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A heavy metal is any of a number of higher atomic weight elements, which has the properties of a metallic substance at room temperature. There are several different definitions concerning which elements fall in this class designation. Alternative terms are 'metal' or 'semi-metal' (according to the element in view). There are nearly 40 known definitions. Among them:

• According to one definition, heavy metals are a group of elements between copper and bismuth on the periodic table of the elements—having specific gravities greater than 4.0.

• A more strict definition increases specificity to metals heavier than the rare earth metals, which are at the bottom of the periodic table. None of these are essential elements in biological systems and additionally, most of the better known elements are toxic in fairly low concentrations. Thorium and uranium are occasionally included in this classification as well, but they are more often referred to as "radioactive metals". See actinides in the environment for further details of these radioactive 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.

• In astronomy, which defines any element heavier than helium a metal, a heavy metal or heavy element includes all elements that were not formed in the big bang; all but hydrogen (and deuterium), helium, and lithium.

• Any toxic metals may be called "heavy metals", irrespective of their atomic mass or density.

As seen in these definitions, "heavy metals" is an ambiguous term, not necessarily associated with any specific set of elements, and therefore not necessarily implying any common set of properties (such as high toxicity, high atomic weight, etc). This has led one author to call for the elimination of the term altogether from scientific discourse (Duffus 2002).

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Living organisms require trace amounts of some heavy metals, including iron, 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 (with one exception for the latter[1]) are toxic metals — they have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammals can cause serious illness. The pathway for toxic effects on humans is normally:

• for the entry of heavy metals into the atmosphere as industrial stack gas
• to enter the soil as a soil contaminant
• to enter groundwater as a water pollutant
• to be deposited in ocean bottoms or bay mud, which materials at a later time be dredged to the surface

In medical usage, the definition is considerably looser and includes all toxic metals irrespective of their atomic weight: "heavy metal poisoning" can include excessive amounts of iron, manganese, aluminium, or beryllium (the seventh-lightest metal); or such a semimetal as arsenic as well as the true heavy metals. It paradoxically excludes bismuth, the heaviest of stable elements because of its non-toxicity.

Heavy metals in a hazardous materials (or "hazmat") setting are for the most part classified in Misc. on the UN model hazard class but, they are sometimes labeled as a poison when being transported.

• Metallicity
• Soil contamination

• Kuhn, Karl F. and Koupelis, Theo (2004) In Quest of the Universe, Fourth Edition. Jones and Bartlett Publishers, Canada. ISBN 0-7637-0810-0
• Duffus, J. H. (2002) "Heavy Metals" – A meaningless term? – Pure Appl. Chem., 74 (5): 793-807, 3 Abb., 4 Tab.; Oxford.

• Survey of meanings of the term (pdf)