Mercury toxicity

Symptoms typically include sensory impairment (vision, hearing, speech), disturbed sensation and a lack of coordination. The type and degree of symptoms exhibited depend upon the individual toxin, the dose, and the method and duration of exposure.

Due to its toxicity, there have been campaigns in many countries to ban mercury altogether, despite it being an element found in deposits worldwide.

Signs and symptoms

An example of desquamation of the hand of a child with severe mercury poisoning acquired by handling elemental mercury is this photograph in Horowitz, et al (2002).^[2]

Causes

Mercury poisoning is caused by sufficient exposure to elemental mercury or mercury compounds. The consumption of fish is by far the most significant source of ingestion-related mercury exposure in humans, although plants and livestock also contain mercury due to bioaccumulation of mercury from soil, water and atmosphere, and due to biomagnification by ingesting other mercury-containing organisms.^[3] Exposure to mercury can occur from breathing contaminated air,^[4] or from improper use or disposal of mercury and mercury-containing objects, for example, after spills of elemental mercury or improper disposal of fluorescent light bulbs.^[5]

Human-generated sources such as coal plants emit approximately half of atmospheric mercury, with natural sources such as volcanoes responsible for the remainder. An estimated two-thirds of human-generated mercury comes from stationary combustion, mostly of coal. Other important human-generated sources include gold production, non-ferrous metal production, cement production, waste disposal, crematoria, caustic soda production, pig iron and steel production, mercury production (mostly for batteries), and biomass burning.^[6]

Mercury and many of its chemical compounds, especially organomercury compounds, can also be readily absorbed through direct contact with bare, or in some cases (such as dimethyl mercury) insufficiently protected, skin. Mercury and its compounds are commonly used in chemical laboratories, hospitals, dental clinics, and facilities involved in the production of items such as fluorescent light bulbs, batteries, and explosives.^[7]

Toxic effects

Mercury damages the central nervous system, endocrine system, kidneys, and other organs, and adversely affects the mouth, gums, and teeth. Exposure over long periods of time or heavy exposure to mercury vapor can result in brain damage and ultimately death. Mercury and its compounds are particularly toxic to fetuses and infants. Women who have been exposed to mercury in pregnancy have sometimes given birth to children with serious birth defects (see Minamata disease).

Mercury exposure in young children can have severe neurological consequences, preventing nerve sheaths from forming properly. Mercury inhibits the formation of myelin, the building block protein that forms these sheaths.^[8]

Mercury poisoning's effects partially depend on whether it has been caused by exposure to elemental mercury, inorganic mercury compounds (as salts), or organomercury compounds.

Elemental mercury

Quicksilver (liquid metallic mercury) is poorly absorbed by ingestion and skin contact. It is hazardous due to its potential to release mercury vapour. Animal data indicate that less than 0.01% of ingested mercury is absorbed through the intact gastrointestinal tract; though it may not be true for individuals suffering from ileus. Cases of systemic toxicity from accidental swallowing are rare, and attempted suicide via intravenous injection does not appear to result in systemic toxicity.^[10] Though not studied quantitatively, the physical properties of liquid elemental mercury limit its absorption through intact skin and in light of its very low absorption rate from the gastrointestinal tract, skin absorption would not be high.^[11] Some mercury vapour is absorbed dermally but uptake by this route is only approximately 1% of that by inhalation.^[12]

In humans, approximately 80% of inhaled mercury vapour is absorbed via the respiratory tract where it enters the circulatory system and is distributed throughout the body.^[13] Chronic exposure by inhalation, even at low concentrations in the range 0.7–42 μg/m³, has been shown in case control studies to cause effects such as tremors, impaired cognitive skills, and sleep disturbance in workers.^[14]^[15]

Inorganic mercury compounds

Mercury occurs inorganically as salts such as mercury(II) chloride. Mercury salts primarily affect the gastro-intestinal tract and the kidneys, and can cause severe kidney damage; however, as they can not cross the blood-brain barrier easily, mercury salts inflict little neurological damage without continuous or heavy exposure.^[16] As two oxidation states of mercury form salts (Hg⁺ and Hg²⁺), mercury salts occur in both mercury(I) (or mercurous) and mercury(II) (mercuric) forms. Mercury(II) salts are usually more toxic than their mercury(I) counterparts because their solubility in water is greater; thus, they are more readily absorbed from the gastrointestinal tract.^[16]

