Heavy Metals as Pollutants

Heavy metal is a poorly defined term that refers to a metallic or metalloid element with a high density and high atomic weight. There is considerable disagreement about whether both or only one of the parameters must be high and about where to set the cutoff values for the parameter(s). Values of 4, 5, and 6 g/cm3 (grams per cubic centimeter) have been used to define high density; 23, 40, and 63 have been used as cutoff values for atomic weight. To remedy these inconsistencies, another parameter, atomic number, has occasionally been used to define heavy metal, but this sometimes resulted in the inclusion of obviously light metals in the list of heavy metals. The absence of a consensus about the definition of the term heavy metal has led some to abandon it altogether and use alternatives such as toxic metals or trace metals. However, these terms also lack a universally accepted definition and do not result in a classification that completely overlaps with the general intent for the meaning of heavy metal. Consequently, heavy metal is still a very widely used term. Metallic elements that are generally considered to be heavy metals include antimony, cadmium, chromium, copper, gold, lead, mercury, nickel, silver, thallium, tin, and zinc. Arsenic and tellurium, both metalloids, and even selenium, a nonmetal, are sometimes included among the heavy metals. Depending on the definition used, the list of heavy metals can have between 40 and 80 elements. Many of these heavy metals are toxic to some degree to humans and other life forms, and therefore, sometimes toxicity is included in the definition of heavy metal. Because of their toxicity, their widespread release into the environment through natural and anthropogenic processes, and their persistence under natural conditions, many heavy metals are of great environmental and human health concern. This entry discusses the sources of heavy metals and their environmental distribution. It then describes their adverse health effects as well as national and international regulatory standards.