It's well-known that smoking and secondhand smoke are dangerous to our health, but are they also harmful to our mental health? A new study suggests they might be.

The study in the Archives of General Psychiatry has drawn a link between secondhand smoke and psychological distress, as well as the risk of future hospitalization for psychiatric illness.

"To our knowledge, this is the first study to demonstrate a prospective association between objectively assessed secondhand smoke exposure and mental health in a representative sample of a general population," write the authors, who were led by Mark Hamer, of the Department of Epidemiology and Public Health at University College London.

For the study, Hamer and his team looked at 5,560 non-smoking adults, whose average age was about 50, and 2,595 smokers, average age 45, who did not have a history of mental illness. All had participated in the Scottish Health Survey in 1998 or 2003.

Participants were assessed with a questionnaire on their psychological state, with questions on their happiness, symptoms of depression and sleep disturbances. Their medical records were also analyzed to track admissions to psychiatric hospitals over six years of follow-up.

The researchers assessed the non-smoking volunteers' exposure to secondhand smoke using saliva levels of cotinine, a substance the body creates when nicotine is broken down by the body. It's considered "a reliable and valid circulating biochemical marker of nicotine exposure," the authors write.

A total of 14.5 per cent of the participants reported psychological distress. Non-smokers with a high exposure to secondhand smoke had higher odds of psychological distress when compared with those who had no detectable cotinine.

After the researchers adjusted for factors that are known to be linked to psychological distress, such as poor social status, high secondhand smoke exposure among non-smokers was associated with 50 per cent higher odds of psychological distress compared to those with little secondhand smoke exposure.

Over the six-year follow-up, 41 individuals were newly admitted to psychiatric hospitals. Smokers and non-smokers with high exposure to secondhand smoke were both more likely than non-smokers with low levels of secondhand smoke exposure to be hospitalized for depression, schizophrenia, delirium or other psychiatric conditions.

The researchers say it's not clear how secondhand smoke might relate to mental health. Animal data have suggested tobacco may induce a negative mood.

For example in rats, nicotine intake during adolescence leads to decreased sensitivity to natural reward and enhanced sensitivity to stress and anxiety-eliciting situations later in life.

Some human studies have also identified a potential association between smoking and depression, with research suggesting nicotine affects the dopamine system and pathways linked to depression.

"Taken together, therefore, our data are consistent with other emerging evidence to suggest a causal role of nicotine exposure in mental health," the authors write.