MERCURY (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Mercury is a transition metal. A transition metal is one of the elements found between Groups 2 (IIA) and 13 (IIIA) on the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Mercury has long been known as quicksilver, because it is a silver liquid. The chemical symbol also reflects this property. The symbol, Hg, comes from the Latin term hydrargyrum, meaning "watery silver."

Mercury has been known for thousands of years. In many cultures, people learned to make mercury metal from its most important ore, cinnabar. When heated cinnabar releases mercury as a vapor (gas). The vapor is cooled and captured as liquid mercury.

SYMBOL
Hg

ATOMIC NUMBER
80

ATOMIC MASS
200.59

FAMILY
Group 12 (IIB)
Transition metal

PRONUNCIATION
MER-kyuh-ree

Some mercury compounds are known to be poisonous. For example, mercuric chloride (corrosive sublimate) was often used to kill pests and, sometimes, people. On the other hand, some mercury compounds have been used as medicines. For instance, mercurous chloride (calomel) was long used as a cure for skin rashes. In the last forty years, the dangers of mercury have become better known. As a result, mercury use is now being phased out.

Discovery and naming

The oldest sample of mercury dates to about the fifteenth or sixteen century b.c. It was found in an Egyptian tomb at Kurna, stored in a small glass container.

Mercury and cinnabar are both mentioned in ancient manuscripts. The Chinese, Hindus, Egyptians, Greeks, and Romans all recorded information about the element and its ore. Greek philosopher Theophrastus (372-287 b.c.), for example, described a method for preparing mercury. Cinnabar was rubbed together with vinegar in a clay dish. Theophrastus wrote that the cinnabar had been found in silver mines. When the metal was first made, he said, people thought it might contain gold. They were misled by the metal's shiny appearance. They soon realized, however, that it was quite different from gold.

Many reports on mercury told of its poisonous effects. Slaves who worked in Roman mercury mines, for example, often died of exposure to mercury. Strangely enough, trees and plants around these mines were not affected. Mercury was sometimes very dangerous and sometimes quite safe. People even drank from streams that ran through mercury mines. Scientists now know that mercury's effects depend on the form in which it occurs.

Mercury amalgams have also been around for a long time. An amalgam is a combination of mercury with at least one other metal. Amalgams are formed when a metal, such as silver, dissolves in mercury. The process is similar to dissolving salt in water. Amalgamation is used in mining to remove silver from ore. The silver dissolves in the mercury and a silver amalgam is formed. Heating the amalgam releases the silver. This method was used by miners as early as the sixteenth century.

Physical properties

Mercury is the only liquid metal. In fact, there is only one other liquid element, bromine. Bromine is a non-metal. Mercury can be frozen (changed into a solid) at a temperature of –38.85°C (–37.93°F). It can be changed into a gas ("boiled") at 365.6°C (690.1°F). Its density is 13.59 grams per cubic centimeter.

Mercury has two physical properties of special interest. First, it has very high surface tension. Surface tension is a property of liquids that make them act like they are covered with a skin.

For example, some water bugs are able to walk on the surface of water. With care, one can float a needle on the surface of water. These incidents are possible because of water's surface tension.

Mercury is also a very good conductor of electricity. This property is used in a number of practical devices. One such device is a mercury switch, such as the kind that turns lights on and off. A small amount of mercury can be placed into a tiny glass capsule. The capsule can be made to tip back and forth. As it tips, the mercury flows from one end to the other. At one end of the capsule, the mercury may allow an electric current to flow through a circuit. At the other end, no mercury is present, so no current can flow. Mercury switches are easy to make and very efficient.

Chemical properties

Mercury is moderately active. It does not react with oxygen in the air very readily. It reacts with some acids when they are hot, but not with most cold acids.

Occurrence in nature

The abundance of mercury in the Earth's crust is estimated to be about 0.5 parts per million. That makes it one of the 20 least common elements. It very rarely occurs as an element. Instead, it is usually found as a compound. Its most common ore is cinnabar, or mercuric sulfide (HgS). Cinnabar usually occurs as a dark red powder. It is often called by the common name of vermillion or Chinese vermillion.

The largest producer of mercury outside the United States is Spain. U.S. production numbers are not announced in order to protect U.S. industries from revealing important company secrets. Other producers after Spain are Kyrgyzstan, Algeria, China, and Finland.

In the United States, mercury is produced as a by-product of gold mining. It comes from eight gold mines in California, Nevada, and Utah.

