Heavy metals is a common toxicological term covering a number of metallic substances that acutely damage human beings and ecosystems, and whose atomic weights fall between and 64 and 201. Those responsible for the most injuries and deaths are lead, mercury, and cadmium. Others with toxic properties—for example zinc, beryllium, chromium, aluminum, bismuth, manganese, and copper—are frequently listed as heavy, but because their atomic weights fall below 64 are not chemically regarded as such. A term better-suited to all these substances might simply be toxic.
Another toxic material, arsenic, is often included among the heavy metals but chemists see arsenic as a semimetal because its chemical and physical properties are only partially metallic. Thus they advocate a separate classification for this substance that since the 1980s has been poisoning well water and damaging the health of hundreds of thousands of villagers in Bangladesh and West Bengal, India.
Origin and Issues
Metals leach into living systems from natural ore deposits. But by far the major sources of toxic entry are emissions and wastes from mining and smelting operations, manufacturing processes, power plant emissions, waste incinerators, and through such consumer items as fuel additives, dental amalgams, toys, paints, light bulbs, plumbing, electronic devices, even vaccines and herbal dietary supplements. Toxic metals are ubiquitous, persistent, and controversial, and because they destroy critical enzymes can be savage in their toxic effects.
Accordingly the regulation of these substances has taken many forms, from public health and consumer protection laws to measures that control air, land, and water contamination. International treaties are probably inevitable, since these metals disperse throughout the ecosphere, cross national boundaries, essentially never degrade, and accumulate to toxic concentrations in fruits, vegetables, farm animals, and seafood. The major practical approach to their control is capture, followed by impounding.
Heavy metals history is replete with stories of environmental injustice and regulatory lethargy. Children and developing fetuses are the most tragic victims, usually suffering from cancer and serious neural disorders such as Parkinsonism and mental retardation. Increasing bodies of evidence indicate that high toxic chemical levels also correlate geographically with high crime rates, raising important legal and ethical questions as to whether polluters should be liable for offenses that promote criminal behavior in persons exposed to metallic emissions.
State, local, and federal regulations over the last three decades of the twentieth century reduced public exposure to these substances. But localized incidents remain frequent in the early-twenty-first century and the legacy of past abuses poses persistent problems through the presence, for example, of industrial waste or Super-fund sites that have not yet been cleaned up (or in technical jargon, remediated). The history of heavy metals toxicity is a particularly tragic one, marked by bitter conflicts over surreptitious dumping, disposal in areas populated by poor people, exposure to children, lack of equitable compensation of victims, and corporations that are unwilling or unable to pay for control and cleanup.
One of the earliest, most heartrending modern instances of heavy metal poisoning was the disaster that occurred in Minamata, Japan, during the 1950s and 1960s when mercury was discharged from a plastics manufacturing plant into the waters of Minamata Bay. The metal, in the form of methyl mercury, accumulated in the bodies of fish that were the food staple for the thousands of persons who lived in that section of southwestern Japan. The pathological result was painful neural disorders that had distressing physiological, social, and psychological effects on the people of Minamata.
Mercury's largest single source is the combustion of coal in power plants, a problem that grows as global industrial economies expand. The challenge is enormous and international health and environmental advisory bodies have urged regulations that call for removal of 90 percent of mercury from such emissions. (Cadmium and lead are also significant emission components.)
Mercury regulation has been a controversial issue in the United States for several years, mainly over government attempts to amend the Clear Air Act in favor of less stringent standards for emissions. Relaxation of standards and regulations has always been under fierce debate in toxic metals regulation, but in the case of mercury, the underlying conflict has been more closely related to the government's market-based approach to regulation as opposed to regulatory procedures specific to conditions near emission sites. The regulatory hope among experts in toxic metals research and regulation is to construct an international treaty similar to the Kyoto protocol that was established to reduce carbon dioxide emissions from industrial operations and thus decrease global warming. In other words, if all industrial operations adhered to low to zero emission standards, environmentalists believe the world would be a much safer place.
In any case, the public and environmental agencies at all levels of government are now acutely aware of the dangers of mercury. Disposal from mining operations remain a problem throughout the world and disputes over health effects and liability generate headlines almost daily. Likewise mercury contamination in ocean fish such as tuna, mackerel, and salmon remains a constant concern. Mercury in dental amalgam was for years a major cause of concern, but due to intense public attention that issue has subsided in recent years.
Lead contamination is more widely recognized than mercury contamination but vigilance over its dangers has helped to establish broad measures to bring exposure under control. A metal widely used since early times and treasured as a decorative and culinary material in ancient Rome, lead's toxic problems have been known for centuries. Since the mid-twentieth century, thousands of children have suffered the effects of lead poisoning by ingesting or absorbing lead from toys, painted household items, playground soil, and refuse left after the demolition of homes and buildings.
But in the broader sense, it was the overall public health implications of lead in gasoline (in the form of tetraethyl lead) that caused most of the initial furor over the need to control it in the environment. The U.S. petroleum and auto industries successfully fought efforts to end its use. However when the auto industry began installing catalytic converters to comply with U.S. air pollution laws, testing determined that lead rendered the devices inactive. The auto industry had no alternative but to demand development of lead-free gasoline. Leaded gasoline, however, is still in use in many countries.
