Studies have proven that second-hand smoke can be physically harmful to non-smokers. But new research shows spending a lot of time around smokers can ruin your mood, too. The study from the University of Miami School of medicine finds non-smokers who are exposed to cigarette smoke at home or at work are twice as likely to suffer from major depression. That's compared with those who don't live or work around smoke. The risk for depression was doubled for even those who worked in places that only allowed smoking in public areas.