Isotopes

Seven naturally occurring isotopes of mercury are known. They are mercury-196, mercury-198, mercury-199, mercury-200, mercury-201, mercury-202, and mercury-204. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Mercury is the only liquid metal.

About a dozen radioactive isotopes of mercury are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Two radioactive isotopes of mercury are used in medicine, mercury-197 and mercury-203. Both isotopes are used to study the brain and the kidneys. The isotopes are injected into the body where they travel to the brain and the kidneys. Inside these two organs, the isotopes give off radiation that is detected by instruments held above the body. The pattern of radiation provides information about how well the brain and kidneys are functioning.

Extraction

Mercury is still prepared as it was hundreds of years ago. Cinnabar is heated in air. The compound breaks down to give mercury metal:

The mercury metal is then purified by distillation. Distillation is the process of heating two or more liquids to their boiling points. Different liquids boil at different temperatures. The liquid that is wanted (such as mercury) can be collected at its boiling point. Mercury that is more than 99 percent pure can be collected by distillation.

Uses

The most important use of mercury is in the preparation of chlorine. Chlorine is produced by passing an electric current through sodium chloride:

There is a problem with using this method, however. Sodium (Na) is a very reactive metal. If any water is present, the sodium will react violently with the water. This reaction makes the production of chlorine much more difficult.

In 1892, two English chemists developed a method for solving this problem. They made a container with a layer of mercury on the bottom. As sodium is produced by the electric current, it dissolves in the mercury, forming an amalgam. The sodium is unable to react with water. For many years, the "mercury cell" invented in 1892 was a very popular method for producing chlorine.

But today, companies are looking for other ways to make chlorine. They are worried about the harmful effects of mercury. They are also concerned that mercury can get into the environment and harm humans, animals, and plants.

The second most important use of mercury in the United States is in switches and other electrical applications. Again, there are increasing concerns about the health effects of mercury. Many companies are switching to electronic switches.

One application in which concerns about mercury have had little effect is fluorescent lamps. A fluorescent lamp contains mercury vapor (gas). When the lamp is turned on, an electric current passes through the mercury vapor, causing it to give off invisible radiation. The radiation strikes the inside of the glass tube, whose walls are coated with a phosphor. A phosphor is a material that gives off visible light when struck by electrons. The tube glows as the radiation strikes the phosphor.

Lamp manufacturers have reduced the amount of mercury in fluorescent lamps by about 60 percent. They developed ways to make the Lamps work just as well with less mercury. However, mercury lamps are much more popular. Each lamp now contains much less mercury. But there are many more lamps than ever before.

For a time, mercury batteries were quite popular. In the early 1980s, more than 1,000 tons of mercury a year were used to make mercury batteries. These batteries are a special environmental problem, however. People tend to just throw them away when they no longer work. The cases split open easily, releasing mercury into the environment. As a result, much less mercury is now being used to make such batteries. In 1996, less than one ton of mercury was used in these batteries. They are now restricted almost entirely to military and medical uses.

Mercury is also used in dental applications, measuring instruments (such as mercury thermometers and barometers), and coatings for mirrors.

Compounds

Mercury compound use is also decreasing because of health concerns. A few of the compounds still in use follow. Notice that two different endings are used for mercury compounds. Those that end in -ous have less mercury than those that end in -ic.

mercuric arsenate (HgHAsO₄): waterproofing paints

mercuric benzoate (Hg(C₇G₅O₂)₂): medicine; used to treat syphilis

mercuric chloride, or mercury bichloride, or corrosive sublimate (HgCl₂): disinfectant, tanning of leather, spray for potato seedlings (to protect from disease), insecticide, preservation of wood, embalming fluid, textile printing, and engraving

mercuric cyanide (Hg(CN)₂): germicidal soaps (soaps that kill germs), photography

mercuric oxide (HgO): red or yellow pigment in paints, disinfectant, fungicide (to kill fungi), perfumes and cosmetics

mercuric sulfide (HgS): red or black pigment in paints

mercurous chloride, or calomel (Hg₂Cl₂): fungicide, maggot control in agriculture, fireworks

mercurous chromate (Hg₂CrO₄): green pigment in paints

mercurous iodide (Hg₂I₂): kills bacteria on the skin

The tragic effects of mercury poisoning

I n a tragic irony, a scientist who was helping to improve the environment died as a result of her efforts. On June 8, 1997, Dartmouth College chemistry professor Karen Wetterhahn died of mercury poisoning. Less than a year earlier, she had been experimenting with dimethyl mercury when she spilled a tiny amount on her hands. Dimethyl mercury is one of the most toxic of mercury compounds.

