Scientists found that non-smoking people exposed to a lot of passive smoke were 50 percent more likely to suffer from mental distress than those who don't stand nearsmoking people.

And their chances of being admitted to apsychiatric clinicover the next five years was nearly tripled (it was almost quadrupled forsmokers).

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 second-hand smoke in 60 percent of no smoking people.

Studies measuring the nicotine byproduct cotinine have made it possible to precisely measuresecondhand smokeexposure and its health effects, they add, but there is "very limited information" on how other people's smoke might affect psychological health.

To investigate, Hamer and his colleagues studied 5,560 non-smoking mature people and 2,595 smoking mature people, none of whom had a history of psychological diseases. The study subjects answered questions about mental distress and admissions to psychiatric clinics were tracked for five years.

Exposure to second-hand smoking among no smoking people was determined using saliva levels of cotinine, which is formed when nicotine is broken down in the body and is an established marker ofnicotineexposure.

A total of 13.5 percent of study subjects reported mental distress. According to the investigators, the higher a person's second-hand smoke exposure, the greater their risk of mental distress, while the risk was highest for people who were themselves smokers.

People with high exposure to second-hand smoke (those with the highest cotinine levels) who didn't actually smoke themselves were 62 percent more likely to report mental distress than those unexposed to passive smoking, while the risk for smokers was 2.55 times greater.

During follow-up, which averaged about six years, 42 people were admitted to psychiatric clinic. The risk of hospitalization was 2.7 times greater for second-hand smokers compared to people not exposed to passive smoking, while it was 3.7 times greater forcigarette consumers.

The effects were stronger for free tobacco people than for ex-smokers, Hamer noted; the fact that former cigarette consumers were able to quit could suggest they were intrinsically less attackable 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 diseases and the likelihood of being exposed to second-hand 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.