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Home , Mercury (element), Mercury poisoning, Metal toxicity, Methylmercury, Poison, Poisoning

Journal of (1999) 13, 651–656  1999 Stockton Press. All rights reserved 0950-9240/99 $15.00 http://www.stockton-press.co.uk/jhh REVIEW of

NJ Langford and RE Ferner West Midlands Centre for Adverse Reaction Reporting, City Hospital, Dudley Road, Birmingham B18 7QH, UK

A ruling by the heralds the demise of can occur if mercury is spilled into those useful clinical instruments, the mercury ther- crevices or cracks in the floorboards. Dentists are mometer and the mercury sphygmomanometer. The occasionally poisoned this way. Mercury easily crosses new laws have been passed because of worries about into the brain, and causes , depression, and . Yet you can drink metallic mercury behavioural disturbances. A second danger from met- and come to no harm. What does it all mean? There are allic mercury is that it is biotransformed into organic three forms of mercury from a toxicological point of mercury, by at the bottom of . This can be view: inorganic mercury ; organic mercury com- passed along the chain and eventually to man. It pounds; and metallic mercury. Inorganic mercury salts was this process that led to the Japanese tragedy at are soluble, irritate the gut, and cause severe kid- Minimata Bay in the late 1950s when over 800 people ney damage. Organic mercury compounds, which are were poisoned. It is the need to reduce mercury con- fat soluble, can cross the brain barrier and cause tamination of the environment which should encourage neurological damage. Mercury metal poses two dan- us to cut the usage of metallic mercury. However, much gers. It can be vaporised: the vapour at room more metallic mercury is spilled as waste by the chemi- is about 100 times the safe amount, so cal industry than is dropped on the floor in the clinic.