Wetterhahn was studying the effects that heavy metals (mercury, chromium, lead, and arsenic) have on living things. She was concerned about how these elements pollute the environment and cause disease in people.

In August 1996, as Wetterhahn was transferring some dimethyl mercury to a tube, the accident occurred. She was wearing latex gloves, but they were not adequate protection against the dangerous chemical. The mercury seeped into her skin. Wetterhahn did not begin to feel the effects of the exposure until six months later. She then started losing her balance, slurring her speech, and suffering vision and hearing loss. Tests showed her system had eighty times the lethal dose of mercury. Wetterhahn died of mercury poisoning on June 8, 1997.

Wetterhahn's death prompted some safety changes. Bright stickers on latex glove boxes should warn against using the gloves with hazardous chemicals. Workshops were held to teach proper glove selection. The dangers of dimethyl mercury were stressed. And scientists were urged to use a less dangerous chemical than dimethyl mercury. Overall, her death heightened awareness in the scientific community of potential laboratory dangers.

Health effects

Mercury metal and most compounds of mercury are highly toxic. Interestingly enough, scientists have become aware of this fact only quite recently. The toxicity of some mercury compounds has been known for many centuries. One form of mercury chloride known as calomel, for example, was sometimes used as a poison to kill people. It was also once used extensively to kill fungi and control maggots in agricultural crops.

But even as recently as fifty years ago, there was relatively little concern about mercury metal and many mercury compounds. High school chemistry students often played with tiny droplets of mercury in the laboratory. They used mercury to coat pennies and other pieces of metal.

Mercury was also widely used in dentistry. It was used to make amalgams, alloys of mercury with other metals, used to fill teeth. Most people even today are likely to have dental fillings that contain a small amount of mercury metal.

In the last fifty years, chemists have learned a great deal more about the toxic effects of both mercury metal and most of its compounds. They now know that mercury itself enters the body very easily. Its vapors pass through the skin into the blood stream. Its vapors can also be inhaled. And, of course, it can also be swallowed. In any of these cases, mercury gets into blood and then into cells. There it interferes with essential chemical reactions and can cause illness and death.

Sometimes, these effects occur over very long periods of time. People who work with mercury, for example, may take in small amounts of mercury over months or years. Health problems develop very slowly. These problems can include inflammation of the mouth and gums; loosening of the teeth; damage to the kidneys and muscles; shaking of the arms and legs; and depression, nervousness, and personality changes.

"Mad as a hatter!"

B ack in the 1800s, most of the negative effects of mercury and its compounds were not yet known. Hatmakers of that time commonly used a mercury compound in their craft. It was used to treat the felt and beaver fur that lined the hats. Eventually, exposure to the mercury began to cause changes in the hatmakers' bodies. Their personalities and behavior became erratic. Recognizing the bizarre personalities of many hatmakers, people often used the expression "mad as a hatter." In fact, author Lewis Carroll (1832-98) created a character for Alice's Adventures in Wonderland that owes its origins to the symptoms of mercury poisoning: The Mad Hatter.

People can also be exposed to large doses of mercury over short periods of time. In such cases, even more serious health problems can arise. These include nausea, vomiting diarrhea, stomach pain, damage to the kidneys, and death in only a week or so.

So is mercury still safe to use in dental fillings? That question is the source of considerable controversy. Some people say that so little mercury is lost from fillings that the metal presents no danger to people. Other people think that dentists should take no chances with this dangerous metal. They should stop using mercury fillings entirely.

Mercury

Background

Mercury is one of the basic chemical elements. It is a heavy, silvery metal that is liquid at normal temperatures. Mercury readily forms alloys with other metals, and this makes it useful in processing gold and silver. Much of the impetus to develop mercury ore deposits in the United States came after the discovery of gold and silver in California and other western states in the 1800s. Unfortunately, mercury is also a highly toxic material, and as a result, its use has severely declined over the past 20 years. Its principal applications are in the production of chlorine and caustic soda, and as a component of many electrical devices, including fluorescent and mercury-vapor lamps.

