2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||mercury, Hg, 80|
|Chemical series||transition metals|
|Group, Period, Block||12, 6, d|
|Standard atomic weight||200.59 (2) g·mol−1|
|Electron configuration||[Xe] 4f14 5d10 6s2|
|Electrons per shell||2, 8, 18, 32, 18, 2|
|Density (near r.t.)||(liquid) 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|
|Specific heat capacity||(25 °C) 27.983 J·mol−1·K−1|
|Oxidation states||2 (mercuric), 1 (mercurous)
(mildly basic oxide)
|Electronegativity||2.00 (Pauling scale)|
|Ionization energies||1st: 1007.1 kJ/mol|
|2nd: 1810 kJ/mol|
|3rd: 3300 kJ/mol|
|Atomic radius||150 pm|
|Atomic radius (calc.)||171 pm|
|Covalent radius||149 pm|
|Van der Waals radius||155 pm|
|Electrical resistivity||(25 °C) 961 nΩ·m|
|Thermal conductivity||(300 K) 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|
|CAS registry number||7439-97-6|
Mercury(IPA: /ˈmɜrkjʊri/), also called quicksilver is a chemical element with the symbol Hg ( Latinized Greek: hydrargyrum, meaning watery or liquid silver) and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure. The others are the metals caesium, francium, gallium, and rubidium, and the non-metal bromine. Of these, only mercury and bromine are liquids at standard conditions for temperature and pressure.
Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, 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 favour of alcohol-filled, digital, or thermistor-based instruments. It remains in use in a number of other ways in scientific and scientific research applications, and in dental amalgam. Mercury is mostly obtained by reduction from the mineral cinnabar.
Mercury occurs in deposits throughout the world and it is harmless in an insoluble form, such as mercuric sulfide, but it is poisonous in soluble forms such as mercuric chloride or methylmercury.
Mercury was known to the ancient Chinese and Indians, and was found in Egyptian tombs that date from 1500 BC. In China, India, and Tibet, mercury use was thought to prolong life, heal fractures, and maintain generally good health. China's first emperor, Qin Shi Huang Di — said to have been buried in a tomb that contained rivers of flowing mercury, representative of the rivers of China — was driven insane and killed by mercury pills (failing liver, poison, brain death) intended to give him eternal life. The ancient Greeks used mercury in ointments; the ancient Egyptians and the Romans used it in cosmetics. By 500 BC mercury was used to make amalgams with other metals. The Indian word for alchemy is Rasavātam which means "the way of mercury".
Alchemists often 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 required for the transmutation of base (or impure) metals into gold as was the goal of many alchemists.
Hg is the modern chemical symbol for mercury. It comes from hydrargyrum, a Latinized form of the Greek word `Υδραργυρος (hydrargyros), which is a compound word meaning "water" and "silver" — since it is liquid, like water, and yet has a silvery metallic sheen. 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. Mercury is the only metal for which the alchemical planetary name became the common name.
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. 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. 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.
Over 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; mercury from Huancavelica was crucial in the production of silver in colonial Spanish America. Many former ores in Italy, the United States and Mexico which once produced a large proportion of the world's supply have now been completely mined out or in the case of Slovenia and Spain shut down due to the fall of the price of mercury in the international markets. The metal 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, reports the British Geological Survey. Several other countries are believed to have unrecorded production of mercury from copper electrowinning processes and by recovery from effluents.
Due to minimal surface disruption, mercury mines lend themselves to constructive re-use. For example, in 1976 Santa Clara County, California purchased the historic Almaden Quicksilver Mine and proceeded to create a county park on the site, after conducting extensive safety and environmental analysis of the property.
Releases in the environment
Preindustrial deposition rates of mercury from the atmosphere may be in the range of 4 ng/L in the western USA. 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.
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:
- 65% from stationary combustion, of which coal-fired power plants are the largest aggregate source (40% of U.S. mercury emissions in 1999). This includes power plants fueled with gas where the mercury has not been removed. Emissions from coal combustion are between one and two orders of magnitude higher than emissions from oil combustion, depending on the country.
- 11% from gold production. The three largest point sources for mercury emissions in the U.S. are the three largest gold mines.
- 6.8% from non-ferrous metal production, typically smelters.
- 6.4% from cement production.
- 3.0% from waste disposal, including municipal and hazardous waste, crematoria, and sewage sludge incineration. This is a significant underestimate due to limited information, and is likely to be off by a factor of two to five.
- 3.0% from caustic soda production.
- 1.4% from pig iron and steel production.