Keywords: mercury; toxicity; ; sphygmomanometer

Toxicity of mercury and organometallic mercury compounds and we review each form separately. A new European Union directive1 will prohibit the use of mercury in sphygmomanometers and clinical . We review here the toxic effect of Metallic mercury mercury and its compounds and discuss their rel- Metallic mercury ( mercury, quicksilver, evance to the environment and to modern . hydrargyrum (hence Hg), elemental mercury), is a The Chinese used mercury (II) sulphide 1000 white metal which melts at −38.7°C. Mercury years before the birth of Christ as the red dye pig- is best known as a , having a vapour ment . It was used similarly in the Graeco- pressure (a measure of the amount of vapour ‘given Roman world, with both Hippocrates and Galen rec- off’) of 0.002 mm Hg at 25°C. This approximately ording its toxic effects. Since then its toxicity has doubles for every 10°C increase in temperature, so become well known in metalworkers, miners, - that heating metallic mercury greatly increases the hat manufacturers, dyers and paint manufacturers. associated risks, as inhalation is the usual route of Despite this, mercury has been incorporated into the toxicity. Inhaled mercury vapour accumulates in the treatment of man’s maladies from ancient times. Its body, and in particular the central , main use has been to treat , from its first which is the site of its major toxic actions. Orally appearance in the West in the 15th century up to ingested elemental mercury rarely causes acute toxic World War II.2 Mercury and its salts have at various effects, as gastro-intestinal absorption is low—less times been used as , ointments, laxa- than 0.01% of the .3 For practical purposes, tives, , bowel washouts for the treatment of of oral elemental mercury as a single dose colorectal cancer, and scabicides. It is still used poses a negligible risk of severe toxicity. The oral today as a for the silver- amalgams used LD10 is reported to be 1429 mg/kg (in man), or in dental fillings. So how toxic is mercury? approximately 100 g for a 70 kg adult. Percutaneous absorption is also low (approximately 2% of the rate The of mercury of uptake by the ).4 There are substantial differences in toxicity of Absorption by inhalation readily occurs, as mer- 5 elemental mercury metal, inorganic mercury salts, cury vapour freely crosses the alveolar membrane with nearly 100% .6 Once absorbed, a proportion of mercury is taken up by the red blood Correspondence: NJ Langford, West Midlands Centre for Adverse , whilst some remains in the bloodstream, Drug Reaction Reporting, City Hospital, Dudley Road, Birmingham. B18 7QH, UK allowing its rapid distribution around the body, Received 10 March 1999; revised 28 May 1999; accepted 10 including the . Within the June 1999 red blood cells, , and central nervous system the Toxicity of mercury NJ Langford and RE Ferner 652 metal is oxidised via the catalase-peroxide pathway browns and blacks of the sulphide compounds. to mercury (II) . The rate at which this reaction They are also commonly used in industry. They occurs is limited in the presence of catalase inhibi- have been used in medicine in powders, tors and competitive substrates such as ethanol. skin-lightening creams and as in cer- Mercury from the body starts almost tain —particularly as eyedrops. Mercury immediately after absorption, following a variety of (I) () was widely used as a purgative routes, though principally by the kidneys.7 Minor in the form of the ‘blue pill’. routes of excretion include the gastro-intestinal tract The toxicity of mercury salts varies with their and saliva into which mercury is actively secreted, . Usually mercury (I) compounds are of as well as from the , in the nails and the low solubility and significantly less toxic than mer- and also from sweat glands. There is a relatively fast cury (II) compounds. Inorganic mercury salts initial , during which a third of the inhaled present a far greater than elemental mercury mercury is rapidly concentrated in the liver and kid- if ingested orally, owing to their greater water solu- neys, and excreted in the bile or . This phase bility.17 For mercury (II) chloride, the has an estimated half- of 2–16 days. A still slower may be as small as 0.5 g, compared with 100 g mer- second phase, reliant on renal accumulation and cury metal. Mercury salts are usually non-volatile probably responsible for the excretion of the solids, so poisoning by inhalation is rare, though majority of the body’s mercury load via the urine, toxicity may arise if are deposited in the has a half-life of 30–60 days.8 A slower third phase lungs. Once adsorbed, the mercury (I) form will of excretion, via the , has been postulated readily react with the groups of amino accounting for ෂ15% of the mercury load.9 Mercury such as . The , func- deposited within the brain has an elimination half- tions as a natural chelator, preventing damage by 10 life that may exceed several years. In workers with mercury and other transitional to thiol- high lifetime exposure to elemental mercury, containing . Once metallothionein proteins deposits of mercury were found in the brain at auto- are saturated, other structurally related proteins can psy, even though exposure to mercury had ceased be damaged. Inorganic mercury salts are not lipid 11 years previously. In , the highest concen- soluble, and so do not cross the blood brain barrier trations of mercury were found in the Purkinje cells in significant amounts. The majority of the dose of of the cerebellum and in certain neurons of the spi- 12 an ingested inorganic mercury accumulates nal cord and midbrain. either in the liver, where it is excreted in the bile, The toxicity of mercury vapour is dose-depen- or in the kidney, where it is excreted in the urine. dent. Exposed patients may initially complain of a The symptoms and signs of inorganic mercury metallic taste. Three to 5 h after a high acute poisoning arise in two phases. Very soon after inges- exposure, cough, dyspnoea, chest tightness, leth- tion, there is burning in the chest, rapid dis- argy, restlessness, fever and signs of pneumonitis colouration of the mucous membranes (secondary to can develop. If the exposure levels are sufficient, precipitation of proteins in the mucosal lining), and and especially when accumulation occurs after gastro-intestinal pain from direct local trauma due repeated exposures, then central nervous symptoms to the salts’ corrosive nature. There can be and signs arise, with tremor, and (behavioural disturbances).13 Vasomotor disturb- and a profuse bloody diarrhoea which can in