Mercury has been found in Egyptian tombs dating to about 1500 b.c., and it was probably used for cosmetic and medicinal purposes even earlier. In about 350 b.c., the Greek philosopher and scientist Aristotle described how cinnabar ore was heated to extract mercury for religious ceremonies. The Romans used mercury for a variety of purposes and gave it the name hydrargyrum, meaning liquid silver, from which the chemical symbol for mercury, Hg, is derived.

Demand for mercury greatly increased in 1557 with the development of a process that used mercury to extract silver from its ore. The mercury barometer was invented by Torricelli in 1643, followed by the invention of the mercury thermometer by Fahrenheit in 1714. The first use of a mercury alloy, or amalgam, as a tooth filling in dentistry was in 1828, although concerns over the toxic nature of mercury prevented the widespread use of this new technique. It wasn't until 1895 that experimental work by G.V. Black showed that amalgam fillings were safe, although 100 years later scientists were still debating that point.

Mercury found its way into many products and industrial applications after 1900. It was commonly used in batteries, paints, explosives, light bulbs, light switches, pharmaceuticals, fungicides, and pesticides. Mercury was also used as part of the processes to produce paper, felt, glass, and many plastics.

In the 1980s, increasing understanding and awareness of the harmful health and environmental effects of mercury started to greatly outweigh its benefits, and usage began to drop sharply. By 1992, its use in batteries had dropped to less than 5% of its level in 1988, and overall use in electrical devices and light bulbs had dropped 50% in the same period. The use of mercury in paints, fungicides, and pesticides has been banned in the United States, and its use in the paper, felt, and glass-manufacturing processes has been voluntarily discontinued.

Worldwide, production of mercury is limited to only a few countries with relaxed environmental laws. Mercury mining has ceased altogether in Spain, which until 1989 was the world's largest producer. In the United States, mercury mining has also stopped, although small quantities of mercury are recovered as part of the gold refining process to avoid environmental contamination. China, Russia (formerly the USSR), Mexico, and Algeria were the largest producers of mercury in 1992.

Raw Materials

Mercury is rarely found by itself in nature. Most mercury is chemically bound to other materials in the form of ores. The most common ore is red mercury sulfide (HgS), also known as cinnabar. Other mercury ores include corderoite (Hg₃S₂Cl₂), livingstonite (HgSb₄S₈), montroydite (HgO), and calomel (HgCl). There are several others. Mercury ores are formed underground when warm mineral solutions rise towards the earth's surface under the influence of volcanic action. They are usually found in faulted and fractured rocks at relatively shallow depths of 3-3000 ft (1-1000 m).

Other sources of mercury include the dumps and tailing piles of earlier, less-efficient mining and processing operations.

The Manufacturing
Process

The process for extracting mercury from its ores has not changed much since Aristotle first described it over 2,300 years ago. Cinnabar ore is crushed and heated to release the mercury as a vapor. The mercury vapor is then cooled, condensed, and collected. Almost 95% of the mercury content of cinnabar ore can be recovered using this process.

Here is a typical sequence of operations used for the modern extraction and refining of mercury.

Mining

Cinnabar ore occurs in concentrated deposits located at or near the surface. About 90% of these deposits are deep enough to require underground mining with tunnels. The remaining 10% can be excavated from open pits.

• 1 Cinnabar is dislodged from the surrounding rocks by drilling and blasting with explosives or by the use of power equipment. The ore is brought out of the mine on conveyor belts or in trucks or trains.

Roasting

Because cinnabar ore is relatively concentrated, it can be processed directly without any intermediate steps to remove waste material.

• 2 The ore is first crushed in one or more cone crushers. A cone crusher consists of an interior grinding cone that rotates on an eccentric vertical axis inside a fixed outer cone. As the ore is fed into the top of the crusher, it is squeezed between the two cones and broken into smaller pieces.
• 3 The crushed ore is then ground even smaller by a series of mills. Each mill consists of a large cylindrical container laying on its side and rotating on its horizontal axis. The mill may be filled with short lengths of steel rods or with steel balls to provide the grinding action.
• 4 The finely powdered ore is fed into a furnace or kiln to be heated. Some operations use a multiple-hearth furnace, in which the ore is mechanically moved down a vertical shaft from one ledge, or hearth, to the next by slowly rotating rakes. Other operations use a rotary kiln, in which the ore is tumbled down the length of a long, rotating cylinder that is inclined a few degrees off horizontal. In either case, heat is provided by combusting natural gas or some other fuel in the lower portion of the furnace or kiln. The heated cinnabar (HgS) reacts with the oxygen (02) in the air to produce sulfur dioxide (SO₂), allowing the mercury to rise as a vapor. This process is called roasting.