Organic mercury compounds

Compounds of mercury tend to be much more toxic than the element itself, and organic compounds of mercury are often extremely toxic and have been implicated in causing brain and liver damage. The most dangerous mercury compound, dimethyl mercury, is so toxic that even a few micro liters spilled on the skin, or even a latex glove, can cause death.^[17]^[18]

Methyl mercury is the major source of organic mercury for all individuals.^[19] It works its way up the food chain through bioaccumulation in the environment, reaching high concentrations among populations of some species. Larger species of fish, such as tuna or swordfish, are usually of greater concern than smaller species. The U.S. Food and Drug Administration (FDA) and the U.S. Environmental Protection Agency (EPA) advise women of child-bearing age, nursing mothers, and young children to completely avoid swordfish, shark, king mackerel and tilefish (golden bass), to limit consumption of albacore ("white") tuna to no more than 6 oz (170 g) per week, and of all other fish and shellfish to no more than 12 oz (340 g) per week.^[20] However, there is no evidence that moderate consumption of fish in the U.S. poses a significant health hazard.^{citation needed} A 2006 review of the risks and benefits of fish consumption found that for adults the benefits of one to two servings of fish per week outweigh the risks, even (except for a few fish species) for women of childbearing age, and that avoidance of fish consumption could result in significant excess coronary heart disease deaths and suboptimal neural development in children.^[21]

Ethylmercury is a breakdown product of the antibacteriological agent ethylmercurithiosalicylate, which has been used as a topical antiseptic and a vaccine preservative (further discussed under Thiomersal below). Its characteristics have not been studied as extensively as methyl mercury. It is cleared from the blood much more rapidly, with a half-life of 7 to 10 days, and it is metabolized much more quickly than methyl mercury. It probably does not have methyl mercury's ability to cross the blood-brain barrier via a transporter, but instead relies on simple diffusion to enter the brain.^[19]

Other exposure sources of organic mercury include phenylmercuric acetate and phenylmercuric nitrate. These were used in indoor latex paints for their anti-mildew properties, but were removed in 1990 because of cases of toxicity.^[19]

Treatment

Identifying and removing the source of the mercury is crucial. Decontamination requires removal of clothes, washing skin with soap and water, and flushing the eyes with saline solution as needed. Inorganic ingestion such as mercuric chloride should be approached as the ingestion of any other serious caustic. Immediate chelation therapy is the standard of care for a patient showing symptoms of severe mercury poisoning or the laboratory evidence of a large total mercury load.^[19]

Even if the patient has no symptoms or documented history of mercury exposure, a growing number of physicians, particularly those in alternative medicine, use chelation to "rid" the body of mercury, which they believe to cause neurological and other disorders. A common practice is to challenge the patient's body with a chelation agent, collect urine samples, and then use laboratory reports to diagnose the patient with toxic levels of mercury; often no pre-chelation urine sample is collected for comparison. The patient is then advised to undergo further chelation. No controlled studies show that this procedure actually improves outcome. Inappropriate use of chelators may present untoward danger to the patient, or (in the case of pregnant patients) to a developing fetus.^[22]

Chelation for acute inorganic mercury poisoning can be done with DMSA, 2,3-dimercapto-1-propanesulfonic acid (DMPS), D-penicillamine (DPCN), or dimercaprol (BAL).^[19] Only DMSA is FDA-approved for use in children for treating mercury poisoning. However, several studies found no clear clinical benefit from DMSA treatment for poisoning due to mercury vapor.^[22] No chelator for methylmercury or ethylmercury is approved by the FDA; DMSA is the most frequently used for severe methylmercury poisoning, as it is given orally, has fewer side effects, and has been found to be superior to BAL, DPCN, and DMPS.^[19] Alpha-lipoic acid (ALA) has been shown to be protective against acute mercury poisoning in several mammalian species when it is given soon after exposure; correct dosage is required, as inappropriate dosages increase toxicity. Although it has been hypothesized that frequent low dosages of ALA may have potential as a mercury chelator, studies in rats have been contradictory.^[23] Glutathione and N-acetylcysteine (NAC) are recommended by some physicians, but have been shown to increase mercury concentrations in the kidneys and the brain.^[23] Experimental findings have demonstrated an interaction between selenium and methylmercury, but epidemiological studies have found little evidence that selenium helps to protect against the adverse effects of methylmercury.^[24]

Some of the toxic effects of mercury are in some cases partially or wholly reversible, either through specific therapy or through natural elimination of the metal after exposure has been discontinued. However, heavy or prolonged exposure can do irreversible damage, particularly in fetuses, infants, and young children.

