Exposure to second-hand smoke may lead to higher risk of mental illness, study says

From MedPage Today:

Healthy adults exposed to secondhand smoke appear to be at higher risk of suffering psychological distress and future psychiatric illness requiring hospitalization, according to a major Scottish population study.

The study, which tracked more than 8,000 adults over six years found that nonsmokers exposed to high levels of secondhand smoke, as measured by salivary levels of the nicotine breakdown product cotinine, were at a 49% higher adjusted risk of psychological distress (OR 1.49; 95% CI 1.13 to 1.97) compared with nonsmokers who had undetectable salivary levels of cotinine, according to the report published online June 7 in the Archives of General Psychiatry.

High exposure to secondhand smoke (a salivary cotinine level of greater than 0.70 μg/L and less than 15.00 μg/L) raised the risk of future hospitalization for psychiatric treatment nearly threefold for nonsmokers exposed to high levels of secondhand smoke (HR 2.84; 95% CI 1.07 to 7.59) and nearly four-fold for smokers (HR 3.74; 95% CI 1.55 to 8.98), after adjustment for multiple variables.

In the U.S., an estimated 60% of nonsmokers have some biological evidence of exposure to secondhand smoke. "Even a low level of risk may have a major public health impact," Mark Hamer, PhD, of University College London, and colleagues wrote.

Among the entire study cohort, 14.5% of smokers and nonsmokers reported psychological distress. "We found a robust dose-response association between objectively assessed nicotine exposure and psychological distress, which was apparent at low levels of secondhand smoke exposure and was strongest in current smokers," the authors commented. "This association was replicated in prospective analyses that demonstrated an association between secondhand smoke exposure, active smoking, and risk of psychiatric episodes over six years of follow-up."

Hamer and colleagues noted that a growing body of research has linked secondhand smoke with adverse effects on physical health, but much of this evidence is based on crude, self-report measures, such as exposure in the workplace or through family members who smoke.

"Recent studies using valid objective biochemical markers of secondhand smoke have reported associations with various health outcomes, including markers of inflammation, glucose control, and cardiovascular disease risk," the authors noted. "There is, however, very limited information on the association between objectively assessed secondhand smoke exposure and mental health in humans." Animal data suggest that tobacco may induce negative mood, and some human studies have identified a potential association between smoking and depression.

To provide more evidence based on more objective measures, Hamer and colleagues studied 5,560 nonsmoking adults and 2,595 smokers who had participated in the Scottish Health Survey in 1998 or 2003. At the time of enrollment, participants did not have a history of mental illness. Smoke-free legislation was also not in effect in Scotland at that time.

Nonsmokers with higher cotinine levels were significantly younger, had lower socioeconomic status, higher BMI, more chronic illness, less physical activity, and higher alcohol consumption than those with undetectable continine levels.

The participants initially completed the General Health Questionnaire in 1998 as part of the health survey, which included questions to evaluate psychological distress and mental illness. At that time, participants' exposure to secondhand smoke was assessed using saliva levels of cotinine, the main product formed when nicotine is broken down by the body. In 2003, the participants completed the survey again, which allowed the researchers to evaluate changes in their mental health, including levels of psychological distress and admissions to psychiatric hospitals.

"The prospective nature of our study adds considerably to the current evidence base," the authors wrote. "In our analyses, the association between nicotine exposure and risk of psychiatric events persisted despite adjustment for psychological distress at baseline, which was in itself strongly associated with psychiatric admissions."

The authors cautioned that although they collected data on psychological distress using the questionnaire, they did not account for cases of psychiatric illness that may have required treatment but not hospitalization. They also noted that they did not collect follow-up measurements of cotinine levels and were thus unable to objectively assess participants' changes in smoking status.