Condensing

• 5 The mercury vapor rises up and out of the furnace or kiln along with the sulfur dioxide, water vapor, and other products of combustion. A considerable amount of fine dust from the powdered ore is also carried along and must be separated and captured.
• 6 The hot furnace exhaust passes through a water-cooled condenser. As the exhaust cools, the mercury, which has a boiling point of 675° F (357° C), is the first to condense into a liquid, leaving the other gases and vapors to be vented or to be processed further to reduce air pollution.
• 7 The liquid mercury is collected. Because mercury has a very high specific gravity, any impurities tend to rise to the surface and form a dark film or scum. These impurities are removed by filtration, leaving a liquid mercury that is about 99.9% pure. The impurities are treated with lime to separate and capture any mercury, which may have formed compounds.

Refining

Most commercial-grade mercury is 99.9% pure and can be used directly from the roasting and condensing process. Higher purity mercury is needed for some limited applications and must be refined further. This ultrapure mercury commands a premium price.

• 8 Higher purity can be obtained through several refining methods. The mercury may be mechanically filtered again, and certain impurities may be removed through oxidation with chemicals or air. In some cases the mercury is refined through an electrolytic process, in which an electric current is passed through a tank of liquid mercury to remove the impurities. The most common refining method is triple distillation, in which the temperature of the liquid mercury is carefully raised until the impurities either evaporate or the mercury itself evaporates, leaving the impurities behind. This distillation process is performed three times, with the purity increasing each time.

Shipping

• 9 Commercial-grade mercury is poured into wrought-iron or steel flasks and sealed. Each flask contains 76 lb (34.5 kg) of mercury. Higher purity mercury is usually sealed in smaller glass or plastic containers for shipment.

Quality Control

Commercial-grade mercury with 99.9% purity is called prime virgin-grade mercury. Ultrapure mercury is usually produced by the triple-distillation method and is called triple-distilled mercury.

Quality control inspections of the roasting and condensing process consist of spot checking the condensed liquid mercury for the presence of foreign metals, since those are the most common contaminants. The presence of gold, silver, and base metals is detected using various chemical-testing methods.

Triple-distilled mercury is tested by evaporation or spectrographic analysis. In the evaporation method, a sample of mercury is evaporated, and the residue is weighed. In the spectrographic analysis method, a sample of mercury is evaporated, and the residue is mixed with graphite. Light coming from the resulting mixture is viewed with a spectrometer, which separates the light into different color bands depending on the chemical elements present.

Health and Environmental Effects

Mercury is highly toxic to humans. Exposure may come from inhalation, ingestion, or absorption through the skin. Of the three, inhalation of mercury vapor is the most dangerous. Short-term exposure to mercury vapor can produce weakness, chills, nausea, vomiting, diarrhea, and other symptoms within a few hours. Recovery is usually complete once the victim is removed from the source. Long-term exposure to mercury vapor produces shaking, irritability, insomnia, confusion, excessive salivation, and other debilitating effects.

In normal situations, most exposure to mercury comes from the ingestion of certain foods, such as fish, in which the mercury has accumulated at high levels. Although mercury is not absorbed in great quantities when passing through the human digestive system, ingestion over a long period of time has been shown to have cumulative effects.

In industrial situations, mercury exposure is a far more serious hazard. Mining and processing mercury ore can expose workers to mercury vapor as well as to direct contact with the skin. The production of chlorine and caustic soda can also cause significant mercury exposure hazards. Dentists and dental assistants can be exposed to mercury while preparing and placing mercury amalgam fillings.

Because mercury poses a serious health hazard, its use and release to the environment has come under increasingly tight restrictions. In 1988, it was estimated that 24 million lb/yr (11 million kglyr) of mercury were released into the air, land, and water worldwide as the result of human activities. This included mercury released by mercury mining and refining, various manufacturing operations, the combustion of coal, the discarding of municipal refuse and sewage sludge, and other sources.

In the United States, the Environmental Protection Agency (EPA) has banned the use of mercury for many applications. The EPA has set a goal of reducing the level of mercury found in municipal refuse from 1.4 million Ib/yr (0.64 million kg/yr) in 1989 to 0.35 million lb/yr (0.16 million kg/yr) by 2000. This is to be accomplished by decreasing the use of mercury in products and increasing the diversion of mercury from municipal refuse through recycling.

