Passive smoking is associated with psychological distress and the risk of developing severe psychiatric illness, a cross-sectional and longitudinal study has found.

A total of 8,155 adults, of whom 2,595 were smokers, with no previous history of admission to psychiatric hospital were recruited from the Scottish Health Survey.

Salivary cotinine levels were used as a marker of nicotine exposure. Participants who claimed to be non-smokers but had a cotinine level ≥15µg/L were recategorised as smokers.

Participants were asked to complete the 12-item General Health Questionnaire (GHQ-12), which enquires about general happiness, symptoms of anxiety and depression, and sleep disturbance over the previous four weeks. Data on psychiatric hospital admissions was collected prospectively over an average of 5.9 years.

After adjustment for covariables, exposure to high levels of secondhand smoke (cotinine level 0.71-14.99 µg/L) was found to be significantly associated with psychological distress (GHQ-12 score ≥ 3); odds ratio 1.49 (95% CI: 1.13-1.97). The risk of future admission to psychiatric hospital was also increased; hazard ratio 2.84 (95% CI: 1.07-7.59). The risk associated with active smoking was even higher; hazard ratio 3.74 (95% CI: 1.55-8.98).

The GHQ-12 measures non-specific psychological distress and is designed to detect any psychiatric disorder. In primary care settings, the positive predictive value (PPV) for any depressive disorder is 31%,1 whereas the PPV for major depression is only 12%.2 In a UK community sample, where the point prevalence of major depression is 2.6%, the PPV will be only 6%.

Curiously, whereas the cross-sectional study was concerned with mild psychological distress, the longitudinal study was concerned with severe mental illness. The number of participants admitted to hospital was very small, which is reflected in the wide confidence interval. There was a broad range of diagnoses, and the study was underpowered to detect any specific associations.

Compared with non-smokers, heavy smokers have more than twice the odds of major depression.3 There is evidence of a cause and effect relationship between smoking and depression,4 possibly mediated by effects on dopamine signalling or the HPA axis. It would now appear that smoking can also have a detrimental effect on the mental health of other family members.

However, it may not be the inhaled cigarette smoke that is harmful. Passive smokers are more likely to have a depressed partner, and there is evidence that depression is contagious. One study found that roommates of depressed college students were more likely to suffer from depressive symptoms, and excessive reassurance seeking is suggested as the vehicle whereby distress is passed from one person

to another.5

It is unlikely to be helpful if we suggest to patients that their depression has been caused by their partner's smoking habit. Nevertheless, this study provides a useful reminder that we should consider depression within the context of relationships: depression is often preceded by marital problems, and the effects of the depression on the other partner and his/her reaction to it are likely to have an important influence on the future course of both the depression and the relationship.

Dr Phillip Bland