Researchers in the US found that exposure to tobacco in the womb and to lead during childhood was linked to a particularly high risk for Attention-Deficit/Hyperactivity Disorder (ADHD) in children, suggesting that while we tend to focus on treatment for ADHD, eliminating such exposures might prevent the condition in many hundreds of thousands of children.

The study was the work of senior author Dr Robert Kahn, a physician and researcher at Cincinnati Children’s Hospital Medical Center, Ohio, and colleagues, and was published online on 23 November in the journal Pediatrics.

Kahn and colleagues estimated that up to 35 per cent of cases of ADHD in youngsters aged between 8 and 15 could be reduced by getting rid of both prenatal exposure to tobacco and childhood exposure to lead: in numbers this figure represents some 800,000 children in the US population.

Kahn told the press that while the tendency was to focus on treatment:

“Our study suggests that reducing exposures to environmental toxicants might be an important way to lower rates of ADHD.”

For the study, which was funded by grants from the National Institutes of Health and the Academic Pediatrics Association, and a Robert Wood Johnson Generalist Physician Faculty Scholars Award, Kahn and colleagues used data on 2,588 youngsters aged 8 to 15 who had taken part between 2001 and 2004 in the National Health and Nutrition Examination Survey (NHANES).

NHANES is a cross-sectional, nationally representative sample of the population of the US and includes information about health and diet. The survey is administered by the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC).

To assess prenatal exposure to tobacco, the researchers used mothers’ reports on cigarette use during pregnancy. To assess lead exposure, the researchers used current blood lead levels, and the diagnosis for ADHD was based on the Diagnostic and Statistical Manual for Mental Disorders, Fourth Edition, considered the “gold standard” for defining certain mental health conditions.

The results showed that:

  • 8.7 per cent of the children met the diagnostic criteria for ADHD (95 per cent confidence interval [CI] ranging from 7.3 to 10.1 per cent).

  • Children exposed prenatally to tobacco smoke were 2.4 times more likely to have ADHD (based on adjusted odds ratio [aOR], with 95 per cent CI ranging from 1.5 to 3.7).

  • Children whose blood levels were in the top third (upper tertile) for lead were 2.3 times more likely to have ADHD (based on aOR, with 95 per cent CI ranging from 1.5 to 3.8).

  • Children with both prenatal tobacco exposure and in the top third for blood lead levels were 8.1 times more likely to have ADHD compared to children with neither exposures (based on aOR, with 95 per cent CI ranging from 3.5 to 18.7).

  • The increased risk of ADHD from both types of exposure was even greater than expected by multiplying the independent risks (tobacco-lead exposure interaction term, P < .001).

The authors concluded that:

“Prenatal tobacco and childhood lead exposures are associated with ADHD in US children, especially among those with both exposures.”

They suggested that reducing exposure to tobacco in the womb and lead in childhood may be important ways to prevent ADHD.

Lead author Dr Tanya Froehlich, a physician in the Division of Developmental and Behavioral Pediatrics at Cincinnati Children’s said:

“Tobacco and lead exposure each have their own important adverse effect.”

“But if children are exposed to both lead and prenatal tobacco, the combined effect is synergistic,” she explained.

ADHD is a common behavioral disorder that affects around 8 to 10 per cent of school-age children.

Boys are about three times more likely than girls to be diagnosed with ADHD, though why is still a mystery.

Children with ADHD are hyperactive, act without thinking, and find it hard to focus. While they may understand what is expected of them, they have difficulty carrying it out because they struggle to pay attention, take in details and be still.

While these characteristics describe many children, particularly very young ones and especially when they get excited, children with ADHD are like this for longer periods.

ADHD hampers children’s social and academic development and often makes life very hard for families.

“Association of Tobacco and Lead Exposures With Attention-Deficit/Hyperactivity Disorder.”
Tanya E. Froehlich, Bruce P. Lanphear, Peggy Auinger, Richard Hornung, Jeffery N. Epstein, Joe Braun, and Robert S. Kahn.
Pediatrics, Published online November 23, 2009.
DOI:10.1542/peds.2009-0738

Sources: Cincinnati Children’s Hospital Medical Center, Kidshealth (Nemours).

Written by: Catharine Paddock, PhD