Children with lead exposure below what the government considers safe maystill score lower on intelligence tests, suggests a study publishedtodayin the New England Journal of Medicine. Previous studies have linkedlowerIQ test results in children with blood lead levels above 10 microgramsperdeciliter -- the safety threshold established by the Centers for DiseaseControl and Prevention. But a recent study found that children with leadconcentrations of 10 micrograms per deciliter had IQ test scores thatwere7 points less on average than children with blood lead levels of 1microgram per deciliter.
Intellectual Impairment in Children with Blood Lead Concentrationsbelow 10 µg per Deciliter
Despite dramatic declines in children's blood lead concentrations and a lowering of the Centers forDisease Control and Prevention's level of concern to 10 µg per deciliter(0.483 µmol per liter), little is known about children's neuro behavioral functioning at lead concentrations below this level.
We measured blood lead concentrations in 172 children at 6, 12,18, 24, 36, 48, and 60 months of age and administered the Stanford Binet Intelligence Scale at the agesof 3 and 5 years. The relation betweenIQ and blood lead concentration was estimated with the use of linear andnonlinear mixed models, with adjustment for maternal IQ, quality of the home environment, and other potential confounders.
The blood lead concentration was inversely and significantly associated with IQ. In the linear model,each increase of 10 µg per deciliter in the lifetime average blood lead concentration was associated with a4.6-point decrease in IQ (P=0.004), whereas for the subsample of 101children whose maximal lead concentrations remained below 10 µg per deciliter, the change in IQassociated with a given change in leadconcentration was greater. When estimated in a nonlinear model with thefull sample, IQ declined by 7.4 pointsas lifetime average blood lead concentrations increased from 1 to 10 µgper deciliter.
Blood lead concentrations, even those below 10 µg perdeciliter, are inversely associated with children's IQ scores at three and five years of age, and associated declines in IQ are greater at these concentrations than at higher concentrations. These findings suggest that more U.S. children may be adversely affected by environmental lead than previously estimated.
Written by: New England Journal of Medicine, Richard L. Canfield, Ph.D., Charles R. Henderson, Jr., M.A., DeborahA. Cory-Slechta, Ph.D., Christopher Cox, Ph.D., Todd A. Jusko, B.S., and Bruce P.Lanphear, M.D., M.P.H.
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