WASHINGTON, Dec. 20 (UPI) -- Researchers have again reported a link between environmental exposures and the development of attention-deficit/hyperactivity disorder in U.S. children, although some experts say the evidence is still shaky.
In a study of 4,704 children ages 4 to 15, researchers calculated that prenatal exposure to tobacco smoke could account for 270,000 excess cases of ADHD in U.S. children, and lead exposure could cause 290,000 excess cases.
"It's pretty obvious the findings underscore the profound behavioral impact that these prenatal exposures have on children's health," said study author Dr. Bruce P. Lanphear, director of the Environmental Health Center at Cincinnati Children's Hospital Medical Center.
"They should strengthen our efforts to reduce these exposures," he added.
The study, published in the December issue of Environmental Health Perspectives, did not find a connection between post-natal tobacco smoke and ADHD in children.
Lanphear and colleagues obtained data from the National Health and Nutrition Examination Survey between 1999 and 2002. The researchers relied on parents to report their children's exposure to tobacco; they also measured the kids' blood-lead concentration. For the purpose of the study, a child with ADHD was on stimulant medication, and his parents had confirmed a doctor's diagnosis.
Of the sample, 4.2 percent of the kids had ADHD, which mirrors the rate of prevalence nationally. ADHD, which usually manifests in preschool and early school years, affects between 3 percent and 5 percent of children in the United States, or about 2 million kids, according to the National Institute of Mental Health. Children with ADHD find it difficult to control their behavior and pay attention.
Little research has been done on lead and ADHD, although some studies have reported a relationship between lead and impulsive behavior, attention problems and hyperactivity -- hallmarks of ADHD.
Lanphear and colleagues' research, the first to specifically test blood-lead levels and ADHD diagnosis, supports a connection between childhood lead exposure and ADHD.
Most kids in the United States encounter lead through lead-based paint in homes; smaller numbers might get exposed through candy wrappers or water containing the metal. Also, fetuses can get high doses of lead during pregnancy, when their mother's bones -- where lead is stored -- are broken down for extra calcium.
This often raises the blood-lead level of a child after birth; the level peaks at around age 2. In the range that Lanphear and colleagues studied, between 4 and 15, the blood lead was relatively low. Lanphear adjusted for age-related blood-lead levels in his analysis.
In previous research on tobacco, "compelling and consistent" studies have found children exposed to prenatal tobacco are at a two-fold or higher risk for ADHD, said Lanphear. The Environmental Health Perspectives study went a step further, looking at a specific population of kids with diagnosed ADHD -- and allowing Lanphear to "say with confidence" that a link exists.
It's also possible lead and tobacco exposure could jointly raise ADHD risk -- a kind of double whammy -- although the study didn't find that to be true, Lanphear said.
The mechanisms of how lead and tobacco damage a developing fetus or a child are not fully explained. But researchers believe the substances disturb dopamine, a neurotransmitter associated with proper functioning of the central nervous system.
The study was limited by its cross-sectional nature -- the researchers were only able to study the kids at one point in time. Likewise, the reliance on parental reports could create recall bias, the authors acknowledged. For instance, a mother could fail to report tobacco use due to guilt or social stigma, the authors wrote.
Since the researchers drew on national data, the results can be generalized to the U.S. population of ADHD-affected children, Lanphear said.
Yet the study does not fully represent the landscape of childhood ADHD in the United States, said psychologist Steven Kurtz, the clinical director of the ADHD Institute at the New York University Child Studies Center.
Many U.S. kids with ADHD go undiagnosed, and many who are diagnosed don't receive the kind of medication outlined in Lanphear and colleagues' study.
Kurtz also confirmed recall bias could be a problem. For instance, the mother of a kid with major behavioral problems may overestimate how much she exposed her child to tobacco during pregnancy, compared to a parent of a kid without behavior problems.
Dr. Mina Dulcan, head of the Department of Child and Adolescent Psychiatry at Children's Memorial Hospital in Chicago, agreed relying on parental reports from the past can be inaccurate. A prospective study -- where researchers follow their subjects for a length of time -- provides better measurements.
Dulcan also said the conclusions of Lanphear and colleagues' research "go way beyond the data."
For instance, observational studies cannot prove causation, period, Dulcan said, yet Lanphear and colleagues' study does use some causal language.
Other factors come into play that could influence the results, she said. For one thing, kids with ADHD might be more likely to be exposed to lead: They tend to run around and touch things, which is how many children come into contact with lead dust. She was also troubled by the use of blood lead as a marker. Since blood-lead level changes over time, it's hard to know the effects of lead on kids ages 4 to 15.
Lastly, people with ADHD are at a higher risk of becoming smokers. Since ADHD is highly heritable, could that be the reason why a kid gets ADHD? The answer is unknown, Dulcan said, but the study's associations aren't as strong as the researchers purport.
However, all researchers agreed reducing a child's exposure to both lead and tobacco smoke is nothing but beneficial.
