There is no agreed upon chemical definition of the group of elements commonly described as “heavy metals.” Heavy metals are a loosely defined group of elements with variable chemical and physical properties (for example, atomic weight and position on the periodic table) all of which exhibit some of the characteristics of metals. They are also sometimes defined as having a specific gravity (a measure of density) that is at least five times that of water. Many heavy metals are essential for life in some form or lower concentrations yet toxic in higher concentrations. When in toxic concentrations they cause heavy metal poisoning in the soft tissues of the body.
Whether based on their physical or chemical properties, the distinction between heavy metals and nonmetals is not sharp. For example, arsenic (As), germanium (Ge), selenium (Se), tellurium (Te), and antimony (Sb) possess chemical properties of both metals and nonmetals. Defined as metalloids, they are often loosely classified as heavy metals. The category heavy metal is, therefore, somewhat arbitrary and highly nonspecific because it can refer to approximately eighty of the 103 elements in the periodic table. The term trace element is commonly used to describe substances that cannot be precisely defined but most frequently occur in the environment in concentrations of a few parts per million (ppm) or less. Only a relatively small number of heavy metals such as cadmium (Cd), copper (Cu), iron (Fe), cobalt (Co), zinc (Zn), mercury (Hg), vanadium (V), lead (Pb), nickel (Ni), chromium (Cr), manganese (Mn), molybdenum (Mo), silver (Ag), and tin (Sn) as well as the metalloids arsenic and selenium are associated with environmental, plant, animal, or human health problems. Heavy metals that are within a hazardous material are classified as “Misc.” with respect to the United Nations' hazardous classification. When being transported, however, they are classified as a poison.
Some chemical forms of heavy metals are persistent environmental contaminants. Natural processes such as bedrock and soil weathering, wind and water erosion, volcanic activity, sea salt spray, and forest fires release heavy metals into the environment. While the origins of anthropogenic releases of heavy metals are lost in antiquity, they probably began as prehistoric human ancestors learned to recover metals such as gold, silver, copper, and tin from their ores and to produce bronze. The modern age of heavy metal pollution has its beginning with the Industrial Revolution, which began in England in the late eighteenth century. The rapid development of industry, intensive agriculture, transportation, and urbanization since the Industrial Revolution, however, has been the precursor of today's environmental contamination problems. Anthropogenic utilization has also increased heavy metal distribution by removing the substances from localized ore deposits and transporting them to other parts of the environment. Heavy metal by-products result from many activities including the following: ore extraction and smelting; fossil fuel combustion; dumping of industrial wastes into landfills; exhausts from leaded gasoline products; steel, iron, cement and fertilizer production; or refuse and wood combustion. Heavy metal cycling has also increased through activities such as farming, deforestation, construction, dredging of harbors, and the disposal of municipal sludges and industrial wastes on land.
Thus, anthropogenic processes, especially combustion, have substantially supplemented the natural atmospheric emissions of selected heavy metals or metalloids such as selenium, mercury, arsenic, and antimony. They can be transported as gases or adsorbed on particles. Other metals such as cadmium, lead, and zinc are transported atmospherically only as particles. In either state, heavy metals may travel long distances before being deposited on land or water.
The heavy metal contamination of soils is a far more serious problem than either air or water pollution because heavy metals are usually tightly bound by the organic components in the surface layers of the soil and may, depending on conditions, persist for centuries or millennia. Consequently, the soil is an important geochemical sink that accumulates heavy metals rapidly and usually depletes them very slowly by leaching into groundwater aquifers or bioaccumulating into plants. Bioaccumulation is the accumulation of a substance in an organism. However, heavy metals can also be very rapidly translocated through the environment by erosion of the soil particles to which they are adsorbed or bound and redeposited elsewhere on the land or washed into rivers, lakes, or oceans to the sediment.
The interactions of heavy metals in aquatic systems are complicated because of the possible changes due to many dissolved and particulate components and nonequilibrium conditions. For example, the speciation of heavy metals is controlled not only by their chemical properties but also by environmental variables such as:
In addition, various species of bacteria can oxidize arsenate or reduce arsenate (As5+) to arsenite (As3+), or oxidize ferrous iron (Fe2+) to ferric iron (Fe3+), or convert mercuric ion (Hg2+) to elemental mercury or the reverse. Various enzyme systems in living organisms can biomethylate a number of heavy metals. While it had been known for at least sixty years that arsenic and selenium could be biomethylated, microorganisms capable of converting inorganic mercury into monomethyl ([CH3Hg]+) and dimethylmercury ([CH3]2Hg) in lake sediments were not discovered until 1967. Since then, numerous heavy metals such as lead, tin, cobalt, antimony, platinum, gold, tellurium, thallium (Tl), and palladium (Pd) have been shown to be biomethylated by bacteria and fungi in the environment.
