The independent and joint effects of prenatal tobacco and childhood lead exposures on ADHD in a national sample of US children were studied by researchers at Cincinnatti Children's Hospital, OH; British Columbia Children's Hospital, Vancouver, Canada; and University of North Carolina, Chapel Hill, NC. Using data from a 2001-2004 National Health and Nutrition Examination Survey, a total of 8.7% of children 8 -15 years of age met criteria for ADHD. Prenatal tobacco exposure and third tertile (1.3-5 mcg/dL) current blood lead concentrations were independently associated with ADHD. (Children with lead levels >5mcg/dL were excluded). Compared with children with neither exposure, children with both exposures had an even greater risk of ADHD than if the independent risks were multiplied (p<0.001). Children who attended preschool and boys (vs girls) had increased likelihoods of ADHD. Mexican American and black children had lower risks compared to non-Hispanic white children. 
COMMENT. The association of prenatal tobacco exposure and ADHD has been demonstrated in previous studies, one involving a national data base population, but this is the first indication of a potentiation of effect when combined with childhood low level lead exposure. A similar association with ADHD is reported with a combination tobacco and alcohol prenatal exposure. Patients with ADHD are 2.5 times more likely to be exposed to alcohol in utero and 2.1 times more likely to be exposed to tobacco. Alcohol is a risk factor for ADHD that is independent of prenatal exposure to nicotine and other familial risk factors. (Mick E et al, 2002). Genes that influence the risk of alcohol and nicotine use may also influence vulnerability to ADHD. Advice regarding hazards of nicotine and alcohol exposure and monitoring of blood count and thyroid function during pregnancy are particularly important for patients with a family history of ADHD.