of cotinine, which is formed when nicotine is broken down in the body and is an established marker of nicotine exposure.
A total of 14.5 percent of study subjects reported psychological distress. According to the investigators, the higher a person's secondhand smoke exposure, the greater their risk of psychological distress, while the risk was highest for people who were themselves smokers.
People with high exposure to secondhand smoke (those with the highest cotinine levels) who didn't actually smoke themselves were 62 percent more likely to report psychological distress than those unexposed to secondhand smoke, while the risk for smokers was 2.45 times greater.
During follow-up, which averaged about six years, 41 people were admitted to psychiatric hospitals. The risk of hospitalization was 2.8 times greater for secondhand smokers compared to people not exposed to secondhand smoke, while it was 3.7 times greater for smokers.
The effects were stronger for never-smokers than for ex-smokers, Hamer noted; the fact that former smokers were able to quit could suggest they were intrinsically less vulnerable to the effects of nicotine.
Studies like the current one can't prove that something caused something else, Hamer said in an interview. However, he added, the link remained even after he and his colleagues accounted for social status, alcohol intake and other factors that could influence both the risk of mental health problems and the likelihood of being exposed to secondhand smoke. "We did see pretty robust associations that remained after those adjustments," he said.
Moreover, Hamer and colleagues note that animal studies have hinted that tobacco may depress a person's mood and some human studies have also suggested a potential link 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 investigators conclude.
Archives of General Psychiatry, June 7, 2010.