Mercury (element)

gold ← mercury → thallium
Cd
↑
Hg
↓
Cn
Appearance
silvery


Spectral lines of mercury (UV not seen)
General properties
Name, symbol, number mercury, Hg, 80
Pronunciation /ˈmɜrkjəri/
or /ˈmɜrkəri/ mer-k(y)ə-ree
alternatively /ˈkwɪksɪlvər/
or /haɪˈdrɑrdʒɨrəm/ hye-drar-ji-rəm
Element category transition metal
Group, period, block 12, 6, d
Standard atomic weight 200.59(2)
Electron configuration [Xe] 4f14 5d10 6s2
Electrons per shell 2, 8, 18, 32, 18, 2 (Image)
Physical properties
Phase liquid
Liquid density at m.p. 13.534 g·cm−3
Melting point 234.32 K, -38.83 °C, -37.89 °F
Boiling point 629.88 K, 356.73 °C, 674.11 °F
Critical point 1750 K, 172.00 MPa
Heat of fusion 2.29 kJ·mol−1
Heat of vaporization 59.11 kJ·mol−1
Molar heat capacity 27.983 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 315 350 393 449 523 629
Atomic properties
Oxidation states 4, 2 (mercuric), 1 (mercurous)
(mildly basic oxide)
Electronegativity 2.00 (Pauling scale)
Ionization energies 1st: 1007.1 kJ·mol−1
2nd: 1810 kJ·mol−1
3rd: 3300 kJ·mol−1
Atomic radius 151 pm
Covalent radius 132±5 pm
Van der Waals radius 155 pm
Miscellanea
Crystal structure rhombohedral
Magnetic ordering diamagnetic
Electrical resistivity (25 °C) 961nΩ·m
Thermal conductivity 8.30 W·m−1·K−1
Thermal expansion (25 °C) 60.4 µm·m−1·K−1
Speed of sound (liquid, 20 °C) 1451.4 m·s−1
CAS registry number 7439-97-6
Most stable isotopes
Main article: Isotopes of mercury
iso NA half-life DM DE (MeV) DP
194Hg syn 444 y ε 0.040 194Au
195Hg syn 9.9 h ε 1.510 195Au
196Hg 0.15% 196Hg is stable with 116 neutrons
197Hg syn 64.14 h ε 0.600 197Au
198Hg 9.97% 198Hg is stable with 118 neutrons
199Hg 16.87% 199Hg is stable with 119 neutrons
200Hg 23.1% 200Hg is stable with 120 neutrons
201Hg 13.18% 201Hg is stable with 121 neutrons
202Hg 29.86% 202Hg is stable with 122 neutrons
203Hg syn 46.612 d β− 0.492 203Tl
204Hg 6.87% 204Hg is stable with 124 neutrons
· r

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum (from "hydr-" water and "argyros" silver). A heavy, silvery d-block element, mercury is the only metal that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is bromine, and metals such as caesium, francium, gallium, and rubidium melt just above room temperature. With a freezing point of −38.83 °C and boiling point of 356.73 °C, mercury has one of the narrowest ranges of its liquid state of any metal.[1][2][3]

Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion is mostly obtained by reduction from cinnabar. Cinnabar is highly toxic by ingestion or inhalation of the dust. Mercury poisoning can also result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury), inhalation of mercury vapor, or eating seafood contaminated with mercury.

Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, some electrical switches, and other scientific apparatus, though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of alcohol-filled, galinstan-filled, digital, or thermistor-based instruments. It remains in use in scientific research applications and in amalgam material for dental restoration. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-wave ultraviolet light which then causes the phosphor to fluoresce, making visible light.

Contents

Properties

Physical properties

Mercury is a heavy, silvery-white metal. As compared to other metals, it is a poor conductor of heat, but a fair conductor of electricity.[4] Mercury has an exceptionally low melting temperature for a d-block metal. A complete explanation of this fact requires a deep excursion into quantum physics, but it can be summarized as follows: mercury has a unique electronic configuration where electrons fill up all the available 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d and 6s subshells. As such configuration strongly resists removal of an electron, mercury behaves similarly to noble gas elements, which form weak bonds and thus easily melting solids. The stability of the 6s shell is due to the presence of a filled 4f shell. An f shell poorly screens the nuclear charge that increases the attractive Coulomb interaction of the 6s shell and the nucleus (see lanthanide contraction). The absence of a filled inner f shell is the reason for the somewhat higher melting temperature of cadmium and zinc, although both these metals still melt easily and, in addition, have unusually low boiling points. Metals such as gold have atoms with one less 6s electron than mercury. Those electrons are more easily removed and are shared between the gold atoms forming relatively strong metallic bonds.[2][5] At its freezing point (−38.86 °C), the density of mercury is[6] 13.534 g/cm3.

