Exposure to even low levels of lead can damage children's intelligence, U.S. researchers say.

Researchers found children with far less lead in their blood than allowed by current guidelines had lower IQ scores. Much of the damage seemed to occur at very low levels of exposure.

"These findings suggest that more U.S. children may be adversely affected by environmental lead than previously estimated," the researchers wrote in their report, which appears in Thursday's issue of The New England Journal of Medicine.

Children can be exposed to lead from old paint

The World Health Organization and the U.S. Centers for Disease Control and Prevention both say blood levels above 10 micrograms of lead per decilitre of blood put children at risk of cognitive damage.

A research team led by Richard Canfield of Cornell University followed 172 children in New York and measured lead levels in their blood at 6, 12, 18, 24, 36, 48 and 60 months of age. Researchers tested for IQ at three and five years of age.

The team found as blood levels increased from one to 10, a child's IQ fell by an average of 7.4 points. The decline was sharper than researchers had seen at higher lead levels. Most earlier studies looked for effects at 10 to 30 micrograms of lead per decilitre of blood.

"It implies that there is no safety margin at existing exposures," Walter Rogan of the National Institute of Environmental Health Sciences and James Ware of the Harvard School of Public Health wrote in a commentary.

The researchers adjusted for factors like maternal education and IQ, household income and prenatal exposure to tobacco.

Children can be exposed to lead from old paint and lead water pipes. The CDC says only two per cent of U.S. children under five years old exceed the WHO/CDC lead limit, but 10 per cent have levels between 5.0 and 10 micrograms/decilitre or higher.

As evidence mounted about the harmful effects of lead, the acceptable amount was lowered from 60 in the 1960s to 30 in the late 1970s, and 10 since the early 1990s.

Canada prohibited the production, importation and sale of most leaded gasoline in 1990. In their commentary, Rogan and Ware say there's little doctors can do to treat lead exposure, so people should be more of aware of potential exposure in children.