ITHACA, N.Y. -- A five-year study has found that lead is harmful to children at concentrations in the blood that are typically considered safe.
Reporting in the latest issue (April 17) of The New England Journal of Medicine , two Cornell University scientists say that children suffer intellectual impairment at a blood-lead concentration below the level of 10 micrograms per deciliter (mcg/dl) -- about 100 parts per billion -- currently considered acceptable by the Centers for Disease Control and Prevention (CDC). "We also found that the amount of impairment attributed to lead was most pronounced at lower levels," says Richard Canfield, lead author of the journal paper and a senior researcher in Cornell's Division of Nutritional Sciences.
The study followed 172 children in the Rochester, N.Y., area whose blood lead was assessed at 6, 12, 18, 24, 36, 48 and 60 months and who were tested for IQ at both 3 and 5 years of age. The study was conducted by researchers at Cornell, the Cincinnati Children's Hospital, the University of Rochester, the National Institutes of Health and the University of Washington.
Before 1970, childhood lead poisoning was defined by a blood-lead concentration greater than 60 mcg/dl. The level considered acceptable was set at 40 mcg/dl in 1970 and reduced to 25 mcg/dl in 1985. The current level of 10 mcg/dl was established in 1991 based on findings linking lead at this level to lowered intelligence and diminished school performance.
An important feature of the study is its focus on children with blood-lead levels below 10 mcg/dl. Most previous research examined the effects of lead in the 10 to 30 mcg/dl range. But the new study finds lead-related impairments at lower levels.
"In our sample, most of the damage to intellectual functioning occurs at blood-lead concentrations that are below 10 mcg/dl," says Canfield. The amount of impairment was also much greater than the researchers had expected. "Given the relatively low exposure levels, we were surprised to find that the IQ scores of children with blood-lead levels of 10 mcg/dl were about 7 points lower than for children with lead levels of 1 mcg/dl," Canfield says.At the same time, the study found that an increase in blood lead from 10 to 30 mcg/dl is associated with only a small additional decline in IQ of about 2 to 3 points. "Because most prior research focused on children with higher exposures than in our sample, we suspect those investigators could estimate only the damage that occurs after blood lead has reached 10 mcg/dl -- unaware that substantial impairment may occur at lower levels," says Charles Henderson, a senior researcher in the Department of Human Development at Cornell and second author of the paper.
"While these findings are based on a single sample and will need to be replicated in further studies," says Henderson, "we found that the relation between lead and IQ was very consistent at 3 and 5 years of age." He notes that the researchers controlled for maternal education, IQ, income, prenatal exposure to tobacco and level of intellectual stimulation in the home.
Children's blood-lead concentrations have fallen by more than 80 percent in the past 30 years, but Canfield notes that undue lead exposure is an especially important problem among children living in impoverished communities. For example, the CDC recently reported that children ages 1 to 5 years who were enrolled in Medicaid accounted for 60 percent of all U.S. children with blood-lead levels greater than 10 mcg/dl. Furthermore, more than 80 percent of children enrolled in Medicaid do not typically receive blood-lead tests. Many such children live in housing built before about 1950, which is more likely to contain paint having high levels of lead. If the paint cracks or peels, lead particles can fall onto floors and onto children's toys. Children ingest the lead particles when they put contaminated toys and fingers into their mouths.
According to new CDC figures, approximately 1 out of every 50 children in the United States between the ages of 1 and 5 years has a blood-lead level above 10 mcg/dl, whereas 1 in every 10 children has blood-lead levels of 5 mcg/dl or higher. "Given the current CDC recommendations and the findings from our study, it appears that many children are passing their lead test but failing to escape the adverse consequences of low-level lead exposure," Canfield says.
Other authors of the report are Bruce Lanphear of Children's Hospital Medical Center in Cincinnati, Deborah Cory-Slechta of the University of Rochester, Christopher Cox of the National Institute of Child Health and Human Development, and Todd Jusko of the University of Washington. The research was funded by the National Institute of Environmental Health Sciences, the Cornell Bronfenbrenner Life Course Center and U.S. Department of Hatch funds.
Related World Wide Web sites: The following site provides additional information on this news release. It is not part of the Cornell University community, and Cornell has no control over its content or availability.
o Information on Richard Canfield, contact information for other researchers and staff members involved in the project, and links to sites with more information about childhood lead exposure: http://cdnl.human.cornell.edu/contacts.htm