Chemical properties

Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate, nitrate, and chloride salts. Like silver, mercury reacts with atmospheric hydrogen sulfide. Mercury even reacts with solid sulfur flakes, which are used in mercury spill kits to absorb mercury vapors (spill kits also use activated carbon and powdered zinc).[7]

Amalgams

Mercury dissolves to form amalgams with gold, zinc and many other metals. Because iron is an exception, iron flasks have been traditionally used to trade mercury. Other metals that do not form amalgams with mercury include tantalum, tungsten and platinum. Sodium amalgam is a common reducing agent in organic synthesis.

Mercury readily combines with aluminium to form a mercury-aluminium amalgam when the two pure metals come into contact. Since the amalgam reacts with air to give aluminium oxide, small amounts of mercury corrode aluminium. For this reason, mercury is not allowed aboard an aircraft under most circumstances because of the risk of it forming an amalgam with exposed aluminium parts in the aircraft.[8]

Isotopes

Main article: Isotopes of mercury

There are seven stable isotopes of mercury with 202Hg being the most abundant (29.86%). The longest-lived radioisotopes are 194Hg with a half-life of 444 years, and 203Hg with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lives that are less than a day. 199Hg and 201Hg are the most often studied NMR-active nuclei, having spins of 1⁄2 and 3⁄2 respectively.[4]

History

Mercury was found in Egyptian tombs that date from 1500 BC.[9]

In China and Tibet, mercury use was thought to prolong life, heal fractures, and maintain generally good health, although it is now known that exposure to mercury leads to serious adverse health effects.[10] One of China's emperors, Qín Shǐ Huáng Dì — allegedly buried in a tomb that contained rivers of flowing mercury on a model of the land he ruled, representative of the rivers of China — was killed by drinking a mercury and powdered jade mixture formulated by Qin alchemists (causing liver failure, mercury poisoning, and brain death) who intended to give him eternal life.[11][12]

The ancient Greeks used mercury in ointments; the ancient Egyptians and the Romans used it in cosmetics which sometimes deformed the face. In Lamanai, once a major city of the Maya civilization, a pool of mercury was found under a marker in a Mesoamerican ballcourt.[13][14] By 500 BC mercury was used to make amalgams (Medieval Latin amalgama, "alloy of mercury") with other metals.[15]

Alchemists thought of mercury as the First Matter from which all metals were formed. They believed that different metals could be produced by varying the quality and quantity of sulfur contained within the mercury. The purest of these was gold, and mercury was called for in attempts at the transmutation of base (or impure) metals into gold, which was the goal of many alchemists.[16]

Hg is the modern chemical symbol for mercury. It comes from hydrargyrum, a Latinized form of the Greek word Ύδραργυρος (hydrargyros), which is a compound word meaning "water-silver" (hydr- = water, argyros = silver) — since it is liquid like water and shiny like silver. The element was named after the Roman god Mercury, known for speed and mobility. It is associated with the planet Mercury; the astrological symbol for the planet is also one of the alchemical symbols for the metal; the Indian word for alchemy is Rasavātam which means "the way of mercury".[17] Mercury is the only metal for which the alchemical planetary name became the common name.[16]

The mines in Almadén (Spain), Monte Amiata (Italy), and Idrija (now Slovenia) dominated the mercury production from the opening of the mine in Almadén 2500 years ago until new deposits were found at the end of the 19th century.[18]

Occurrence

Mercury is an extremely rare element in the Earth's crust, having an average crustal abundance by mass of only 0.08 parts per million (ppm).[19] However, because it does not blend geochemically with those elements that constitute the majority of the crustal mass, mercury ores can be extraordinarily concentrated considering the element's abundance in ordinary rock. The richest mercury ores contain up to 2.5% mercury by mass, and even the leanest concentrated deposits are at least 0.1% mercury (12,000 times average crustal abundance). It is found either as a native metal (rare) or in cinnabar, corderoite, livingstonite and other minerals, with cinnabar (HgS) being the most common ore.[20] Mercury ores usually occur in very young orogenic belts where rock of high density are forced to the crust of the Earth, often in hot springs or other volcanic regions.[21]

