Heavy metals
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The term heavy metal may have various more general or more specific meanings.
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. 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. The pathway for toxic effects on humans is normally: (i) for the entry of heavy metals into the atmosphere as industrial stack gas; (ii) or to enter the soil as a soil contaminant; (iii) or to enter groundwater as a water pollutant; or to be deposited in ocean bottoms or bay mud, which materials at a later time be dredged to the surface.
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". For further details of these radioactive metals please see Actinides in the environment.
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.
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 only Hydrogen (and Deuterium), Helium, and Lithium.
Sources
In Quest of the Universe, Fourth Edition, Karl Kuhn Theo Koupelis. Jones and Bartlett Publishers, Canada. 2004. ISBN 0-7637-0810-0See also
External links
- [Survey of meanings of the term] (pdf)
- [Overview of heavy metals in food and their health effects]
- [Heavy metal poisoning information]
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