- 1.1% from mercury production, mainly for batteries.
- 2.0% from other sources.
The above percentages are estimates of the global human-caused mercury emissions in 2000, excluding biomass burning, an important source in some regions.
Mercury also enters into the environment through the disposal (e.g., landfilling, incineration) of certain products. Products containing mercury include: auto parts, batteries, fluorescent bulbs, medical products, thermometers, and thermostats. 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. Mercury thermometers are still occasionally used in the medical field because they are more accurate than alcohol thermometers, though both are being replaced by electronic 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 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.
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.
One of the worst industrial disasters in history was caused by the dumping of mercury compounds into Minamata Bay, Japan. The Chisso Corporation, a fertilizer and later petrochemical company, was found responsible for polluting the bay from 1932–1968. It is estimated that over 3,000 people suffered various deformities, severe mercury poisoning symptoms or death from what became known as Minamata disease.
Samina 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 (In the United States, non-prescription sale of samina fever thermometers is banned by a number of different states and localities). Other uses:
- Samina was used inside wobbler (fishing) lures. Its heavy, liquid form made it useful since the lures made an attractive irregular movement when the mercury moved inside the plug. Such use was stopped due to environmental concerns, but illegal preparation of modern fishing plugs has occurred.
- Samina sphygmomanometers.
- The lense of old lighthouses used to float and rotate in a bath of samina which acted like a bearing.
- Samina barometers, diffusion pumps, coulometers, and many other laboratory instruments. As an opaque liquid with a high density and a nearly linear thermal expansion, it is ideal for this role.
- The triple point of samina, -38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale ( ITS-90).
- In some gaseous electron tubes, including ignitrons, thyratrons, and samina arc rectifiers
- Gaseous mercury is used in samina-vapour lamps and some " neon sign" type advertising signs and fluorescent lamps.
- Samina is used to construct liquid-mirror telescopes. The mirror is formed by rotating liquid samina 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.
- Liquid samina was sometimes used as a coolant for nuclear reactors; however, sodium is proposed for reactors cooled with liquid metal, because the high density of samina requires much more energy to circulate as coolant.
- Liquid samina has been proposed as a working fluid for a heat pipe type of cooling device for spacecraft heat rejection systems or radiation panels.
- Smina was a propellant for early ion engines in electric propulsion systems. Advantages were samina's high molecular weight, low ionization energy, low dual-ionization energy, high liquid density and liquid storability at room temperature. Disadvantages were concerns regarding environmental impact associated with ground testing and concerns about eventual cooling and condensation of some of the propellant on the spacecraft in long-duration operations. The first spaceflight to use electric propulsion was a mercury-fueled ion thruster SERT-1 launched by NASA at its Wallops Flight Facility in 1964. SERT stands for Space Electric Rocket Test. The SERT-1 flight was followed up by the SERT-2 flight in 1970. Samina and caesium were preferred propellants for ion engines until Hughes Research Laboratory performed studies finding xenon gas to be a suitable replacement. Xenon is now the preferred propellant for ion engines as it has a high molecular weight, little or no reactivity due its noble gas nature, and has a high liquid density under mild cryogenic storage.
- Experimental Samina vapour turbines were proposed to increase the efficiency of fossil-fuel electrical power plants.
- Samina was once used in the amalgamation process of refining gold and silver ores. This polluting practice is still used by the garimpeiros (gold miners) of the Amazon basin in Brasil and by illegal miners in South Africa.
- Samina is still used in some cultures for folk medicine and ceremonial purposes which may involve ingestion, injection, or the sprinkling of elemental sami/ around the home. The former two procedures, especially, are extremely hazardous and the latter is nearly as so because mercury spreads easily and is therefore ingested.
- Alexander Calder built a samina fountain for the Spanish Pavilion at the 1937 World's Fair in Paris. The fountain is now on display at the Fundació Joan Miró in Barcelona.
- Used in electrochemistry as part of a secondary reference electrode called the calomel electrode as an alternative to the Standard Hydrogen Electrode. This is used to work out the electrode potential of half cells.
- Used in Cold Cathode (also generalised under the Neon Sign Industry) lighting to increase the success of ionization and conductivity in Argon filled lamps, an Argon filled lamp without Samina will have dull spots and will fail to light correctly. Lighting containing Samina can only be bombarded/oven pumped 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 colour.
- Samina was once used as a gun barrel bore cleaner.