Beginning in 1558, with the invention of the patio process to extract silver from ore using mercury, mercury became an essential resource in the economy of Spain and its American colonies. Mercury was used to extract silver from the lucrative mines in New Spain and Peru. Initially, the Spanish Crown's mines in Almaden in Southern Spain supplied all the mercury for the colonies.[22] Mercury deposits were discovered in the New World, and more than 100,000 tons of mercury were mined from the region of Huancavelica, Peru, over the course of three centuries following the discovery of deposits there in 1563. The patio process and later pan amalgamation process continued to create great demand for mercury to treat silver ores until the late 19th century.[23]

Former mines in Italy, the United States and Mexico which once produced a large proportion of the world supply have now been completely mined out or, in the case of Slovenia (Idrija) and Spain (Almadén), shut down due to the fall of the price of mercury. Nevada's McDermitt Mine, the last mercury mine in the United States, closed in 1992. The price of mercury has been highly volatile over the years and in 2006 was $650 per 76-pound (34.46 kg) flask.[24]

Mercury is extracted by heating cinnabar in a current of air and condensing the vapor. The equation for this extraction is

HgS + O2 → Hg + SO2

In 2005, China was the top producer of mercury with almost two-thirds global share followed by Kyrgyzstan.[25] Several other countries are believed to have unrecorded production of mercury from copper electrowinning processes and by recovery from effluents.

Because of the high toxicity of mercury, both the mining of cinnabar and refining for mercury are hazardous and historic causes of mercury poisoning.[26] In China, prison labor was used by a private mining company as recently as the 1950s to create new cinnabar mercury mines. Thousands of prisoners were used by the Luo Xi mining company to establish new tunnels.[27] In addition, worker health in functioning mines is at high risk.

The European Union directive calling for compact fluorescent bulbs to be made mandatory by 2012 has encouraged China to re-open deadly cinnabar mines to obtain the mercury required for CFL bulb manufacture. As a result, environmental dangers have been a concern, particularly in the southern cities of Foshan and Guangzhou, and in the Guizhou province in the south west.[27]

Abandoned mercury mine processing sites often contain very hazardous waste piles of roasted cinnabar calcines. Water run-off from such sites is a recognized source of ecological damage. Former mercury mines may be suited for constructive re-use. For example, in 1976 Santa Clara County, California purchased the historic Almaden Quicksilver Mine and created a county park on the site, after conducting extensive safety and environmental analysis of the property.[28]

Chemistry

Mercury exists in two main oxidation states, I and II. Higher oxidation states are unimportant, but have been detected, e.g., mercury(IV) fluoride (HgF4) but only under extraordinary conditions.[29]

Compounds of mercury(I)

Different from its lighter neighbors, cadmium and zinc, mercury forms simple stable compounds with metal-metal bonds. The mercury(I) compounds are diamagnetic and feature the dimeric cation, Hg2+
2. Stable derivatives include the chloride and nitrate. Treatment of Hg(I) compounds complexation with strong ligands such as sulfide, cyanide, etc. induces disproportionation to Hg2+ and elemental mercury.[30] Mercury(I) chloride, a colorless solid also known as calomel, is really the compound with the formula Hg2Cl2, with the connectivity Cl-Hg-Hg-Cl. It is a standard in electrochemistry. It reacts with chlorine to give mercuric chloride, which resists further oxidation.

Indicative of its tendency to bond to itself, mercury forms mercury polycations, which consist of linear chains of mercury centers, capped with a positive charge. One example is Hg32+(AsF6–)2.[31]

Compounds of mercury(II)

Mercury(II) is the most common oxidation state and is the main one in nature as well. All four mercuric halides are known. The form tetrahedral complexes with other ligands but the halides adopt linear coordination geometry, somewhat like Ag+ does. Best known is mercury(II) chloride, an easily sublimating white solid. HgCl2 forms coordination complexes that are typically tetrahedral, e.g. HgCl42–.

