High levels of lead in human bone, and not just blood, are dangerous to one's health, according to a study published recently by researchers at the Harvard School of Public Health and the Medical School.

"This study is the first study to demonstrate that lead that is accumulated in your bones, and not just what shows up in your blood, may be exerting a toxic effect," said Associate Professor of Occupational Medicine Howard Hu, who led the team of researchers which produced the findings.

Hu explained that bones are constantly regenerating, especially in the bodies of pregnant or lactating women or in those of elderly people with osteoporosis.

In the past, some researchers had speculated that lead in bone might seep into the bloodstream during that process and pose a risk.

But they had no means of proving their hypothesis--until the development of the X-ray fluorescence instrument, which measures levels of lead in bone, according to Hu.

With this device, researchers in Sweden and Britain showed that lead in the bones of workers exposed to high levels of lead may result in kidney damage, Hu said. But his team was the first to show that modest exposure to lead among the general population may also harm the body.

In their study, which was published in the Journal of the American Medical Association, Harvard researchers demonstrated a connection between lead in the mineral matrix of the bone and deficiencies in red blood cells and hemoglobin in the bloodstream.

"Holy smokes! The stuff that's hanging around bones that we thought was harmless might be doing something," Hu said.

The researcher said that lead in the bone can come out into the blood cells decades after the initial exposure.

A number of other research teams in the United States and Europe are also studying the effects of lead con centrations in bone using the new technology, said Dr. Philip J. Landrigan of the Mount Sinai School of Medicine in New York.

Hu's research has given the scientific community a greater understanding about the effects of lead on the body, Landrigan said.

"He's made some important breakthroughs," Landrigan said.

One significant finding in Hu's research, Landrigan said, was that thyroid disease can cause the skeletal system to release lead into the body.

Hu is currently documenting the effect of high levels of lead on organ systems such as the fetal nervous system. In addition, he is trying to prove a correlation between lead and problems such as dementia in the elderly, hypertension and kidney failure.

Lead Poisoning

People can get lead poisoning from a number of sources, Hu said. In Cambridge, old houses frequently use lead paint.

"Any old house built before 1970 has a high probability of having lead paint in it," he said.

"Lead in water is a big deal" as well, Hu said. "Our plumbing systems are big risk factors for putting lead in water" because the pipes are old.

"There's a whole panoply of ways that people are being exposed," Hu said