Even Lower Lead Levels Harmful
Study finds IQ test scores falling when levels are half current limitApr 17, 2003 | Pittsburgh Post Gazette
Exposure to levels of lead that have been regarded as safe actually can cause dramatic drops in the IQ scores of young children, a new study shows.
The findings suggest that there is no acceptable level of exposure to lead, said Dr. Herbert Needleman of the University of Pittsburgh School of Medicine, whose pioneering research made the first definitive link between lead and IQ. "Any amount of lead has toxic impact," he said.
It also suggests that far more children may have lowered IQs because of lead than previously suspected.
The U.S. Centers for Disease Control and Prevention estimates one out of 50 children ages 1 to 5 have blood lead at or above the level considered "elevated" 10 micrograms per deciliter. But one in every 10 children have blood lead levels at or above five micrograms per deciliter, a level at which the new study suggests children may be suffering ill effects.
In the study, published in today's New England Journal of Medicine, researchers led by Richard Canfield, director of Cornell University's cognitive development and neurotoxicology laboratory, checked the levels of lead in the blood of 172 children at 6, 12, 18 months and yearly from the ages of 2 to 5 years. The children completed standard IQ tests when they were 3 and 5 years old.
"What we found was quite surprising to us," Canfield said. "For children whose blood levels were never measured as high as the CDC's level of concern there was still substantial decline in IQ as lead [levels] increased."
The study showed that children whose average lead level was 10 micrograms per deciliter had IQ scores that were about seven points lower than those whose levels averaged 1 microgram per deciliter. But an increase of blood lead from 10 to 30 micrograms per deciliter was associated with a drop of only 2 to 3 points in IQ scores.
"It seems that most of the IQ drop that's associated with lead occurs within the first 10 micrograms per deciliter," Canfield said.
Even a small drop in IQ scores can have a long-term impact, he said.
"It's not trivial because decisions in our society are made on the basis of people's scores on academic-like tests," he said. An IQ test is "an indicator, albeit an imperfect one, of the child's ability to perform well on tests that make a difference in life."
Needelman said that there was no treatment for lead levels lower than 45 micrograms per deciliter, so preventing exposure is the only course of action. These days, the contamination occurs in old houses where lead-based paints were used.
"You don't have to eat paint in order to drive your blood lead level up," Needleman said. "If you have lead in household paint, it eventually ends up in the dust in the household and gets into children that way."
He noted that lead levels in the general population have declined because of public health measures such as the U.S. Environmental Protection Agency's ban of leaded gasoline. In 1975, a level of 15 micrograms per deciliter of blood was average. Now, it's about 2 micrograms per deciliter.
"It shows that a good public health policy pays off," Needleman said. "The next step is to take lead out of housing, which is very expensive."
He added that analyses indicate that the cost of lead clean-up would be outweighed by the savings in health and special education costs.
In 1979, Needleman and his colleagues published a landmark study demonstrating the neurotoxic effects of lead at blood levels higher than 10 micrograms per deciliter, the amounts commonly seen in children then. Lower exposures could not be assessed because, for the most part, they didn't exist.
Needleman was on the CDC committee that set the currently accepted value. In 1960, lead poisoning was defined as a blood level greater than 60 micrograms per deciliter. As more data become available, that threshold fell to 40 in 1970 and then 25 in 1985. The CDC reviews new data as it becomes available and could reset the limit again, he said.
Some of Needleman's other studies showed juvenile delinquents were four times more likely than their non-delinquent peers to have high blood lead levels, and that lead-poisoned children were seven times more likely to fail high school and six times more likely to have a reading disability.
Another study in the New England Journal examined data from a national CDC survey and found that higher blood lead levels were associated with signs of delayed puberty in African-American and Mexican-American girls. Scientists don't know whether such a delay has any health implications. It is plausible that lead effects on the brain extend to secretion of hormones that influence puberty.
"It may be just that puberty is a sensitive marker for some kinds of environmental exposures," said investigator Sherry Selevan, a reproductive epidemiologist with the EPA. "It raises questions."