The Future

Mercury is still an important component in many products and processes, although its use is expected to continue to decline. Improved handling and recycling of mercury are expected to significantly reduce its release to the environment and thereby reduce its health hazard.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th Edition. McGraw-Hill, 1997.

Heiserman, David L. Exploring Chemical Elements and Their Compounds. TAB Books, 1992.

Kroschwitz, Jacqueline I., executive editor, and Mary Howe-Grant, editor. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.

Stwertka, Albert. A Guide to the Elements. Oxford University Press, 1996.

Periodicals

Raloff, J. "Mercurial Airs: Tallying Who's to Blame." Science News (February 19, 1994): 119.

Spencer, Peter, and G. Murdoch. "Mercury in Paint." Consumers' Research Magazine (January 1991): 2.

Stone, R. "Mercurial Debate." Science (March 13, 1992): 1356-1357.

Other

http://www.intercorr.com/periodic/80.htm [This website contains a summary of the history, sources, properties, and uses of mercury.]

—Chris Cavette

MERCURY

Mercury (Hg) is a naturally occurring silvery metal that has been associated with adverse health effects throughout history. Elemental mercury is a liquid at room temperature, and, because of this, Aristotle named mercury "quicksilver." There are three forms of mercury: elemental mercury (Hg⁰), organic mercury (e.g., methylmercury), and inorganic mercury (e.g., Hg⁺, Hg²⁺). Many different organic and inorganic mercury compounds are found in nature because of mercury's ability to form covalent or ionic bonds with other chemicals. Mercury has numerous commercial uses—including its use in the extraction of gold from ores—and is an ingredient in alkaline batteries (approximately 0.025% of battery content), mercury vapor lamps, thermostats, and mercury amalgam fillings (in the United States, 50% of a dental filling is made of mercury). Humans can be exposed to mercury compounds via the oral, inhalation, and dermal routes. The primary source of exposure to mercury compounds is attributed to the ingestion of fish and other seafood (marine mammals, crustaceans) that have bioaccumulated mercury compounds. Dental amalgams, which leach mercury, are another source.

Adverse health effects from elemental and inorganic mercury compounds have been observed, particularly in occupational settings. Health consequences commonly observed from exposure to compounds such as elemental mercury vapor and mercuric chloride include tremors, bleeding gums, abdominal pain, vomiting, and kidney damage.

Health effects from organic mercury compounds have also been well-documented, primarily because of the tragic mass poisonings from organic mercurials in locations such as Minamata, Japan, and in Iraq. These mass poisonings were primarily associated with central nervous system toxicity and death. Adverse health effects observed in poisoned individuals and their offspring included ataxia, dysarthria, impaired vision and hearing, and death. Methylmercury is particularly toxic because 95 percent of an ingested dose is absorbed into the bloodstream and can cross the blood-brain and placental barriers, causing adult and fetal neurotoxicity. One of the reasons that offspring are particularly susceptible is that methylmercury can accumulate at 30 percent higher levels in fetal red blood cells than in maternal red blood cells. Besides damaging the brain and peripheral nervous system, methylmercury may also adversely affect the adult and fetal cardiovascular systems.

Research continues to be performed on the potential neurodevelopment effects of ingesting low levels of mercury in seafood. Three particularly important, ongoing studies involve residents of New Zealand and the Seychelles and Faroe Islands who consume significant portions of seafood as part of their normal diets. Analyses performed to date on mother-offspring pairs from the Seychelles identified adverse neurodevelopmental impact in offspring attributable to maternal methylmercury exposure from seafood. Mild developmental effects were also reported among offspring of New Zealand and Faroe Island residents who ingested seafood containing relatively high levels of methylmercury. These studies are particularly pertinent to assessing potential health effects among Native Arctic populations who consume marine mammals (beluga whales, ringed seals) as part of their normal diet. An increased level of mercury has been noted in the Arctic environment since the 1970s, possibly due to anthropogenic sources such as fossil fuel combustion, or possibly from increased natural releases of mercury from geologic sources. It is hypothesized that the cold Arctic climate acts as a sink for mercury; a particularly troublesome prospect for Native Arctic populations who continue to consume mercury-laden mammals and seafood.

