Despite government efforts to reduce lead contamination in the environment, 3 million to 4 million American children still are exposed to dangerous levels of the highly toxic metal, a new report shows.

In addition, an estimated 400,000 fetuses each year are threatened by lead poisoning of their mothers, said the report presented to Congress Monday by the Agency for Toxic Substances and Disease Registry.The agency, an arm of the Health and Human Services Department, noted lead contamination has been substantially reduced in recent decades by government regulations reducing the use of lead in gasoline and curtailing lead emissions from smelters and other industrial sources.

But it said, "Exposure to lead continues to be a serious potential public health problem - particularly for the young child and fetus."

The agency also said the problem may be more difficult to address than previously believed because medical researchers are finding that lead can have harmful effects at lower exposure levels than past studies indicated.

"Thus, despite some progress in reducing the average level of lead exposure in this country, it is increasingly apparent that the scope of the childhood lead poisoning problem has been, and continues to be, much greater than was previously realized," the report said.

A highly toxic metal, lead is particularly damaging to the developing brain and central nervous system, causing delayed mental development, reduced IQ scores and impaired hearing. Such damage is persistent and may be irreversible.

Depending on the amount of lead absorbed in the bloodstream, the kidney can also be damaged and production of heme in red blood cells can be diminished. Severe exposure can lead to coma, convulsions and even death.

The agency said the incidence of elevated blood levels was highest among inner-city, underprivileged children, but it added, "No economic or racial subgrouping is exempt from the risk of having blood lead levels sufficiently high to cause adverse health effects.

"Indeed, sizable numbers of children from families with incomes above the poverty level have been reported with blood lead levels above (those considered dangerous)."