Prevention and regulation

Mercury poisoning can be prevented (or minimized) by eliminating or reducing exposure to mercury and mercury compounds. To that end, many governments and private groups have made efforts to avoid common hazards or to ban mercury altogether. For example, the export from the European Union of mercury and some mercury compounds will be prohibited from 2011-03-15.^[25] The variability among regulations and advisories is at times confusing for the lay person as well as scientists.

Occurrences of mercury poisoning

Acrodynia epidemic

Acrodynia is a type of mercury poisoning in children characterized by pain and pink discoloration of the hands and feet. The word is derived from the Greek, where ακρος means high (as in:in an extremity) and οδυνη means pain. As such, it might be (erroneously) used to indicate that a patient has pain in the hands or feet. However, acrodynia is a disease rather than a symptom.^[2] Also known as pink disease, erythredema, Selter's disease, or Swift-Feer disease, acrodynia was relatively commonplace amongst children in the first half of the 20th century.^[34] Initially, the cause of the acrodynia epidemic among infants and young children was unknown; however, mercury poisoning, primarily from calomel in teething powders, began to be widely accepted as its cause in the 1950s and 60s.^[34] The prevalence of acrodynia decreased greatly after calomel was excluded from most teething powders in 1954.^[34]

Medical procedures

Because elemental mercury often passes through the GI tract without being absorbed, it was used medically for various purposes until the dangers of mercury poisoning became known. For example, elemental mercury was used to mechanically clear intestinal obstructions (due to its great weight and fluidity), and it was a key ingredient in various medicines throughout history, such as blue mass. The toxic effects often were either not noticed at all, or so subtle or generic that they were attributed to other causes and were not recognized as poisoning caused by mercury. While the usage of mercury in medicine has declined, mercury-containing compounds are still used medically in vaccines and dental amalgam, both of which have been the subject of controversy regarding their potential for mercury poisoning.

Thiomersal

Thiomersal, a preservative that contains mercury, has been added to vaccines to prevent their deterioration since the 1930s.^[10] No adverse effects of thiomersal have ever been proven, although some allergic reactions have been noted.^{citation needed} Its use in vaccines has been hypothesized as a cause of autistic behaviors.^[35] This hypothesis is controversial, as much evidence suggests that about 90% of autism is explained by genetics.^[36] The hypothesis has not been confirmed by reliable studies.^[37] However, organizations such as the American Academy of Pediatrics have recommended that the use of thiomersal be reduced as a precautionary measure. With the exception of some flu vaccines, it is no longer used as a preservative in routinely recommended childhood vaccines in the United States; it is still in limited use as a preservative in multi-dose flu and tetanus vaccines and a few other non-childhood vaccines.^[38]

Dental amalgam

Dental amalgam, an alloy of about 50% elemental mercury, has been used to fill decayed teeth since 1830 in the U.S. Although this amalgam is a source of low-level exposure to mercury, no scientific evidence links it as a cause of clinically significant toxic effects, except for the rare local hypersensitivity reaction. The National Institutes of Health has stated that amalgam fillings pose no personal health risk, and that replacement by non-amalgam fillings is not indicated.^[19]

Cosmetics

Some skin whitening products contain the toxic chemical mercury(II) chloride as the active ingredient. When applied, the chemical readily absorbs through the skin into bloodstream.^[39] The use of mercury in cosmetics is illegal in the United States. However, cosmetics containing mercury are often illegally imported. Following a certified case of mercury poisoning resulting from the use of an imported skin whitening product, the FDA warned against the use of such products.^[40]^[41] Symptoms of mercury poisoning have resulted from the use of various mercury-containing cosmetic products.^[42]^[10]^[43] The use of skin whitening products is especially popular amongst Asian women.^[44] In Hong Kong in 2002, two products were discovered to contain between 9,000 to 60,000 times the recommended dose.^[45]

Fluorescent lamps

Fluorescent lamps contain mercury which is released when bulbs are broken. Mercury in bulbs is typically present as either elemental mercury liquid, vapor or both since the liquid evaporates at ambient temperature.^[46] When broken indoors, bulbs may emit sufficient mercury vapor to present health concerns. Breakage of multiple bulbs presents a more significant concern. A 1987 report described a 23-month-old toddler who suffered anorexia, weight loss, irritability, profuse sweating, and peeling and redness of fingers and toes. This case of acrodynia was traced to exposure of mercury from a carton of 8-foot fluorescent light bulbs that had broken in a potting shed adjacent to the main nursery. The glass was cleaned up and discarded, but the child often used the area for play.^[47]

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