Mercury(II) oxide, the main oxide of mercury, arises when the metal is exposed to air for long periods at elevated temperatures. It reverts to the elements upon heating near 400 °C, as was demonstrated by Priestly in an early synthesis of pure oxygen.[7] Hydroxides of mercury are poorly characterized, as they are for its neighbors gold and silver.

Being a soft metal, mercury forms very stable derivatives with the heavier chalcogens. Preeminent is mercury(II) sulfide, HgS, which occurs in nature as the ore cinnabar and is the brilliant pigment vermillion. Like ZnS, HgS crystallizes in two forms, the reddish cubic form and the black zinc blende form.[4] Mercury(II) selenide (HgSe) and mercury(II) telluride (HgTe) are also known, these as well as various derivatives, e.g. mercury cadmium telluride and mercury zinc telluride being semiconductors useful as infrared detector materials.[32]

Mercury(II) salts form a variety of complex derivatives with ammonia. These include Millon's base (Hg2N+), the one-dimensional polymer (salts of HgNH2+)n), and "fusible white precipitate" or [Hg(NH3)2]Cl2. Known as Nessler's reagent, potassium tetraiodomercurate(II) (HgI42–) is still occasionally used to test for ammonia owing to its tendency to form the deeply colored iodide salt of Millon's base.

Mercury fulminate, (a detonator widely used in explosives);[4]

Compounds of mercury(IV)

Mercury(IV) is the rarest oxidation state of mercury which is known to exist. The only known mercury(IV) compound is mercury(IV) fluoride.

Organomercury compounds

Main article: Organomercury compound

Organic mercury compounds are historically important but are of little industrial value in the western world. Mercury(II) salts are a rare examples of simple metal complexes that react directly with aromatic rings. Organomercury compounds are always divalent and usually two-coordinate and linear geometry. Unlike organocadmium and organozinc compounds, organomercury compounds do not react with water. They usually have the formula HgR2, which are often volatile, or HgRX, which are often solids, where R is aryl or alkyl and X is usually halide or acetate. Methylmercury, a generic term for compounds with the formula CH3HgX is a dangerous family of compounds that is found in some a polluted water.[33] They arise by a process known as biomethylation.

Applications

Mercury is used primarily for the manufacture of industrial chemicals or for electrical and electronic applications. It is used in some thermometers, especially ones which are used to measure high temperatures. A still increasing amount is used as gaseous mercury in fluorescent lamps, while most of the other applications are slowly phased out due to health and safety regulations and is in some applications replaced with less toxic but considerably more expensive Galinstan alloy.

Medicine

See also: Amalgam (dentistry)

Mercury and its compounds have been used in medicine, although they are much less common today than they once were, now that the toxic effects of mercury and its compounds are more widely understood. The element mercury is an ingredient in dental amalgams. Thiomersal (called Thimerosal in the United States) is an organic compound used as a preservative in vaccines, though this use is in decline.[34] Another mercury compound Merbromin (Mercurochrome) is a topical antiseptic used for minor cuts and scrapes is still in use in some countries.

Since the 1930s some vaccines have contained the preservative thiomersal, which is metabolized or degraded to ethyl mercury. Although it was widely speculated that this mercury-based preservative can cause or trigger autism in children, scientific studies showed no evidence supporting any such link.[35] Nevertheless thiomersal has been removed from or reduced to trace amounts in all U.S. vaccines recommended for children 6 years of age and under, with the exception of inactivated influenza vaccine.[36]

Mercury in the form of one of its common ores, cinnabar, is used in various traditional medicines, especially in traditional Chinese medicine. Review of its safety has found cinnabar can lead to significant mercury intoxication when heated, consumed in overdose or taken long term, and can have adverse effects at therapeutic doses, though this is typically reversible at therapeutic doses. Although this form of mercury appears less toxic than others, its use in traditional Chinese medicine has not yet been justified as the therapeutic basis for the use of cinnabar is not clear.[37]

Today, the use of mercury in medicine has greatly declined in all respects, especially in developed countries. Thermometers and sphygmomanometers containing mercury were invented in the early 18th and late 19th centuries, respectively. In the early 21st century, their use is declining and has been banned in some countries, states and medical institutions. In 2002, the U.S. Senate passed legislation to phase out the sale of non-prescription mercury thermometers. In 2003, Washington and Maine became the first states to ban mercury blood pressure devices.[38] Mercury compounds are found in some over-the-counter drugs, including topical antiseptics, stimulant laxatives, diaper-rash ointment, eye drops, and nasal sprays. The FDA has "inadequate data to establish general recognition of the safety and effectiveness", of the mercury ingredients in these products.[39] Mercury is still used in some diuretics, although substitutes now exist for most therapeutic uses.

