Smokers are known to suffer from high rates of depression and other mental health problems, and now a study reports that even people exposed to secondhand smoke are at significantly increased risk — and more likely to be hospitalized for mental illness.

The study analyzed data from the Scottish Health Survey of 1998 and 2003, a periodic look at a nationally representative sample of about 5,560 nonsmoking adults and 2,595 smokers. Nonsmokers exposed to secondhand smoke were 1.5 times as likely to suffer from symptoms of psychological distress as unexposed nonsmokers, the study found. The risk increased with greater exposure. And though psychiatric hospitalizations were rare over all, they were almost three times as common for the exposed nonsmokers, according to the study, published online June 7 in Archives of General Psychiatry.

While the association between smoking and mental health problems has long been known, researchers have never been able to establish whether either one causes the other, said the paper’s lead author, Mark Hamer, a senior research fellow at University College London.

“This research goes some way toward suggesting nicotine is having some sort of impact on mental health,” Dr. Hamer said. “But of course, we need to do further work.”