According to one definition, the heavy metals are a group of elements between copper and lead on the periodic table of the elements—having atomic weights between 63.546 and 200.590 and specific gravities greater than 4.0. Living organisms require trace amounts of some heavy metals, including cobalt, copper, manganese, molybdenum, vanadium, strontium, and zinc, but excessive levels can be detrimental to the organism. Other heavy metals such as mercury, lead and cadmium have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammals can cause serious illness.

A stricter definition restricts the term to those metals heavier than the rare earth metals, at the bottom of the periodic table. None of these are essential elements in biological systems; all of the more well-known elements with the exception of bismuth and gold are horribly toxic. Thorium and uranium are sometimes included as well, but they are more often called simply "radioactive metals".

In medical usage, the definition is considerably looser, and "heavy metal poisoning" can include excessive amounts of iron, manganese, aluminium, or beryllium (the second-lightest metal) as well as the true heavy metals.

Also, often the elements beyond mercury, e.g., the actinides such as uranium and plutonium, are not excluded from the heavy metals. In the context of nuclear power plants, tHM means tons of heavy metal.

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Lead Therapy Won't Help Most Kids - chelation therapy found ineffective for most childrern with lead poisoning - Brief Article
From Science News, 5/12/01 by J. Raloff

Drugs exist that will bind to lead and remove the toxic heavy metal from a person's blood. This therapy, called chelation, has saved the lives of people suffering from acute poisoning. However, a major study now finds that for children who have had moderate exposure to lead, chelation fails to prevent brain impairments.

Behavioral problems, difficulty with reasoning, and a permanently diminished IQ are hallmarks of excessive lead. Federal guidelines define as excessive any childhood exposures that result in lead concentrations of at least 10 micrograms per deciliter ([micro]g/dl) of blood.

Nearly 1 million U.S. children under 6 years of age show such concentrations in their blood, mostly from exposure to lead-based paint. However, even youngsters whose lead concentrations peaked at well below 10 [micro]g/dl may suffer notable IQ drops, recent studies have shown (SN: 5/5/01, p. 277).

In hopes of preventing toddlers from developing the permanent brain changes that underlie cognitive impairments, researchers launched a multicenter study of 1- to 3-year-olds. The 780 participants lived in deteriorating inner-city housing and initially had 20 to 44 [micro]g/dl lead in their blood. Although excessive, these concentrations are not life threatening.

Half the children received succimer, a lead-binding drug, three times a day. Each course of treatment lasted 26 days--and, depending on lead concentrations in blood afterward, was repeated up to two times. The remaining children received drugfree capsules that resembled succimer. Before and at several intervals after treatment, scientists administered developmental, behavioral, and IQ tests to the children.

Youngsters receiving the drug performed no better on those tests than kids receiving the sham capsules did. Moreover, treated children exhibited slightly more behavioral problems than did kids in the placebo group, and they grew a bit less in height. The findings appear in the May 10 NEW ENGLAND JOURNAL OF MEDICINE.

While the aggressive drug therapy effectively removed lead from blood, the researchers found that the children's bones, which store lead, compensated by releasing more of the metal. Long-term, the drug therapy lowered circulating lead concentrations only by about 4.5 [micro]g/dl.

Still, "that's about as good as you can get with chelation [in kids]," observes study leader Walter J. Rogan of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

Though some heavily exposed adults have received chelation for years, he says, lengthy treatments are impractical for children. The drug is expensive and "nasty"--with a sulfurous stink--he notes. Moreover, mixing the contents of the capsules into food and getting this combo into toddlers three times a day proved "tedious, taking a big chunk out of families' lives," Rogan adds.

John F. Rosen, who heads the lead program at Children's Hospital at Montefiore in New York City, has observed 19,000 kids. His findings suggest that once blood lead reaches 20 [micro]g/dl, "children are irrevocably brain damaged," he says. "What's critically important about the Rogan paper is that there are now data to confirm this--and they're unequivocal."

It appears "no longer tenable to focus our primary lead-screening efforts on children," says Bruce P. Lanphear of Children's Hospital Medical Center in Cincinnati. By the time lead poisoning shows up in blood tests, it's usually too late to reverse it, he explains.

Public policy, Lanphear argues, "has been to use children as biological indicators of substandard housing." He and Rosen would prefer that buildings be routinely screened--and lead problems fixed--before children move in.

For federally assisted housing, the U.S. Department of Housing and Urban Development is taking steps in that direction. Last September, HUD enacted a rule that renovations to such structures must "control" the release of dust from lead-based paint. Afterward, tests must confirm that any lead dust released has been removed.

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