Production of chlorine and caustic soda

Chlorine is produced from sodium chloride (common salt, NaCl) using electrolysis to separate the metallic sodium from the chlorine gas. Usually the salt is dissolved in water to produce a brine. By-products of any such chloralkali process are hydrogen (H2) and sodium hydroxide (NaOH), which is commonly called caustic soda or lye. By far the largest use of mercury[40][41] in the late 20th century was in the mercury cell process (also called the Castner-Kellner process) where metallic sodium is formed as an amalgam at a cathode made from mercury; this sodium is then reacted with water to produce sodium hydroxide.[42] Many of the industrial mercury releases of the 20th century came from this process, although modern plants claimed to be safe in this regard.[41] After about 1985, all new chloralkali production facilities that were built in the United States used either membrane cell or diaphragm cell technologies to produce chlorine.

Laboratory uses

Some medical thermometers, especially those for high temperatures, are filled with mercury; however, they are gradually disappearing. In the United States, non-prescription sale of mercury fever thermometers has been banned since 2003.[43]

Mercury is also found in liquid mirror telescopes. The mirror is formed by rotating liquid mercury on a disk, the parabolic form of the liquid thus formed reflecting and focusing incident light. Such telescopes are cheaper than conventional large mirror telescopes by up to a factor of 100, but the mirror cannot be tilted and always points straight up.[44][45][46]

Liquid mercury is a part of popular secondary reference electrode (called the calomel electrode) in electrochemistry as an alternative to the standard hydrogen electrode. The calomel electrode is used to work out the electrode potential of half cells.[47] Last, but not least, the triple point of mercury, −38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale (ITS-90).[4]

Niche uses

Gaseous mercury is used in mercury-vapor lamps and some "neon sign" type advertising signs and fluorescent lamps. Those low-pressure lamps emit very spectrally narrow lines, which are traditionally used in optical spectroscopy for calibration of spectral position. Commercial calibration lamps are sold for this purpose; however simply reflecting some of the fluorescent-lamp ceiling light into a spectrometer is a common calibration practice.[48] Gaseous mercury is also found in some electron tubes, including ignitrons, thyratrons, and mercury arc rectifiers.[49] It is also used in specialist medical care lamps for skin tanning and disinfection (see pictures).[50] Gaseous mercury is added to cold cathode argon-filled lamps to increase the ionization and electrical conductivity. An argon filled lamp without mercury will have dull spots and will fail to light correctly. Lighting containing mercury can be bombarded/oven pumped only once. When added to neon filled tubes the light produced will be inconsistent red/blue spots until the initial burning-in process is completed; eventually it will light a consistent dull off-blue color.[51]

Cosmetics

Mercury, as thiomersal, is widely used in the manufacture of mascara. In 2008, Minnesota became the first state in the US to ban intentionally added mercury in cosmetics, giving it a tougher standard than the federal government.[52]

A study in geometric mean urine mercury concentration identified a previously unrecognized source of exposure (skin care products) to inorganic mercury among New York City residents. Population-based biomonitoring also showed that mercury concentration levels are higher in consumers of seafood and fish meals.[53]

Historic uses

Many historic applications made use of the peculiar physical properties of mercury, especially as a dense liquid and a liquid metal:

Others applications made use of the chemical properties of mercury:

Historic medicinal uses

Mercury(I) chloride (also known as calomel or mercurous chloride) has been used in traditional medicine as a diuretic, topical disinfectant, and laxative. Mercury(II) chloride (also known as mercuric chloride or corrosive sublimate) was once used to treat syphilis (along with other mercury compounds), although it is so toxic that sometimes the symptoms of its toxicity were confused with those of the syphilis it was believed to treat.[72] It is also used as a disinfectant. Blue mass, a pill or syrup in which mercury is the main ingredient, was prescribed throughout the 19th century for numerous conditions including constipation, depression, child-bearing and toothaches.[73] In the early 20th century, mercury was administered to children yearly as a laxative and dewormer, and it was used in teething powders for infants. The mercury-containing organohalide merbromin (sometimes sold as Mercurochrome) is still widely used but has been banned in some countries such as the U.S.[74]

