A new study published in this month's issue of the Archives of Pediatrics and Adolescent Medicine concludes that secondhand smoke exposure among children causes mental illness, including depression, attention deficit disorder, and conduct disorder (see: Bandiera FC, Richardson AK, Lee, DJ, He J-P, Merikangas KR. Secondhand smoke exposure and mental health among children and adolescents. Archives of Pediatrics and Adolescent Medicine 2011; 165(4):332-338).
The research involved a cross-sectional study of the relationship between secondhand smoke exposure, measured by serum cotinine levels, and symptoms of mental illness based on data from the National Health and Nutrition Examination Survey in 2001 through 2004. The researchers found that: "Among nonsmokers, serum cotinine level was positively associated with symptoms of DSM-IV major depressive disorder, generalized anxiety disorder, attention-deficit/hyperactivity disorder, and conduct disorder after adjusting for survey design, age, sex, race/ethnicity, poverty, migraine, asthma, hay fever, maternal smoking during pregnancy, and allostatic load."
The article concludes: "The findings presented herein provide additional evidence on the harmful effects of SHS exposure on children and adolescents. Our results are consistent with data from previous cross-sectional and prospective studies in adults and suggest that exposure to SHS may precipitate the onset of or exacerbate mental disorder symptoms."
The Rest of the Story
The glaring limitation of this study is that it failed to control for mental health problems among the children's parents, which have been demonstrated to be associated both with smoking and with mental health problems among their children. Thus, this is a likely confounder of the observed association between secondhand smoke exposure and mental health problems among children and adolescents.
This flaw does not invalidate the overall finding of an association between secondhand smoke exposure and mental health problems in children, but it does preclude a conclusion that it is the secondhand smoke exposure that is causing the mental health problems. It may simply be that parents with mental health problems are both more likely to smoke and more likely to have children with mental health problems.
What is most striking about this article, however, is not any of the above. What is most striking is that the authors acknowledge this limitation, yet they still conclude that there is a causal relationship between secondhand smoke exposure and mental health problems in children.
The authors write: "Another potential confounder that we were unable to control for in our analysis is maternal psychiatric history. That is, children with depressed mothers are more likely to have poor mental health." They cite three studies which demonstrate an association between mental health problems in children and mental health problems in their mothers.
Despite this limitation, however, the authors go on to conclude that the association observed in the study is a causal one ("The findings presented herein provide additional evidence on the harmful effects of SHS exposure on children and adolescents.") In fact, they go so far as to suggest that if parents smoke outside the home, it will lower population rates of mental illness.
There is nothing wrong with publishing a study that has limitations. The art of research, however, is drawing solid conclusions given the limitations in your study. In this case, one wonders why the authors have drawn a causal conclusion despite the major limitation in the methodology, which they readily acknowledge and highlight for the reader. Unfortunately, it leaves the impression that the researchers had a pre-determined conclusion and that they reached this conclusion regardless of the strength of the actual scientific evidence.
As my readers know, this is not an isolated case. For some reason, many recent studies on secondhand smoke appear to fall into this category. For this reason, I have come to the conclusion that the scientific rigor in the tobacco control movement has deteriorated. Investigators seem so determined to demonstrate the adverse effects of secondhand smoke that they overlook their own clearly stated assertions and conclude that there is an effect of secondhand smoke anyway, despite their own acknowledgment that such a conclusion is unwarranted based on the evidence.