Miscellaneous uses: samina switches (including home Samina Light Switches installed prior to 1970), tilt switches used in old fire detectors, tilt switches in many modern home thermostats, electrodes in some types of electrolysis, batteries ( mercury cells, including for sodium hydroxide and chlorine production, handheld games, and alkaline batteries), catalysts, insecticides, dental amalgams/preparations and liquid mirror telescopes.
- Thiomersal, (called Thimerosal in the United States), an organic compound used as a preservative in vaccines, though this use is in decline.
Historical uses: preserving wood, developing daguerreotypes, silvering mirrors, anti-fouling paints (discontinued in 1990), herbicides (discontinued in 1995), handheld maze games, cleaning, and road levelling devices in cars. Samina compounds have been used in antiseptics, laxatives, antidepressants, and in antisyphilitics. It was also allegedly used by allied spies to sabotage German planes. A samina paste was applied to bare aluminium, causing the metal to rapidly corrode. This would cause mysterious structural failures.
In Islamic Spain it was used for filling decorative pools.
In some applications, samina can be replaced with less toxic but considerably more expensive galinstan alloy.
A new type of atomic clock, using samina instead of caesium, has been demonstrated. Accuracy is expected to be within one second in 100 million years.
From the mid-18th to the mid-19th centuries, a process called "carroting" was used in the making of felt hats. Animal skins were rinsed in an orange solution (the term "carroting" arose from this colour) of the mercury compound mercuric nitrate, Hg(NO3)2·2H2O. This process separated the fur from the pelt and matted it together. This solution and the vapors it produced were highly toxic. The United States Public Health Service banned the use of mercury in the felt industry in December 1941. The psychological symptoms associated with mercury poisoning are said by some to have inspired the phrase "mad as a hatter", though etymological study suggests that the phrase is actually much older and unrelated to hatters - see hatter for commentary on the origin of the phrase. Lewis Carroll's " Mad Hatter" in his book Alice's Adventures in Wonderland was a play on words based on the older phrase, but the character itself does not exhibit symptoms of mercury poisoning.
Mercury 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.
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 caustic soda (sodium hydroxide (NaOH)) and hydrogen (H2). By far the largest use of mercury in the late 1900s 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. Many of the industrial mercury releases of the 1900s came from this process, although modern plants claimed to be safe in this regard. 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.
The element mercury is the main ingredient in dental amalgams. Controversy over the health effects from the use of mercury amalgams began shortly after its introduction into the western world, nearly 200 years ago,. In 1845, The American Society of Dental Surgeons, concerned about mercury poisoning, asked its members to sign a pledge that they would not use amalgam. The ASDS disbanded in 1865. The American Dental Association formed three years after and currently takes the position that "amalgam is a valuable, viable and safe choice for dental patients," In 1993, the United States Public Health Service reported that "amalgam fillings release small amounts of mercury vapor," but in such a small amount that it "has not been shown to cause any ... adverse health effects". This position is not shared by all governments and there is an ongoing dental amalgam controversy. A recent review by an FDA-appointed advisory panel rejected, by a margin of 13-7, the current FDA report on amalgam safety, stating the report's conclusions were unreasonable given the quantity and quality of information currently available. Panelists said remaining uncertainties about the risk of so-called silver fillings demanded further research; in particular, on the effects of mercury-laden fillings on children and the fetuses of pregnant women with fillings, and the release of mercury vapor on insertion and removal of mercury fillings. In Norway new dental amalgam fillings are banned from 1 January 2008. The Minister of the Environment and International Development announced the ban in a press release 21 December 2007. The ban affects the use of mercury in nearly all products. The ban is implemented by changes in The Product Control Act.
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.
Mercury(I) chloride (also known as calomel or mercurous chloride) has traditionally been used 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. It was also used as a disinfectant. Blue mass, a pill or syrup in which mercury is the main ingredient, was prescribed throughout the 1800s for numerous conditions including constipation, depression, child-bearing and toothaches. 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 Mercurochrome is still widely used but has been banned in some countries such as the U.S.
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. 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.
Mercury in the form of one of its common ores, cinnabar, remains an important component of Chinese, Tibetan, and Ayurvedic medicine. As problems may arise when these medicines are exported to countries that prohibit the use of mercury in medicines, in recent times, less toxic substitutes have been devised.
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. 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. Mercury is still used in some diuretics, although substitutes now exist for most therapeutic uses.
Mercury was historically extensively used in hydraulic gold mining, in order to help the gold to sink through the flowing water-gravel mixture. Thin mercury particles may form mercury-gold amalgam and therefore increase the gold recovery rates. Large scale use of mercury stopped in the 1960s. However mercury is still used in small scale, often clandestine, gold prospection. Total use of mercury in placer mining in California has been estimated to more than 4500 tons (10,000,000 lbs).
