Heavy Metals

HEAVY METALS

"Heavy metals" is an inexact term used to describe more than a dozen elements that are metals or metalloids (elements that have both metal and nonmetal characteristics). Examples of heavy metals include chromium, arsenic, cadmium, lead, mercury, and manganese. Generally, heavy metals have densities above 5 g/cm³. Because they cannot be degraded or destroyed, heavy metals are persistent in all parts of the environment. Human activity affects the natural geological and biological redistribution of heavy metals through pollution of the air, water, and soil. The primary

anthropogenic sources of heavy metals are point sources such as mines, foundries, smelters, and coal-burning power plants, as well as diffuse sources such as combustion by-products and vehicle emissions. Humans also affect the natural geological and biological redistribution of heavy metals by altering the chemical form of heavy metals released to the environment. Such alterations often affect a heavy metal's toxicity by allowing it to bioaccumulate in plants and animals, bioconcentrate in the food chain, or attack specific organs of the body.

Heavy metals are associated with myriad adverse health effects, including allergic reactions(e.g., beryllium, chromium), neurotoxicity (e.g., lead), nephrotoxicity (e.g., mercuric chloride, cadmium chloride), and cancer (e.g., arsenic, hexavalent chromium). Humans are often exposed to heavy metals in various ways—mainly through the inhalation of metals in the workplace or polluted neighborhoods, or through the ingestion of food (particularly seafood) that contains high levels of heavy metals or paint chips that contain lead.

The three heavy metals commonly cited as being of the greatest public health concern are cadmium, lead, and mercury. There is no biological need for any of these three heavy metals. Cadmium has many commercial applications, including electroplating and the manufacture of batteries. Exposure to cadmium can occur in the workplace or from contaminated foodstuffs and can result in emphysema, renal failure, cardiovascular disease, and perhaps cancer.

Humans discovered lead more than 8,500 years ago, and over time have used lead in artwork, plumbing, gasoline, batteries, and paint. Modern-day exposure to lead occurs in the workplace or through the ingestion of lead-contaminated items such as paint chips. The primary adverse health effect from exposure to lead is permanent neurological impairment (particularly in children). Other adverse health effects associated with lead include sterility in males and nephrotoxicity.

Mercury is equally toxic. Depending on its chemical form (elemental, inorganic, or organic) mercury is able to cause a myriad of adverse health effects including neurotoxicity (elemental mercury, methylmercury), nephrotoxicity (elemental mercury, mercuric salts such as mercuric chloride), teratogenicity (methylmercury), and death (elemental mercury, methylmercury). The major source of human exposure to mercury compounds is through the consumption of seafood that contains high levels of organic mercury compounds.

The international community is beginning to recognize the adverse health effects of heavy metals. In 1998, the United Nations proposed the Protocol to the Convention on Long-range Trans-boundary Air Pollution on Heavy Metals. This protocol is designed to reduce worldwide air emissions of cadmium, lead, and mercury, but has yet to be officially adopted.

Margaret H. Whitaker

Bruce A. Fowler

(see also: Arsenic; Lead; Mercury )

Bibliography

Goyer, R. A. (1996). "Toxic Effects of Metals." In Casarett & Doull's Toxicology: Basic Science of Poisons, ed. C. D. Klaassen. New York: McGraw-Hill.

Hawkes, S. J. (1997). "What Is a Heavy Metal?" Journal of Chemical Education 74:1374.

United Nations (1998). Protocol to the 1979 Convention on Long-range Transboundary Air Pollution on Heavy Metals. Available at http://www.unece.org/env/lrtap/protocol/98hm.htm.