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All you have to do is look around at the growing number of smoking bans in workplaces, restaurants, bars and airplanes to appreciate how seriously officials take the health risks of smoking. And in public places, that applies as much to the nonsmokers who are exposed to the potentially dangerous effects of cigarette smoke as it does to the smokers themselves.

Now researchers at University College of London have quantified another health risk for those exposed to secondhand smoke: mental-health ills. In a study of 8,155 men and women in the Scottish Health survey, published in the Archives of General Psychiatry, researchers led by Mark Hamer at University College of London documented a 50% greater risk of psychological distress in nonsmokers with the highest levels of nicotine residue in their blood, compared with those with the lowest levels.

"Clearly something is going on even at very low levels, which is quite worrying," says Hamer.

Previous data have suggested a link between smoking and mood disorders, and nicotine exposure in animals is known to trigger depressive symptoms, stress, anxiety and a dampening of feelings of reward and satisfaction. Hamer's group extended those findings to nonsmokers who passively inhale cigarette smoke, and while the study does not establish that the exposure directly causes changes in mood, it does suggest a strong association.

"I don't think it's the be-all and end-all, 100% solid proof, but it's a very reasonable inference," says Dr. Norman Edelman, chief medical officer of the American Lung Association and a professor of preventive medicine at Stony Brook University. "It's a very important conclusion that they draw."

In the study, the authors tested subjects' saliva for levels of cotinine, a well-established marker for nicotine exposure in the past 24 hours, and asked them to complete a 12-item questionnaire to assess their mental state. Hamer accounted for potential confounding factors such as socioeconomic status, which is independently linked to both being exposed to cigarette smoke and a higher risk of depression and anxiety. But even after controlling for this factor, he found that the greater the levels of cotinine, the greater likelihood that the subject would show signs of psychological distress. That, he says, suggests there may be more to the relationship than mere chance.

Edelman points out that the findings are particularly relevant in group-home and mental-health settings, where many caretakers allow patients to smoke as a way to keep them calm and help control their behavior. "What this study is inferring is that just the opposite may be happening, that secondhand smoke may be making their mental-health problems worse," he says. "The risk may be greatest in settings where we manage people with mental-health problems."

What's particularly worrisome is that for the majority of the population, even transient exposure to secondhand smoke is sufficient to have an effect; those with the lowest cotinine levels, equivalent to the amount that might be absorbed during occasional forays to a smoke-filled bar or restaurant, still increased their risk of depressive symptoms and anxiety by 25%, compared with those with negligible exposure.

The survey also revealed that most passive smoke exposure is occurring in the home. Thanks to aggressive legislation to ban smoking in public places, most smokers are lighting up at home, where nonsmokers, including children, are more likely to be exposed on a chronic and intense level. "Passive smoke is very harmful, and if you smoke and live with a nonsmoking partner, we need to help them realize that they are not only doing themselves quite a lot of harm, but they could also be affecting people living with them," says Hamer.

• Alice Park
• A new study has found that the link between smoking and mood disorders may extend to passive, or secondhand, smoke as well