A Little Lead Can Lower Kids' IQ
IQ Loss, Delayed Puberty Suggests Lead Unsafe at Any Level
April 16, 2003 -- Kids' IQ test scores drop -- permanently -- if they get lead in their systems. Now it looks as though much of this damage comes at blood-lead levels once thought safe.
The CDC says that lead in kids' blood reaches a "level of concern" when it hits 10 micrograms per deciliter of blood (10 mcg/dL). That may soon change.
By the time blood lead levels reach 10 mcg/dL, kids' IQ test scores already have dropped a very significant 7.4 points, report Cornell University researcher Richard L. Canfield, PhD, and colleagues.
"Our findings were quite surprising in that we found quite robust effects of lead -- adverse in nature -- within a range below what the CDC calls a 'level of concern,'" Canfield tells WebMD. "There was a substantial adverse effect on IQ below that range. It was so concerning and unexpected that this would be so large that we retested the children at age 5. We found no difference in the lead effect between age 3 and age 5."
Canfield's research team followed 172 inner-city kids from birth to 5 years of age. They tested their blood-lead levels at ages 6, 12, and 18 months and 2, 3, 4, and 5 years. The kids took IQ tests at ages 3 and 5. The result: Kids with lifetime blood-lead levels of 10 mcg/dL scored 7.4 points lower on IQ tests than kids whose lead levels were only 1 mcg/dL. The findings hold true even when the researchers controlled for many of the factors that affect a child's intelligence.
Most of the alarm over lead has been for blood levels much higher than 10 mcg/dL. Before government regulation forced the removal of lead additives from gasoline, U.S. kids averaged 15 mcg/dL. That's now dropped to a national average of 2 mcg/dL. As of 2000, there were an estimated 454,000 kids with lead levels above 10 mcg/dL -- but by the time levels get that high, most of the damage may be done.
"We found if you compare the amount of IQ loss over the first 10 mcg/dL, it is somewhere about three times as large as the damage that occurs from 10 to 20 mcg/dL," Canfield says. "That was one of the most surprising findings. Almost all studies have concentrated on children with blood-lead levels between 10 and 30 mcg/dL. The effects they report represented only the additional damage that occurred after levels reached 10 mcg/dL. Much more damage occurred at lower levels."
James Ware, PhD, Harvard School of Public Health, is co-author of an editorial appearing alongside Canfield's report in the April 17 issue of TheNew England Journal of Medicine. As a biostatistician, he's uncomfortable making specific conclusions based on the relatively small number of kids in the Canfield study.