Margaret H. Whitaker

Bruce A. Fowler

(see also: Environmental Determinants of Health; Foods and Diets; Heavy Metals; Minamata Disease; Occupational Safety and Health )

Bibliography

Agency for Toxic Substances and Disease Registry (1999). Toxicological Profile for Mercury (Update). Washington, DC: U.S. Department of Health and Human Services.

Arctic Monitoring and Assessment Programme (1999). Arctic Pollution Issues: A State of the Arctic Environment Report. Available at http://www.amap.no.

National Research Council (2000). Toxicological Effects of Methyl Mercury. Washington, DC: Committee on the Toxicological Effects of Mercury. Board on Environmental Studies and Toxicology. Commission on Life Sciences.

Tenenbaum, D. J. (1998). "Northern Overexposure." Environmental Health Perspective 106(2): A64–A69.

U.S. Environmental Protection Agency (1997). Report to Congress on Mercury. Available at http://www.epa.gov/oar/mercury.html.

World Health Organization (1990). Methyl Mercury, Vol. 101. Geneva: International Programme on Chemical Safety, WHO.

—— (1990). Inorganic Mercury, Vol. 118. Geneva: International Programme on Chemical Safety, WHO.

Mercury

Mercury is a metal with chemical similarities to zinc and cadmium. The metal is liquid at room temperature, with a freezing point at –31°C, and it is one of the most volatile metals. It occurs as the element Hg0 and as the mercuric ion Hg++, which has a great affinity for reduced sulfur (sulfide, S=). Most mercury ore deposits consist of the very insoluble mineral cinnabar (HgS), with little droplets of elemental Hg. Mercury also occurs as impurities in many other ore minerals, creating mercury contamination when these minerals are mined or processed. Most common rocks have very low Hg contents, about ten to one hundred parts per billion (ppb) Hg . Elemental mercury is barely soluble in pure water, with only twenty-five ppb Hg dissolving at room temperature, but it is more soluble at higher temperatures. The mercuric ion is very soluble in most ambient waters, but very insoluble in the presence of sulfide. Natural enrichments of mercury occur in and around ore deposits and in geothermal hot spring areas and volcanoes. Bacteria in coastal waters convert inorganic Hg ions back into the elemental state, which then evaporate from the water back into the atmosphere. The physical transport of mercury from ore regions and the vapor transport from geothermal areas and the oceans provide the natural background contamination of mercury.

Mercury is a toxic element that damages the human nervous system and brain. Elemental mercury is less dangerous when it is ingested than when it is inhaled. The use of mercury in felt-making led to widespread elemental mercury poisoning of hatmakers ("mad as a hatter"), which was expressed by tremor, loss of hair and teeth, depression, and occasional death. The organic forms of mercury—methylmercury compounds, CH3Hg+ and (CH3)2Hg—are very bioavailable or are easily taken up by living organisms and rapidly enter cells, and are therefore the most hazardous. Minamata disease was an episode of mercury poisoning of a small coastal community in Japan (1954) through the direct industrial release of methylmercury in the bay. Another infamous episode of mercury contamination occurred in Iraq, where people ate wheat that was treated with a mercury-containing fungicide. The continuous flux of mercury from the atmosphere results in the low level of mercury pollution nationwide. A small fraction of the Hg++ from atmospheric deposition is converted by bacteria into the very dangerous methylmercury form. The methylmercury is then taken up by the lowest life forms and makes its way up the food chain and bioaccumulates in the larger fish. As a result, large predator fish such as bass, tuna, shark, and swordfish have the highest levels of Hg in the methylmercury form. Most states in the United States have advisories for eating only limited amounts of freshwater fish. Limiting intake of mercury-contaminated fish is especially important for pregnant women and young children. The current U.S. legal limit for Hg in fish for consumption is 1 ppm. Limits for Hg in soils vary from state to state but generally range from 10 to 20 ppm, whereas the Environmental Protection Agency's limit for drinking water is 2 ppb Hg. The Occupational Safety and Health Administration limits for Hg in the air in the workplace (for an eight-hour average) are 0.01 mg organic Hg/m3 air.

Modern sources of mercury contamination from human activities are subdivided into the following groups:

1. High-temperature combustion processes such as coal-fired power plants, incineration of solid household waste, medical waste, sewage sludge, and ore smelting.
2. Industrial waste effluents, such as from chlor-alkali plants that use liquid mercury as electrodes.
3. Effluents of wastewater treatment plants.
4. Point sources of specific industries, many of them no longer active today (such as hat making, explosives, mercury lights, herbicides, and plastics).