Toxicity and safety

See also: Mercury poisoning

Mercury and most of its compounds are extremely toxic and must be handled with care; in cases of spills involving mercury (such as from certain thermometers or fluorescent light bulbs), specific cleaning procedures are used to avoid exposure and contain the spill.[75] Protocols call for physically merging smaller droplets on hard surfaces, combining them into a single larger pool for easier removal with an eyedropper, or for gently pushing the spill into a disposable container. Vacuum cleaners and brooms cause greater dispersal of the mercury and should not be used. Afterwards, fine sulfur, zinc, or some other powder that readily forms an amalgam (alloy) with mercury at ordinary temperatures is sprinkled over the area before itself being collected and properly disposed of. Cleaning porous surfaces and clothing is not effective at removing all traces of mercury and it is therefore advised to discard these kinds of items should they be exposed to a mercury spill.

Mercury can be inhaled and absorbed through the skin and mucous membranes, so containers of mercury are securely sealed to avoid spills and evaporation. Heating of mercury, or of compounds of mercury that may decompose when heated, is always carried out with adequate ventilation in order to avoid exposure to mercury vapor. The most toxic forms of mercury are its organic compounds, such as dimethylmercury and methylmercury. However, inorganic compounds, such as cinnabar are also highly toxic by ingestion or inhalation.[76] Mercury can cause both chronic and acute poisoning.

Releases in the environment

Preindustrial deposition rates of mercury from the atmosphere may be about 4 ng /(1 L of ice deposit). Although that can be considered a natural level of exposure, regional or global sources have significant effects. Volcanic eruptions can increase the atmospheric source by 4–6 times.[77]

Natural sources, such as volcanoes, are responsible for approximately half of atmospheric mercury emissions. The human-generated half can be divided into the following estimated percentages:[78][79][80]

The above percentages are estimates of the global human-caused mercury emissions in 2000, excluding biomass burning, an important source in some regions.[78]

Current atmospheric mercury contamination in outdoor urban air is (0.01–0.02 µg/m3) indoor concentrations are significantly elevated over outdoor concentrations, in the range 0.0065–0.523 µg/m3 (average 0.069 µg/m3).[82]

Mercury also enters into the environment through the improper disposal (e.g., land filling, incineration) of certain products. Products containing mercury include: auto parts, batteries, fluorescent bulbs, medical products, thermometers, and thermostats.[83] Due to health concerns (see below), toxics use reduction efforts are cutting back or eliminating mercury in such products. For example, most thermometers now use pigmented alcohol instead of mercury, and galinstan alloy thermometers are also an option. Mercury thermometers are still occasionally used in the medical field because they are more accurate than alcohol thermometers, though both are commonly being replaced by electronic thermometers and less commonly by galinstan thermometers. Mercury thermometers are still widely used for certain scientific applications because of their greater accuracy and working range.

The United States Clean Air Act, passed in 1990, put mercury on a list of toxic pollutants that need to be controlled to the greatest possible extent. Thus, industries that release high concentrations of mercury into the environment agreed to install maximum achievable control technologies (MACT). In March 2005 EPA rule[84] added power plants to the list of sources that should be controlled and a national cap and trade rule was issued. States were given until November 2006 to impose stricter controls, and several States are doing so. The rule was being subjected to legal challenges from several States in 2005 and decision was made in 2008. The Clean Air Mercury Rule was struck down by a Federal Appeals Court on February 8, 2008. The rule was deemed not sufficient to protect the health of persons living near coal-fired power plants. The court opinion cited the negative impact on human health from coal-fired power plants' mercury emissions documented in the EPA Study Report to Congress of 1998.[85]

The EPA announced new rules for coal-fired power plants on December 22, 2011.[86] Cement kilns that burn hazardous waste are held to a looser standard than are standard hazardous waste incinerators in the United States, and as a result are a disproportionate source of mercury pollution.[87]

Historically, one of the largest releases was from the Colex plant, a lithium-isotope separation plant at Oak Ridge. The plant operated in the 1950s and 1960s. Records are incomplete and unclear, but government commissions have estimated that some two million pounds of mercury are unaccounted for.[88]

A serious industrial disasters was the dumping of mercury compounds into Minamata Bay, Japan. It is estimated that over 3,000 people suffered various deformities, severe mercury poisoning symptoms or death from what became known as Minamata disease.[89]

Occupational exposure

Due to the health effects of mercury exposure, industrial and commercial uses are regulated in many countries. The World Health Organization, OSHA, and NIOSH all treat mercury as an occupational hazard, and have established specific occupational exposure limits. Environmental releases and disposal of mercury are regulated in the U.S. primarily by the United States Environmental Protection Agency.

