Even low lead in kids has a high IQ cost.

At exposures far lower than the limit deemed acceptable by the U.S. government, lead can damage a young child's ability to learn and reason, researchers report this week. Moreover, a small increase in lead concentration exerts a more potent effect on the child's IQ when the blood shows a low lead concentration than when the concentration is high.

"This is a bombshell," says study leader Bruce P. Lanphear. When combined with an earlier analysis by his team, the new results indicate there is "no threshold for the adverse effects of lead on cognition," he says. An epidemiologist at Children's Hospital Medical Center in Cincinnati, Lanphear unveiled the findings Monday in Baltimore at the Pediatric Academic Societies' annual meeting.

Lanphear and his coworkers tested blood concentrations of lead in 276 children from infancy. When the children reached age 5, the team administered a standard IQ test. The researchers found a decreasing trend in IQ in 5-year-olds' whose blood lead exceeded 5 micrograms per deciliter (micro]g/dl), which is half the federally acceptable concentration. The result held even after the researchers accounted for such potentially confounding factors as parents' IQ.

At blood concentrations between 5 and 10 [micro]g/dl of lead, mean IQ falls on average by 1.1 point for each additional 1 [micro]g/dl. Above 10 [micro]g/dl, the relative impact of each increase appears only half as large.

These effects mirror those that Lanphear's team reported for a cross-section of 4,850 U.S. children in the November/December 2000 PUBLIC HEALTH REPORTS. However, the single measurement of lead available for those children--age 6 to 16--offered no insight on infancy and toddlerhood, the most vulnerable periods.

Since early lead toxicity can permanently impair the brain's "hard wiring," even subtle poisoning can have lifelong impacts, notes Joel Schwartz, of the Harvard School of Public Health in Boston.

Years ago, Schwartz conducted a risk-benefit analysis of lead's impact on IQ and economic productivity. The result, he says, has become a standard research reference. "It showed that if you lower the mean IQ of the U.S. population by one point, you lower the productivity of the economy by about 1 percent," Schwartz says. "That's a lot," he adds.

Public health officials err today by deeming low even 2 [micro]g/dl of lead in blood, Schwartz argues. A millennium ago, body concentrations of lead were typically one-hundredth to one-thousandth what is seen today (SN: 1/21/89, p. 44).

Having banned lead in gasoline, paint, and water-pipe welds, policy makers now should target smaller sources, Schwartz says. One potentially significant example is the lead weights placed on vehicle tires to balance them.

In the October 2000 ENVIRONMENTAL HEALTH PERSPECTIVES, retired Albuquerque scientist Robert A. Root reported that the quantity of lead shed from these weights each year can exceed 50 kilograms per kilometer of road. As the weights weather, he notes, they make lead dust that can be inhaled or tracked by shoes and pets onto carpets where babies crawl.