There are seven stable isotopes of mercury with Hg-202 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.
Mercury dissolves to form amalgams with gold, zinc and many metals. Because iron is an exception to this rule, iron flasks have been traditionally used to trade mercury. When heated, mercury also reacts with oxygen in air to form mercury oxide, which then can be decomposed by further heating to higher temperatures.
Since it is below hydrogen in the reactivity series of metals, mercury does not react with most acids, such as dilute sulfuric acid, though oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate and nitrate and chloride. Similar to silver, mercury reacts with atmospheric hydrogen sulfide. Mercury even reacts with solid sulfur flakes, which is used in mercury spill kits to absorb mercury vapors (spill kits also use activated charcoal and powdered zinc).
The most important salts are
- Mercury(I) chloride ( calomel) is sometimes still used in medicine, acousto-optical filters and as a standard in electrochemistry;
- Mercury(II) chloride (which is very corrosive, sublimes and is a violent poison);
- Mercury fulminate, (a detonator widely used in explosives);
- Mercury(II) oxide, the main oxide of mercury;
- Mercury(II) sulfide (found naturally as the ore cinnabar, still used in oriental medicine, or vermilion which is a high-grade paint pigment);
- Mercury(II) selenide a semiconductor;
- Mercury(II) telluride a semiconductor; and
- Mercury cadmium telluride and mercury zinc telluride, infrared detector materials.
Laboratory tests have found that an electrical discharge causes the noble gases to combine with mercury vapor. These compounds are held together with van der Waals forces and result in Hg·Ne, Hg·Ar, Hg·Kr, and Hg·Xe (see exciplex). Organic mercury compounds are also important. Methylmercury is a dangerous compound that is widely found as a pollutant in water bodies and streams.
The discovery of Mercury(IV) fluoride (HgF4) was announced in September 2007.
Mercury and most of its compounds are extremely toxic and are generally 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 toxic exposure. It 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 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.
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 vapour even at low concentrations in the range 0.7–42 μg/m3.
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.
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.
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. 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.
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. This is because mercury is stored in the muscle tissues of fish, and when a predatory fish eats another fish, it assumes the entire body burden of mercury in the consumed fish. 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, or more, than the species they consume. This process is called biomagnification. The first occurrence of widespread mercury poisoning in humans occurred this way in Minamata, Japan, now called Minamata disease.
The complexities associated with mercury transport and environmental fate are described by USEPA in their 1997 Mercury Study Report to Congress. Because methylmercury and high levels of elemental mercury can be particularly toxic to unborn or young children, organizations such as the U.S. EPA and FDA recommend that women who are pregnant or plan to become pregnant within the next one or two years, as well as young children avoid eating more than 6 ounces (one average meal) of fish per week.
In the United States the FDA has an action level for methyl mercury in commercial marine and freshwater fish that is 1.0 parts per million (ppm), and in Canada the limit for the total of mercury content is 0.5 ppm.
Species with characteristically low levels of mercury include shrimp, tilapia, salmon, pollock, and catfish (FDA March 2004). The FDA characterizes shrimp, catfish, pollock, salmon, and canned light tuna as low-mercury seafood, although recent tests have indicated that up to 6 percent of canned light tuna may contain high levels.
Mercury and aluminium
Mercury readily combines with aluminium to form a mercury-aluminium amalgam when the two pure metals come into contact. However, when the amalgam is exposed to air, the aluminium oxidizes, leaving behind mercury. The oxide flakes away, exposing more mercury amalgam, which repeats the process. This process continues until the supply of amalgam is exhausted, and since it releases mercury, a small amount of mercury can “eat through” a large amount of aluminium over time, by progressively forming amalgam and relinquishing the aluminium as oxide.
Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide, which is not porous to oxygen. Mercury coming into contact with this oxide does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it, starting the process described above, and potentially damaging a large part of the aluminium before it finally ends.
For this reason, restrictions are placed on the use and handling of mercury in proximity with aluminium. In particular, mercury is not allowed aboard aircraft under most circumstances because of the risk of it forming amalgam with exposed aluminium parts in the aircraft.
In the European Union, 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.
Besides, in the European Union there are restriction 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).
In July 2007, 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 healthcare sector and a two-year grace period for manufacturers of barometers.
In Norway, there is a total ban on use of mercury in the manufacturing as well as import or export of mercury products. The ban is effective as of January 1, 2008. Lakes in Norway are already polluted by mercury.