Case control studies have shown effects such as tremors, impaired cognitive skills, and sleep disturbance in workers with chronic exposure to mercury vapor even at low concentrations in the range 0.7–42 μg/m3.[90][91] A study has shown that acute exposure (4 – 8 hours) to calculated elemental mercury levels of 1.1 to 44 mg/m3 resulted in chest pain, dyspnea, cough, hemoptysis, impairment of pulmonary function, and evidence of interstitial pneumonitis.[92] Acute exposure to mercury vapor has been shown to result in profound central nervous system effects, including psychotic reactions characterized by delirium, hallucinations, and suicidal tendency. Occupational exposure has resulted in broad-ranging functional disturbance, including erethism, irritability, excitability, excessive shyness, and insomnia. With continuing exposure, a fine tremor develops and may escalate to violent muscular spasms. Tremor initially involves the hands and later spreads to the eyelids, lips, and tongue. Long-term, low-level exposure has been associated with more subtle symptoms of erethism, including fatigue, irritability, loss of memory, vivid dreams and depression.[93][94]

Treatment

Research on the treatment of mercury poisoning is limited. Currently available drugs for acute mercurial poisoning include chelators N-acetyl-D, L-penicillamine (NAP), British Anti-Lewisite (BAL), 2,3-dimercapto-1-propanesulfonic acid (DMPS), and dimercaptosuccinic acid (DMSA). In one small study including 11 construction workers exposed to elemental mercury, patients were treated with DMSA and NAP.[95] Chelation therapy with both drugs resulted in the mobilization of a small fraction of the total estimated body mercury. DMSA was able to increase the excretion of mercury to a greater extent than NAP.[96]

Fish

Main article: Mercury in fish

Fish and shellfish have a natural tendency to concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury. Species of fish that are high on the food chain, such as shark, swordfish, king mackerel, albacore tuna, and tilefish contain higher concentrations of mercury than others. As mercury and methylmercury are fat soluble, they primarily accumulate in the viscera, although they are also found throughout the muscle tissue.[97] When this fish is consumed by a predator, the mercury level is accumulated. Since fish are less efficient at depurating than accumulating methylmercury, fish-tissue concentrations increase over time. Thus species that are high on the food chain amass body burdens of mercury that can be ten times higher than the species they consume. This process is called biomagnification. Mercury poisoning happened this way in Minamata, Japan, now called Minamata disease.

Regulations

In the United States, the Environmental Protection Agency is charged with regulating and managing mercury contamination. Several laws give the EPA this authority, including the Clean Air Act, the Clean Water Act, the Resource Conservation and Recovery Act, and the Safe Drinking Water Act. Additionally, the Mercury-Containing and Rechargeable Battery Management Act, passed in 1996, phases out the use of mercury in batteries, and provides for the efficient and cost-effective disposal of many types of used batteries.[98] North America contributed approximately 11% of the total global anthropogenic mercury emissions in 1995.[99]

In the European Union, the directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (see RoHS) bans mercury from certain electrical and electronic products, and limits the amount of mercury in other products to less than 1000 ppm.[100] There are restrictions for mercury concentration in packaging (the limit is 100 ppm for sum of mercury, lead, hexavalent chromium and cadmium) and batteries (the limit is 5 ppm).[101] In July 2007, the European Union also banned mercury in non-electrical measuring devices, such as thermometers and barometers. The ban applies to new devices only, and contains exemptions for the health care sector and a two-year grace period for manufacturers of barometers. [102]

Norway enacted a total ban on the use of mercury in the manufacturing and import/export of mercury products, effective January 1, 2008.[103] In 2002, several lakes in Norway were found to have a poor state of mercury pollution, with an excess of 1 mg/g of mercury in their sediment.[104]

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