ADHD and smoking? Prenatal Exposure to cigarettes and lead may increase risk for ADHD

By Nestor Lopez-Duran PhD

The current issue of the journal Pediatrics includes a large epidemiological analysis of the association between prenatal exposure to tobacco and later risk for ADHD. The analysis was relatively simple and elegant. They examined a national representative sample of 2,588 US adolescents. Three variables were of interest: 1) whether the teen was exposed to tobacco before birth as determined by the mothers self-report of cigarette smoking during pregnancy; 2) the teens current blood levels of lead; and 3) whether the child had ADHD as determined by a structured psychiatric interview. This last point is key because some have argued that ADHD has been overdiagnosed due to the use of informal and arguably inappropriate assessment procedures (e.g., a pediatrician asking a few questions). Thus, the use of a this diagnostic interview would improve the accuracy of the diagnoses. Finally, in addition to these 3 main variables, the authors examined a number of variables that could provide alternative explanations of the findings. These variables included: the sex of the child, income of the family, age, race, the mothers age at birth, birth weight, birth complications (Neonatal Intensive Care Unit admission), post-natal cigarette smoke exposure, and whether the kid went to preschool.

The Results:

1. 8.7% of the entire sample met diagnostic criteria for ADHD. That is, the rate of ADHD, as diagnosed by a structure diagnostic clinical interview, was close to 10% of US teens.

2. While controlling for all other variables, prenatal exposure to tobacco increased the risk of having ADHD by 140%.

3. While controlling for all other variables, current lead levels also increased the risk for having ADHD and this effect was linear in relation to lead levels. Specifically, having lead levels in the second tertile (0.09 1.2 ug/dl ) increased the risk by 70%. Having lead levels in the third tertile (>1.3 ug/dl) increased the risk by 130%.

4. Other variables associated with an increased risk included: being a boy, being non-Hispanic white, and having attended preschool.

5. The combination of prenatal tobacco exposure and having high lead levels increased the risk for ADHD by 700%.

This study is very compelling given the number of potential explanatory variables that were included in the analysis. The results strongly point towards an association between tobacco exposure before birth and ADHD risk. However, one important explanatory variable was omitted: parental history of ADHD. We know that there is a strong familial risk for ADHD, in that children of parents who have ADHD are a significantly higher risk of having ADHD themselves. It is then possible that those ADHD mothers are more likely to smoke during pregnancy, and that it is the maternal history of ADHD and not the tobacco exposure that explains some of these results. The authors correctly noted this issue but argued that other studies have were able to control for maternal history of ADHD have obtained similar results, making this alternative explanation unlikely.

The reference:
Froehlich, T., Lanphear, B., Auinger, P., Hornung, R., Epstein, J., Braun, J., & Kahn, R. (2009). Association of Tobacco and Lead Exposures With Attention-Deficit/Hyperactivity Disorder PEDIATRICS, 124 (6) DOI: 10.1542/peds.2009-0738