"That's a whopping percentage," says Andrew Adesman, MD, chief of developmental and behavioral pediatrics at Schneider Children's Hospital in New Hyde Park, N.Y. Schneider was not associated with the study.
"This is further proof-positive that prenatal smoking has long-term developmental effects on kids, perhaps leading to an additional 270,000 cases of ADHD," Adesman tells WebMD. "Certainly the first take-home message is that pregnant women shouldn't smoke."
But the study's most innovative finding, Adesman tells WebMD, is that childhood exposure to lead, even at low levels that the government finds acceptable, also has detrimental effects on kids, perhaps leading to an additional 290,000 excess cases of ADHD.
2 Important Questions
This raises two important questions: Should the government lower its current threshold of an acceptable blood level, which is defined as higher than 10 micrograms per deciliter? And should public health agencies be more aggressive in eliminating lead from consumer products and taking steps to reduce high levels of lead found in homes and environments where young children play?
"What is suggested is that we as a society need to work harder to further eradicate potential sources of lead and be more vigilant about possible lead exposures in young children," Adesman says.
"Housing surveys estimate that 25 million American housing units contain one or more lead hazards," says researcher Bruce Lanphear, MD, director of the Cincinnati Children's Environmental Health Center at the Cincinnati Children's Hospital Medical Center.
"The most common source of lead exposure is from lead-based paint that disintegrates and contaminates house dust," Lanphear tells WebMD. Other sources include lead-glazed pottery and lead-containing cosmetics. Residences near highways may also have high levels of lead in air and dirt.
The study was published online Sept. 19 in the journal Environmental Health Perspectives.
ADHD Numbers May Be Even Higher
Lanphear and colleagues studied more than 4,700 children aged 4-15 and found that 4.2% were reported to have ADHD treated with medication. This finding is in line with national studies showing that 3% to 8% of U.S. children have ADHD.
The researchers also found that an additional 4% of the children had parent-reported ADHD but were not on medication. But they were not included in the researchers' analysis because parent reports are considered an imprecise measure of ADHD.
"Half of the children who were labeled as ADHD were excluded because they were never treated with medication," Adesman says. "That means that the estimated number of ADHD cases attributable to prenatal smoking and lead exposure may be even higher. I don't know that the numbers would necessarily be double, but they would be substantially higher."
Lanphear found that children born to women who smoked during pregnancypregnancy have a 2.5-fold higher risk of ADHD than children born to nonsmoking women. He also found that children with blood levels of 2 micrograms per deciliter to 5 micrograms per deciliter -- which is only one-fifth to one-half of the current acceptable level -- were four times as likely to have ADHD. The higher the lead level, the higher the risk of ADHD, the researchers found.
Previous studies have shown a strong link between maternal smoking and ADHD. "But nobody has previously been able to test whether lead exposure per se is associated with a diagnosis of ADHD," Lanphear says.
Lead exposure in early childhood is known to affect brain development. Previous research has linked lead exposure during early childhood to a lower IQ. However, these studies have not shown that lead exposure is linked to behavioral issues, such as those characteristic of ADHD.
Get the Lead Out
Because mounting evidence shows that blood lead levels below 10 micrograms per deciliter are harmful to children, the government should consider lowering its acceptable threshold to at least 5 micrograms per deciliter, Lanphear says.
Since it's difficult to control the amount of lead in the environment, however, reducing its effects on children is mostly a societal responsibility, Lanphear says. "This should force us to be much more aggressive in eliminating lead from consumer products."
That said, there are steps parents can take to reduce young children's exposure to lead in the home, Lanphear adds. Commercially available home test kits, available for about $10 each, can show if a home contains hazardous levels of lead.
Lead abatement can be a costly proposition. Lanphear's team is currently conducting a study of 400 children in which families are randomly assigned to receive up to $10,000 worth of home lead abatement. "We're hoping this will lead to lower blood levels of lead in children," he says.