Even 'safe' lead levels in blood may be unsafe

If additional studies verify the findings reported in today's New England Journal of Medicine, many American children may be at risk for academic difficulties caused by exposure to even low levels of lead, a toxic metal found in the environment and especially in older homes with chipping lead-based paint.

Richard Canfield at Cornell University in Ithaca, N.Y., and his colleagues knew that previous studies had linked poor academic performance to blood lead levels above 10 micrograms per deciliter, the federal threshold for unsafe exposure. But no one had focused on lead levels below that amount.

The researchers studied 172 children in the Rochester area who had their blood drawn and tested for the metal starting at 6 months old. The team also gave the kids a standard intelligence test at age 3 and again at age 5.

The team found that children with blood lead concentrations of 10 micrograms per deciliter had IQ test scores that were on average 7 points less than kids with levels at 1 microgram per deciliter.

The team found another, smaller drop in IQ at 30 micrograms of lead per deciliter of blood.

The findings could be a fluke: The link between low levels of lead and IQ test performance might be caused by some other factor and not lead, says Walter Rogan of the National Institute of Environmental Health Sciences, which helped pay for the study.

Experts know that children often are exposed to lead when they crawl or play on the floor, where lead from dust often settles. At very high levels, lead can cause a dangerous swelling of the brain and even death. But this study, and other evidence, suggests that low levels of lead exposure also can cause health problems.

A second study, in the same journal, shows that low lead levels can delay puberty in girls.

How much lead is too much? No one is sure.

The Centers for Disease Control and Prevention in Atlanta says children with blood lead levels of less than 10 micrograms are safe. Under CDC guidelines, it isn't until 15 micrograms that local health experts are asked to find out how the child is being exposed to lead. The agency won't change its lead guidelines based on the new study, which must be verified, says CDC's Tom Matte. But the CDC will review the evidence on the health effects of low lead exposure this fall, he says.