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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:
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. 
Relationship to living organisms
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) 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:
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Heavy_metals". A list of authors is available in Wikipedia.|