Nonsmokers who are exposed to secondhand smoke have twice the risk of major depression as individuals who are not exposed, according to researchers.

USA Today reported March 4 that research unveiled at a recent American Psychosomatic Society meeting examined depression rates among nonsmokers exposed to tobacco smoke at work or home, verifying exposure by measuring blood levels of cotinine, a metabolite for nicotine.

Past research has found higher rates of depression among smokers than nonsmokers, and a previous Japanese study linked depression and secondhand-smoke exposure but did not verify self-reports of exposure with cotinine measurement.

The study was led by Frank Bandiera of the University of Miami School of Medicine, who said that secondhand smoke could raise dopamine levels in the brains of nonsmokers, triggering anxiety and depression.