As environmental factors change the chemical reactivities and speciation of heavy metals, they influence not only the mobilization, transport, and bioavailability, but also the toxicity of heavy metal ions toward biota in both freshwater and marine ecosystems. The factors affecting the toxicity and bioaccumulation of heavy metals by aquatic organisms include the following:
The extent to which most of the methylated metals are bioaccumulated or biomagnified is limited by the chemical and biological conditions and how readily the methylated metal is metabolized by an organism. At present, only methylmercury seems to be sufficiently stable to bioaccumulate to levels that can cause adverse effects in aquatic organisms. All other methylated metal ions are produced in very small concentrations and are degraded naturally faster than they are bioaccumulated. Therefore, they do not biomagnify in the food chain. Biomagnification occurs when a bioaccumulated substance is present in higher concentrations in organisms because of persistence in the environment as well as the inability of organisms to degrade or excrete the substance.
The largest proportion of heavy metals in water is associated with suspended particles, which are ultimately deposited in the bottom sediments where concentrations are orders of magnitude higher than those in the overlying or interstitial waters. The heavy metals associated with suspended particulates or bottom sediments are complex mixtures of the following:
In anoxic waters (those with decreased amounts of oxygen), the precipitation of sulfides may control the heavy metal concentrations in sediments, whereas in toxic waters adsorption, absorption, surface precipitation, and coprecipitation are usually the mechanisms by which heavy metals are removed from the water column. Moreover, physical, chemical, and microbiological processes in the sediments often increase the concentrations of heavy metals in the pore waters, which are released to overlying waters by diffusion or as the result of consolidation and bioturbation, which is the mixing of sediments by bottom-dwelling organisms. Transport by living organisms does not represent a significant mechanism for local movement of heavy metals. However, accumulation by aquatic plants and animals can lead to important biological responses. Even low environmental levels of some heavy metals may produce subtle and chronic effects in animal populations.
Despite these adverse effects, at very low levels, some metals have essential physiological roles as micronutrients. Heavy metals such as chromium, manganese, iron, cobalt, molybdenum, nickel, vanadium, copper, and selenium are required in small amounts to perform important biochemical functions in plant and animal systems. In higher concentrations they can be toxic, but usually some biological regulatory mechanism is available by means of which animals can speed up their excretion or retard their uptake of excessive quantities.
In contrast, nonessential heavy metals are primarily of concern in terrestrial and aquatic systems because they are toxic and persist in living systems. Metal ions commonly bond with sulfhydryl (-SH) and carboxylic acid (CO2H) groups in amino acids, which are components of proteins (enzymes) or polypeptides. This increases their bioaccumulation and inhibits excretion. For example, heavy metals such as lead, cadmium, and mercury bind strongly with sulfur-containing groups such as -SH and -SCH3 in cysteine and methionine and so inhibit the metabolism of the bound enzymes. In addition, other heavy metals may replace an essential element, decreasing its availability and causing symptoms of deficiency.
Uptake, translocation, and accumulation of potentially toxic heavy metals in plants differ widely depending on soil type, pH, redox potential, moisture, and organic content. Public health officials closely regulate the quantities and effects of heavy metals that move through the agricultural food chain to be consumed by human beings. While heavy metals such as zinc, copper, nickel, lead, arsenic, and cadmium are translocated from the soil to plants and then into the animal food chain, the concentrations in plants are usually very low and generally not considered to be an environmental problem. However, plants grown on soils either naturally enriched or highly contaminated with some heavy metals can bioaccumulate levels high enough to cause toxic effects in the animals or human beings that consume them.
The heavy metals most often implicated in accidental human poisoning are lead, mercury, arsenic, and cadmium. More recently, thallium has gained some attention in the media as the poison used in several murder cases in the 1990s. Some heavy metals, such as zinc, copper, chromium, iron, and manganese, are required by the body in small amounts, but these same elements can be toxic in larger quantities.
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 (a system of traditional medicine of India) 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.
In Japan, the Chisso Corporation released methylmercury-containing wastewater into Minimata Bay from 1932 to 1968. Mercury bioaccumulated in fish and shellfish harvested and consumed by humans, causing mercury poisoning and neurological symptoms. Approximately three thousand people were diagnosed with Minimata disease. The disease is caused by mercury poisoning and affects the central nervous system, in some cases resulting in death. Because of the persistence of mercury in the environment, a clean-up project was initiated in 1974 to dredge the bottom sediment in Minimata Bay for removal of mercury. The project took sixteen years to complete and cost forty-eight billion yen (or $437 million 2018 U.S. dollars).
