Traces of lead cause outsize harm. (Environment).

Minute amounts of lead in blood are worse for children than scientists had realized, according to new research. Data now suggest that lead affects development of kids' thinking skills at concentrations below 10 micrograms per deciliters ([micro]g/dl) of blood. Higher concentrations had previously been recognized generally as harmful to the brain.

In fact, microgram for microgram, lead may pack more punch below 10 [micro]g/dl than it does at higher concentrations, according to Richard L. Canfield of Cornell University and his colleagues. They periodically measured blood-lead concentrations in 172 children beginning when the kids were 6 months old and continuing until they were 5 years old. They gave each child an intelligence test at age 3 and at the end of the study.

Putting all these data together, the researchers found that for blood-lead concentrations between 1 and 10 [micro]g/dl, the average effect of each additional 1 [micro]g/dl was a drop of 0.82 IQ point. However, each 1 [micro]g/dl blood lead above 10 [micro]g/dl translated into only a 0.13-point loss, the researchers report in the April 17 New England Journal of Medicine.

A recent study found that 2.2 percent of U.S. children age 1 to 5 have blood-lead concentrations greater than 10 [micro]g/dl, nearly 10 percent have concentrations of at least 5 [micro]g/dl, and 90 percent have at least 1 [micro]g/dl (SN: 2/22/03, p. 120).--B.H.