An overview of modern anthropogenic Hg fluxes into the environment shows that more than 80 percent of mercury is injected into the atmosphere through such combustion processes as coal-fired power plants. The combustion releases mercury as elemental vapor into the atmosphere, where it has an average residence time of about one year before it is oxidized to the mercuric form. The oxidized mercury attaches itself to small dust particles and is removed by wet and dry atmospheric deposition. As a result of this massive injection of Hg into the atmosphere—more than 100 tons of Hg per year in the United States in the late 1990s—the contaminant is distributed all over the globe. Even the polar ice caps show evidence of mercury contamination over the last 150 years, from atmospheric dispersal and deposition from anthropogenic sources. There are almost no places on earth that are not contaminated by anthropogenic mercury.

Mercury contamination is a matter of ongoing concern, and an extensive study was done for the U.S. Congress to summarize the sources, pathways, and sinks of mercury in the outdoor environment. There are several initiatives to limit the anthropogenic flux of Hg from coal-fired power plants, such as switching to mercury-poor coals and scrubbing the stack gases. Limiting or banning the production of mercury-containing materials, including switches, thermometers, thermostats, and manometers, both in the household as well as in the medical profession, would also reduce the mercury recycled back into the atmosphere from garbage incineration.

see also Bioaccumulation; Health, Human; Incineration; Ishimure, Michiko; Medical Waste; Persistent Bioaccumulative Toxic Chemicals (PBTs); Superfund.

internet resource

u.s. environmental protection agency. "mercury study report to congress." available from http://www.epa.gov/oar/mercury.html.

Johan C. Varekamp

The most common exposure to mercury in the home comes when a mercury thermometer is dropped and broken. Children should be removed from the room immediately. DO NOT VACUUM SPILLED MERCURY. Vacuuming will disperse the mercury into the air; inhaling mercury poses high risk. Mercury naturally beads and if it is on a hard surface, it can be scooped up with index cards or a file folder. Seal in a ziplock bag and call the health department or a hospital to arrange safe disposal. Call the health department if mercury has spilled on a carpet or other fabric.

Mercury

melting point: −38.87°C
boiling point: 359.6°C
density: 13.54 g/cm³
most common ions: Hg₂²⁻, Hg²⁺

Mercury is at room temperature a silver-white, volatile liquid metal . It is reputed to have been known in ancient Egypt. Dioscorides, a Greek physician who flourished ca. 60 c.e., recounted the condensation of mercury vapor after the heating of cinnabar, the major ore of mercury. In the modern era mercury is produced via a variation on the procedure used by the ancients: The bright red ore (cinnabar) is now heated in oxygen, with lime, or with iron.

HgS(s) + O₂(g) → SO₂(g) + Hg

HgS(s) + Fe → Hg + FeS

4HgS(s) + 4CaO(s) → 4Hg + 3CaS(s) + CaSO₄(s)

Mercury has three oxidation states: 0, 1+ (mercurous), and 2+ (mercuric). It forms few simple compounds. It does form several simple, water-soluble mercuric compounds: mercuric chloride, HgCl₂; mercuric nitrate, Hg(NO₃)₂; and mercuric acetate, Hg(CH₃COO)₂. The mercurous chloride, Hg₂Cl₂, is insoluble in water. Relatively stable organometallic compounds are formed with aliphatic and organic compounds. Methylmercury (CH₃–Hg⁺) is the major polluting form of mercury. Methylmercury reacts with thiol groups in enzymes.

The mining of mercury has declined in recent decades, as major international concern over the health threat of mercury's extensive pollution of the environment has mounted. Much American freshwater fish is contaminated. The U.S. Environmental Protection Agency estimates 3,000 uses of mercury. Mercury usage is down in the chloroalkali industry, in which mercury is the cathode material used in the electrolysis of sodium chloride solutions, which produce sodium hydroxide and chlorine. An abundance of 500 ppb (0.5 μ g/g) in Earth's crust gives rise to a discharge into the atmosphere of mercury on combustion of fossil fuels and the manufacture of metals and cement.

see also Heavy Metal Toxins; Inorganic Chemistry.

Robert A. Bulman

Bibliography

Magos, L. (1987). "Mercury." In Handbook of the Toxicity of Inorganic Compounds, ed. Hans G. Seiler, Helmut Sigel, and Astrid Sigel. New York: Marcel Dekker.