Lead levels now widely believed to be safe in children actually produce a severe impact on intellectual development, researchers report today.

Blood levels of lead below current federal and international guidelines of 10 micrograms per deciliter produce a surprisingly large drop in IQ of up to 7.4 points, a U.S. team reports in today's New England Journal of Medicine. Researchers estimate that one in every 50 U.S. children has lead levels above that guideline and that one in every 10 has levels of 5 micrograms/deciliter or above — well within the dangerous range.

"People have been asking, 'How low [a lead concentration] is low enough?' " said Dr. Richard Canfield of Cornell University, one of the leaders of the study. "The fact is, in our study, we found no evidence for a safe level. There is no safe level of exposure."

The findings "reflect the growing opinion that low levels of lead are more toxic than we thought," said Dr. Herbert A. Needleman, a prominent lead researcher who was not involved in this study. "When we took the lead out of gasoline ... that left one remaining big source, old houses. Now we have to take the lead out of old houses."

An estimated 38 million houses built before 1950 still have lead-based paints on their walls. In California, exposure also comes from folk medicines and Mexican ceramic pottery. "There is a message for parents in here that goes beyond whatever government policy recommendations should be," Canfield said. Just as parents should protect their children from the effects of smoking and alcohol use, they "should be aware of sources of lead in their environment and, most important, should try to engage in some type of cleanup or abatement so the child never comes in contact with lead."

In a separate paper in the journal, researchers from the Environmental Protection Agency also found that low levels of lead delay puberty for several months in young girls, especially African Americans and Latinas.

Although delaying puberty is not necessarily harmful, the findings suggest that lead is interfering with critical hormonal processes during development.

"That fits in with the increased interest in general with the idea that environmental chemicals can be endocrine disruptors," said Dr. David Bellinger of Harvard Medical School. "Lead has not been considered as prominently as other chemicals. This suggests that we ought to be looking at it more closely."

Lead is a potent poison that adversely affects organs throughout the body. Recent studies have shown that higher levels not only reduce intelligence and slow development, but also can lead to behavioral problems, juvenile delinquency and even criminality.

As these studies have appeared, guidelines for exposure have continued to be lowered.

In the 1960s, doctors diagnosed lead poisoning if blood levels were above 60 micrograms per deciliter (mcg/dl), high enough to cause abdominal spasms, kidney injury and severe brain damage.

After studies in the 1980s and 1990s revealed that lower levels still damaged children's ability to think, concentrate and hear, the Centers for Disease Control and Prevention continued to reduce the allowable level — to 30 mcg/dl in 1975, 25 mcg/dl in 1985 and to the current level of 10 mcg/dl in 1991. The last figure corresponds to about 100 parts per billion. In 1976, when lead was removed from gasoline, the average lead level in children was about 15 mcg/dl.

Today, the average is about 3. "But that's still 10 to 100 times higher than the level in preindustrial humans," said Dr. Bruce Lanphear of the Cincinnati Children's Hospital Medical Center, another study leader. "Three mcg/dl is low by current standards, but from an evolutionary perspective, it is quite high."

Canfield and Lanphear's team studied 172 children in the Rochester, N.Y., area, measuring blood lead levels at ages 6, 12, 18, 24, 36, 48 and 60 months. The children were tested for IQ at 3 and 5 years of age.

They found that a rise in lead levels from 1 mcg/dl to 10 was associated with a 7.4-point drop in IQ. An increase in lead levels from 10 to 30 mcg/dl was associated with an additional drop of only about two to three points, in line with previous studies.

"This really changes the way we think about childhood lead exposure," Lanphear said. "We have to start thinking about how we might identify hazards and reduce them before children are exposed." A 1991 study showed that lead abatement in old houses would cost about $32 billion, but would bring benefits in such areas as special education of more than $60 billion.

Copyright 2003 Los Angeles Times