In 2010 Mèdecins Sans Frontières (MSF) teams treated more than two thousand heavy metal lead-poisoning victims in Zamfara, Nigeria. MSF officials reported treating children with lead blood levels more than a dozen times normal levels. The severity of the poisonings required emergency treatment to alleviate life-threatening and brain-damaging convulsions. MSF and World Health Organization (WHO) officials called the poisoning an “unprecedented environmental emergency” caused by unsafe mining and dangerous environmental disposal practices related to the extraction of gold from lead-rich ore.
For a number of years, non-government organizations (NGOs) have been concerned with the large number of heavy-metal poisonings in China. In April 2010, a group of several of these NGOs discussed the problem with some of the leading electronics companies in the world (such as HP, IBM, Intel, Nokia, Panasonic, and Samsung) because these companies were providing products to China and had leverage to stop China from dumping heavy metals into its waters. In response to these discussions, the Chinese Ministry of Environmental Protection announced the urgency of China to control heavy-metal pollution. In 2011, it was decided that the Chinese State Council would institute a project to combat heavy-metal pollution as part of its 12th Five-Year Plan (from 2011 to 2015). The plan would target 2015 as an emission-reduction target of 15% for the five heavy metals of arsenic, cadmium, chromium, lead, and mercury. According to the February 19, 2011, China Daily article “Project to Tackle Heavy-Metal Pollution,” these five heavy metals were targeted because a Chinese government census and survey of over 110,000 enterprises, in 2010, found that the country discharged 900 tons of the five metals in 2007.
Some heavy metals are toxic or carcinogenic, having adverse effects on the central nervous system of humans. Contamination of soils caused by land disposal of sewage and industrial effluents and sludges may pose the most significant long-term problem. While cadmium and lead are the greatest hazard, other elements such as copper, molybdenum, nickel, and zinc can also accumulate in plants grown on sludge-treated land. High concentrations can under certain conditions cause adverse effects in animals and human beings that consume the plants. For example, when soil contains high concentrations of molybdenum and selenium, these metals can be translocated into edible plant tissue in sufficient quantities to produce toxic effects in ruminant animals. Consequently, the U.S. Environmental Protection Agency has issued regulations that prohibit or tightly regulate the disposal of contaminated municipal and industrial sludges on land to prevent heavy metals, especially cadmium, from entering the food supply in toxic amounts. However, presently, the most serious known human toxicity is not through bioaccumulation from crops but from either mercury in fish or lead in gasoline, paints, and water pipes, and other metals derived from occupational or accidental exposure. One method of removing heavy metals from the environment involves the use of plants that are able to bioaccumulate heavy metals. This method of removal, termed phytoremediation (or bioremediation), is completed by harvesting the plants and incinerating them, thus allowing recovery of the metals if desired.
Heavy metal poisoning is especially dangerous to young children. Chelation therapy, considered an alternative medicine therapy for some purposes, is designed to use chemical binding agents to neutralize heavy metals in the body before they can cause tissue damage. The chelating agents are then excreted with bound heavy metals. Although chelation therapy is considered standard and approved treatment for heavy metal poisoning, studies have failed to support its usefulness in treating other conditions. Moreover, although the chelating agent used to treat heavy metal poisoning is approved in the United States, other chelating agents routinely used in alternative or nonstandard practice await further study.
Exposure to toxic heavy metals is generally classified as acute (exposed 14 days or less), intermediate, (15 to 354 days), and chronic (more than 365 days). Acute toxicity is usually caused by a sudden or unexpected exposure to a high level of the heavy metal. Examples include being carelessly exposed to a heavy metal while handling it or from inadequate safety precautions. An accidental spill or release of toxic material in a laboratory, industrial, or transportation location can also cause acute toxicity of a heavy metal. Chronic toxicity results from repeated or continuous exposure, which leads to a toxic accumulation of a heavy metal in the body. Chronic exposure often comes about from contaminated air, dust, food, or water; living near a hazardous waste site; spending time in areas with deteriorating lead paint; maternal transfer in the womb; or from participating in hobbies that use lead paint or solder.
Heavy metal poisoning may be detected using blood and urine tests, hair and tissue analysis, or x ray. The diagnosis is often overlooked, however, because many of the early symptoms of heavy metal poisoning are nonspecific. The doctor should take a thorough patient history with particular emphasis on the patient's occupation.