Lead from mining has always been an environmental and public health problem and remains so in the early-twenty-first century. A typical industrial example is emissions from the smelter at the Bunker Hill lead mine in Pinehurst, Idaho, during the 1970s. For years fallout from the smelter contaminated the air in the area around Pinehurst. The Center for Disease Control (CDC) tested children in the area for blood-lead levels and found the highest amounts ever recorded in human beings.
On the whole, however, laws, regulations, and a high degree of watchfulness have brought the lead problem relatively under control, though lead poisoning incidents, especially in old housing, continue to be of concern, as do lead emissions from mining and smelting facilities around the world.
Cadmium and Chromium
Cadmium and chromium come from a variety of sources from cigarette smoke to smelting operations to increasingly voluminous waste from electronic devices. They enter living systems from alloys, pigments, batteries, metal coatings, electronic devices, mining operations, and industrial emissions. Cadmium especially affects the kidneys and lungs, but it also causes testicular damage, lung disease, and bone disease.
Chromium, for its part, is an essential nutrient in very small amounts. It is involved in manufacturing chromeplated materials, tanned leather, dyes and pigments, and wood preservatives. It enters living things mainly through the air and underground water. Extended exposure to chromium can cause asthma, lung cancer, and ulcers.
Chemists dislike the term heavy metals because of its inherent imprecision and often urge that it be abandoned. In 2002 the International Union of Pure and Applied Chemistry—the organization that sets the standards for chemistry's precise nomenclature—issued a technical report titled, "Heavy Metals—A Meaningless Term?" that reflected the frustration felt by the chemical community over the term's loose usage by those outside the field of basic chemistry. The term heavy metal, the report pointed out, "has even been applied to semi-metals (metalloids) such as arsenic, presumably because of the hidden assumption that 'heaviness' and 'toxicity' are in some ways identical" (p. 796).
The report bemoaned what it called "the persistence of the term and its continuing use in literature, policy, and regulations" (p. 797). It stated,
There is no similarity in properties between pure tin, which has low toxicity, and tributyltin oxide, which is highly toxic to oysters and dog whelks. Nor is there any similarity in properties between chromium in stainless steel, which is essentially nontoxic, and the chromate ion which has been associated with causing lung cancer. Thus, the tendency to group certain metals and their compounds together for toxicity assessment under the title 'heavy metals' must lead to fuzzy thinking and is another reason to abandon the term. (p. 799).
Ethical issues surrounding the heavy metals parallel those associated with harm caused by toxic substances in general. Tension always exists between producers of these substances and those exposed to them, often leading to tort damage claims and prolonged litigation. Those who believe industry should be held liable for injuries caused by toxic metal emissions have been turning for support to a relatively new legal theory known as the neurotoxity hypothesis. This hypothesis derives from neurochemical research that suggests that criminal behavior in individuals correlates with high levels of lead, manganese, and cadmium in the bodies of those individuals. Further research reinforcing such new insights could lead to changes in tort law that would impose stricter regulatory standards for these substances and more criminally related penalties for violators.
Crawford, Colin. (2000). "Criminal Penalties for Creating a Toxic Environment: Mens rea, Environmental Criminal Liability Standards, and the Neutotoxicity Hypothesis." Boston College Law Review 27(3): 341–290.
Duffus, John H. (2002). "Heavy Metals: A Meaningless Term?" (IUPAC Technical Report). Pure Applied Chemistry 74(5): 793–807.
Nriagu, Jerome O. (1966). "A History of Global Metal Pollution." Science 272(5259): 223–224.
Van der Voet, Ester; Jeroen B. Guinée; and Helias A. Udo de Haes, eds. (2000). Heavy Metals: A Problem Solved?: Methods and Models to Evaluate Policy Strategies for Heavy Metals. Dordrecht, The Netherlands; Boston: Kluwer Academic Publishers.
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
|contaminant||mcl or tt1 (mg/l)2||potential health effects from ingestion of water||sources of contaminant in drinking water|
|maximum contaminant level (mcl) –the highest level of a contaminant that is allowed in drinking water. mcls are set as close to mclgs as feasible using the best available treatment technology and taking cost into consideration. mcls are enforceable standards.|
|treatment technique–a required process intended to reduce the level of a contaminant in drinking water.|
|2units are in milligrams per liter (mg/l) unless otherwise noted. milligrams per liter are equivalent to parts per million.|
|3lead and copper are regulated by a treatment technique that requires systems to control the corrosiveness of their water. if more than 10 percent of tap water samples exceed the action level, water systems must take additional steps. for copper, the action level is 1.3 mg/l, and for lead is 0.015 mg/l.|
|source: U.S. Envionmental Protection Agency. Ground Water and Drinking Water. Available from http://www.epa.gov/safewater/mcl.html#/mcls|
|cadmium||0.005||kidney damage||corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints|
|chromium (total)||0.1||allergic dermatitis||discharge from steel and pulp mills; erosion of natural deposits|
|copper||tt3; action level = 1.3||short term exposure: gastrointestinal distress||corrosion of household plumbing systems; erosion of natural deposits|
|long term exposure: liver or kidney damage|
|people with wilson's disease should consult their personal doctor if the amount of copper in their water exceeds the action level|
|lead||tt3; action level = 0.015||infants and children: delays in physical or mental development; children could show slight deficits in attention span and learning abilities||corrosion of household plumbing systems; erosion of natural deposits|
|adults: kidney problems; high blood pressure|
|mercury (inorganic)||0.002||kidney damage||erosion of natural deposits; discharge from refineries and factories; runoff from landfills and croplands|
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.
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" 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 )
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.