Heavy Metals

The heavy metals, which include copper (Cu), zinc (Zn), lead (Pb), mercury (Hg), nickel (Ni), cobalt (Co), and chromium (Cr), are common trace constituents in the earth crust. Their concentrations in the ambient environment have increased dramatically since the Industrial Revolution, as have lead and copper since Roman times. Many of these metals play an essential role in human physiology. For example, the enzymes that synthesize DNA and RNA contain zinc ions, and cobalt is an integral part of coenzyme B12 and

vitamin B12. It is possible to be deficient in these metals, or to have an optimal or a damaging or lethal intake. However, nonessential elements such as chromium, lead, and mercury have little or no beneficial role in the human body, and the daily intake of these metals is often toxic or lethal. Many heavy metals cause nervous-system damage, with resulting learning disorders in children. Ingestion of mercury can cause the severe breakdown of the nervous system, and metals such as lead and nickel can cause autoimmune reactions. Chromium occurs in a relatively harmless form and a much more dangerous, oxidized hexavalent form. Several studies have shown that chromium (VI) compounds can increase the risk of lung cancer and that ingesting large amounts of chromium (VI) can cause stomach upsets and ulcers, convulsions, kidney and liver damage, and even death, according to the Agency for Toxic Substances and Disease Registry. The dangers of hexa-valent chromium in drinking water were popularized in the movie Erin Brockovich. Many fish are very sensitive to heavy-metal pollution. For example, trout cannot live in waters that contain more than about five parts per billion of copper. Heavy-metal contamination is very widespread, especially lead and mercury.

Most heavy-metal contamination stems from high-temperature combustion sources, such as coal-fired power plants and solid-waste incinerators. Local metal sources may include metal-plating industries and other metal industries. The use of leaded gasoline has led to global lead pollution even in the most pristine environments, from arctic ice fields to alpine glaciers. The metal fluxes from point sources have been strictly regulated, and the introduction of unleaded gasoline has taken a major lead source away. Several sites with severe heavy-metal pollution have become Superfund sites, most of them still under study for decontamination. Site decontamination can be done with large-scale soil removal and metal stripping, or through more gradual methods, like phytoremediation. Nonetheless, even today metals are delivered from the atmosphere to the landscape. In the United States, drinking water is monitored for heavy metals to ensure that their concentration falls below the safe limit or maximum contaminant level (MCL) set by the Environmental Protection Agency. Many urban estuaries like Boston Harbor, San Francisco Bay, and Long Island Sound are severely contaminated with heavy metals. These sedimentary basins will remain polluted for decades, and a small percentage of the sediment-bound metals is released back into the water and occasionally transformed into more dangerous forms.

see also Arsenic; Health, Human; Lead; Mercury; Risk; Superfund.

Internet Resource

U.S. Department of Labor, Occupational Safety and Health Administration. "Safety and Health Topics: Toxic Metals." Available from http://www.osha-slc.gov/SLTC/metalsheavy.

Johan C. Varekamp

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.