In childhood, blood lead levels above 80 milligrams per deciliters (ug/dL) generally indicate lead poisoning; however, significantly lower levels (>30 ug/dL) can cause mental retardation and other cognitive and behavioral problems in affected children. The Centers for Disease Control and Prevention considers a blood lead level of 10 ug/dL or higher in children a cause for concern. In adults, symptoms of lead poisoning are usually seen when blood lead levels exceed 80 ug/dL for a number of weeks.
Blood levels of mercury should not exceed 3.6 ug/dL, while urine levels should not exceed 15 ug/dL. Symptoms of mercury poisoning may be seen when mercury levels exceed 20 ug/dL in blood and 60 ug/dL in urine. Mercury levels in hair may be used to gauge the severity of chronic mercury exposure.
Since arsenic is rapidly cleared from the blood, blood arsenic levels may not be very useful in diagnosis. Arsenic in the urine (measured in a 24-hour collection following 48 hours without eating seafood) may exceed 50 ug/dL in people with arsenic poisoning. If acute arsenic or thallium poisoning is suspected, an x ray may reveal these substances in the abdomen (since both metals are opaque to x rays). Arsenic may also be detected in the hair and nails for months following exposure.
Cadmium toxicity is generally indicated when urine levels exceed 10 ug/dL of creatinine and blood levels exceed 5 ug/dL.
Thallium poisoning often causes hair loss (alopecia), numbness, and a burning sensation in the skin as well as nausea, vomiting, and dizziness. As little as 15–20 mg of thallium per kilogram of body weight is fatal in humans; however, smaller amounts can cause severe damage to the nervous system.
When heavy metal poisoning is suspected, it is important to begin treatment as soon as possible to minimize long-term damage to the patient's nervous system and digestive tract. Heavy metal poisoning is considered a medical emergency, and the patient should be taken to a hospital emergency room.
In cases of acute mercury, arsenic, or thallium ingestion, vomiting may be induced. Activated charcoal may be given in cases of thallium poisoning. Washing out the stomach (gastric lavage) may also be useful. The patient may also require treatment such as intravenous fluids for such complications of poisoning as shock, anemia, and kidney failure.
Patients who have taken arsenic, thallium, or mercury in a suicide attempt will be seen by a psychiatrist as part of emergency treatment.
The Occupational Safety and Health Administration (OSHA), under the U.S. Department of Labor, enforces regulations for workers at risk for exposure to heavy metal poisoning. The Centers for Disease Control and Prevention (CDC) recommends that all children be tested for lead exposure at 12 months of age and again at 24 months. For children at higher risk for being exposed to lead positioning, the testing is recommended by the CDC to begin at six months of age.
The chelation process can only halt further effects of the poisoning; it cannot reverse neurological damage already sustained.
Because arsenic and thallium were commonly used in rat and insect poisons at one time, many countries have tried to lower the rate of accidental poisonings by banning the use of heavy metals in pest control products. Thallium was banned in the United States as a rodent poison in 1984. As a result, almost all recent cases of arsenic and thallium poisoning in the United States were deliberate rather than accidental.
People who use Ayurvedic or traditional Chinese herbal preparations as alternative treatments for various illnesses should purchase them only from reliable manufacturers.
Because exposure to heavy metals is often an occupational hazard, protective clothing and respirators should be provided and worn on the job. Protective clothing should then be left at the work site and not worn home, where it could carry toxic dust to family members. Industries are urged to reduce or replace the heavy metals in their processes wherever possible. Exposure to environmental sources of lead, including lead-based paints, plumbing fixtures, vehicle exhaust, and contaminated soil, should be reduced or eliminated.
See also Hazardous waste ; Heavy metal poisoning ; Lead poisoning ; Mercury poisoning ; National Institute for Occupational Safety and Health ; Occupational Safety and Health Administration .
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American Medical Association, 515 N. State St., Chicago, IL, 60654, (800) 621-8335, http://www.ama-assn.org/ .
Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA, 30333, (800) 232-4636, email@example.com, http://www.cdc.gov/ .
Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Ariel Rios Bldg., Washington, D.C., 20460, 1(202) 272-0167, http://www.epa.gov/ .
Food and Drug Administration, 10903 New Hampshire Ave., Silver Spring, MD, 20993, (888) INFO-FDA (463-6332), http://www.fda.gov/ .
World Health Organization, Avenue Appia 20, Geneva, Switzerland, 1211 27, 41 22 791-2111, Fax: 41 22 791-3111, firstname.lastname@example.org, http://www.who.int/en/ .
Rebecca J. Frey, PhD
Revised by William A. Atkins, BB, BS, MBA