| Heavy Metals |
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The term ''heavy metal'' may have various more general or more specific meanings. 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 Weight s 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 Mammal s 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 Plant s, tHM means tons of heavy metal. External links
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