Roni Caryn Rabin reports in The New York Times June 10, 2010, on a study that analyzed data from the Scottish Health Survey of 1998 and 2003.

Smokers are known to suffer from high rates of depression and other mental health problems, and now a study reports that even people exposed to secondhand smoke are at significantly increased risk — and more likely to be hospitalized for mental illness.

The study analyzed … a nationally representative sample of about 5,560 nonsmoking adults and 2,595 smokers. Nonsmokers exposed to secondhand smoke were 1.5 times as likely to suffer from symptoms of psychological distress as unexposed nonsmokers, the study found.

While Rabin’s article is just a summary, the referenced study is easy to find and moderately easy to read.

The study talks about evidence that tobacco can induce a negative mood.

It also poses the chicken and egg question: are people with mental illnesses more likely to be in an environment with second hand smoke?

As I read the article I look for answers regarding heritability. A nervous parent, that self-medicates with tobacco, could genetically pass on a susceptibility to nervousness for example. But also the way a nervous parent acts around their infant and young child, is going to teach that child a lot about nervousness, and the child is going to develop unhealthy coping skills, for example. Does the study eliminate children of smokers from the study?  Should it?

Another question: is mental health contagious (post partum depression in fathers, Dr. Holland on Science Friday)?  I’ve read that one or two mentally ill apples can spoil a bunch.  You’ve probably seen evidence of this at the office or in your family–when there is one or two people that are negative, or perhaps overly emotional, does that “infect” the behavior of the other people in the group?  Second hand smokers are physically in the same group as smokers.

Another thing I wonder about is socio-economic questions. Do smokers have less money? Are they under stress to make their monthly house payment, rent, grocery bill? We are full of stereotypes about smokers. Are they lonely? Do they drink too much?

All these questions are a trap. If you’ve read this far in the post, I want to make a point that was true in my personal experience, and it may be yours. I do all this analyzing to understand why the person might be depressed–search for the cause. I find a plausible theory that explains it (genetics, low income …) and I ignore the most important fact–the depression (or anxiety) is real and needs to be addressed.

Here is a quote from the study:

The fact that the results of the age-adjusted and fully adjusted models were similar suggests that the associations were not accounted for by measured covariates. Although CIs were relatively large in some analyses, the effect sizes were substantial.

CI stands for Cotinine Level. “Exposure to Second Hand Smoke was assessed using the salivary cotinine level, which is a reliable and valid circulating biochemical marker of nicotine exposure.” The co-variates were “age, sex, social status, BMI (body mass index), chronic illness, psychological distress at baseline, physical activity, and alcohol intake.”

More quotes from the Comment section at end of study that touch on my questions above. (The Comment section is the easiest part to understand.)

In a cohort of Swedish participants, heavy smoking was associated with increased risk of suicide over 26 years of follow-up, but the excess risk of suicide among smokers was almost entirely explained by an increased prevalence of heavy alcohol consumption and low mental well-being among the smokers.

In addition, the dopaminergic system may play a role. Smokers who are genetically predisposed to low resting intrasynaptic dopamine levels have heightened smoking-induced dopamine release, which has been associated with greater depression and anxiety. Thus, this genetic predisposition may also operate in relation to SHS exposure.

The limitations of the study should also be recognized. We did not have sufficient suicide deaths to facilitate a meaningful analysis. Given that much psychiatric illness is managed in primary care or in outpatient clinics, in our prospective analyses we only captured cases severe enough to warrant hospital admission.