Heavy metal ingestion classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


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). Some of the nearly 40 known definitions are:

  • 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.
  • 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", whether or not they are heavy.

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 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) as well as the true heavy metals.

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. Heavy metals may enter the body in food, water, or air, or by absorption through the skin. Once in the body, they compete with and displace essential minerals such as zinc, copper, magnesium, and calcium, and interfere with organ system function. People may come in contact with heavy metals in industrial work, pharmaceutical manufacturing, and agriculture. Children may be poisoned as a result of playing in contaminated soil. Lead poisoning in adults has been traced to the use of lead-based glazes on pottery vessels intended for use with food, and contamination of Ayurvedic and other imported herbal remedies. Arsenic and thallium have been mixed with food or beverages to attempt suicide or poison others.

Another form of mercury poisoning that is seen more and more frequently in the United States is self-injected mercury under the skin. Some boxers inject themselves with mercury in the belief that it adds muscle bulk. Metallic mercury is also used in folk medicine or religious rituals in various cultures. These practices increase the risk of mercury poisoning of children in these ethnic groups or subcultures.

For more information:

Mercury Poisoning

Lead Poisoning

Arsenic Poisoning

Iron Poisoning

Manganese Poisoning

Cadmium Poisoning

Thallium Poisoning

Actinides in the Environment


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