While nerve disorders and other serious effects of lead poisoning in children have become rare, new research indicates that millions could face risks because of blood-lead levels previously believed safe.

"Lead poisoning is still a problem in kids in this country," said Dr. Frank Mitchell, a specialist with the Centers for Disease Control.The CDC on Thursday said screening for lead in children in high-risk areas and reduction of lead in gasoline, air and food have greatly reduced lead poisoning effects such as nerve disorders and kidney damage.

But the agency said long-term effects of lead exposure "are increasingly being observed . . . with lead levels much lower than previously believed harmful."

"The numbers of children exposed to lead at these new lower levels of concern . . . are estimated at several million," the CDC said in its weekly report, summarizing findings in a report to Congress by the U.S. Agency for Toxic Substances and Disease Registry.

Low levels of lead exposure have been linked to learning disabilities in children, while exposure to greater amounts can lead to brain damage and death.

As late as 1985, federal health officials reported that a blood-lead level of anything less than 25 micrograms per deciliter was regarded as safe. A 1985 screening program found that 1.5 percent of the children checked had elevated blood-lead levels by that standard.

But in recent years, many scientists have come to believe that even 15 micrograms of lead per deciliter of blood can have harmful, long-term effects, Mitchell said.

Last December, scientists reported that blood-lead levels of 12 micrograms were linked to a slight hearing loss in children, which could cause learning impairment.

The study, by the Environmental Protection Agency and the Health Effects Research Laboratory at North Carolina's Research Triangle Park, also found that the same blood-lead level could cause delays of up to three months in sitting, walking and talking by young children.

Children, with immature bodies and a habit of getting into dust, dirt and grime, are at greatest risk for adverse effects from lead poisoning. Pregnant women also can transmit lead exposures to a fetus.

Lead residue in soil and dust "is still a major problem," Mitchell said. "That's the one we know less on how to go about cleaning it up. . . . You can't just pick up all the dirt and cart it somewhere."