ances may also occur with excess and extreme cases to hypovolaemic and blushing. Other oral changes include ulceration, . If the patient survives the initial effects of the bleeding gums and loosening of the teeth.14 , the systemic effects predominate, with mer- Chronic poisoning markedly affects the central curial stomatitis (glossitis, ulcerative ginigivitis, nervous system and kidney. Tremor, initially hypersalivation, and a metallic taste), loosening of involving facial muscles and eyelids, is present at the teeth, and renal damage. The renal damage arises rest, but aggravated by intention. It gradually secondary to accumulation of the mercury salt in the becomes more pronounced and also starts to affect proximal convoluted tubules, causing a transient the limbs. Handwriting becomes illegible, with polyuria, proteinuria (in severe cases a nephrotic omission of letters and eventually whole words; syndrome), haematuria, and renal acidosis. erethism is manifest as excessive , loss of Chronic inorganic mercury poisoning often occurs confidence, vague fears, irritability, insecurity, and in combination with elemental mercury poisoning, suicidal melancholia. The patient becomes unable where the central nervous system effects predomi- to perform simple tasks such as dressing.10 Renal nate. Chronic poisoning from pure inorganic problems are relatively rare considering that the mercury salt is rare. Its symptoms, though pre- major route of elimination is via the kidneys. The viously described in children as or ‘Pink commonest signs are of proteinuria-reflecting glom- ’, were only formally attributed to inor- erular damage. With high doses a frank nephrotic ganic mercury compounds in 1951 by Warkany syndrome can develop, that has been seen both in and Hubbard18 following a study of mercury con- industry and medicine.15,16 taining substances used in teething powders, worm pills, ammoniated mercury ointment, and mercury (II) chloride. It is characterised by severe Inorganic mercury salts leg cramps, irritability, paraesthesia (a sensation of Inorganic mercury salts are present in nature in vari- pricking in the skin), pink extremities and exfoli- ous colours ranging from the white to the ation of skin. Toxicity of mercury NJ Langford and RE Ferner 653 Organic mercury compounds in the production of as an electrolytic cata- lyst, as well as its use in calibration instruments and Organic mercury compounds are an important cause fluorescent lights. Even with modern techniques of of poisoning. Mass outbreaks have occurred industrial hygiene, workers exposed to mercury and throughout the world, either inadvertently second- their families27 have increased mercury concen- ary to , or via direct ingestion of organic trations in the urine, though no symptoms of overt mercury compounds. In the developed world, poisoning, with normal industrial exposure.28 More exposure is most commonly via the aquatic food bizarre sources include crematoria.29 As nearly 70% chain, where micro- convert elemental of people who now die are cremated in the UK, it mercury to organic mercury before being eaten by is estimated that a single crematorium emits larger invertebrates and so on up the chain, ending approximately 5.5 kg of mercury a year. However with man. the largest source of mercury pollution is emissions The organic mercury compounds are very lipid from fired power . soluble: 90–100% of an oral dose is absorbed. The Organic mercury and human exposure from it has exact distribution within the body is uncertain. A mirrored environmental pollution. The first case of large proportion of a dose is transformed to inor- organic mercury poisoning arose amongst research ganic compounds. This however fails to explain the workers at St Bartholomew’s Hospital in 1865 as characteristic central nervous system pattern of tox- they tried to manufacture organic mercurial com- icity. Specific damage is seen in the cerebellum and pounds. Three technicians had significant methyl- visual cortex. The excretion of organic mercury 19 mercury exposure and two subsequently died as a compounds is initially in the bile via the liver. result.30 A professor in the However they undergo enterohepatic recirculation, was similarly poisoned in 1997.31 However, micro- leading to reabsorption and uptake into the red organisms are the prime producers as they blood cells where they are metabolised, forming biotransform the metallic mercury in their environ- inorganic salts and follow pathways previously dis- ment to organic mercury compounds.32 The extent cussed. of the problem was only realised following the The toxicity of organic mercury exposure may be Japanese disaster at Minamata where a factory’s delayed for weeks to months, with predominant - 20 effluent was discharged into the local fishing tro-intestinal and central nervous system effects. grounds. By a process of biomagnification, the The gastro-intestinal effects include nausea, vomit- accumulated to sufficient amounts ing and abdominal pain; higher doses can cause dia- in predatory fish33 to poison over 800 people who rrhoea and an exposure-related colitis. Other symp- relied on the fish for their nutrition. A number were toms include the discolouration of the gums (similar in utero.34 This problem is worldwide, lead- to poisoning with inorganic compounds), sia- ing to restrictions on fishing in the in lorrhoea and perioral paraesthaesia. Central nervous the United States and some commentators to con- effects after slight exposure include numbness in the clude that all fish are polluted with methylmer- limbs; as the level of exposure increases, there are cury.32 Other studies have linked fish consumption , , dysarthria, and visual field constric- and increased methylmercury levels in man, though tions. without overt symptoms of poisoning.35,36 (Further Organic mercury poisoning became notorious studies are presently underway.) Other more direct after the Minimata tragedy, where an epidemic of a from methlymercury have arisen owing condition resembling was ascribed to to its use as a particularly in Second and its teratogenic effect.21 Milder forms are present with 22 Third World countries. Inadvertent ingestion of lower levels of exposure, with features including treated corn has caused mass outbreaks of poison- mental retardation, spasticity, , chorea, tre- ing. The largest occurred in Iran in the autumn of mors, cataracts, small size, , and renal dys- 23 1971, resulting in 6530 cases of poisoning with 459 function. Other effects seen from poisoning .37 Smaller outbreaks have also occurred in include cardiac arrhythmias, hepatic dis- Ghana38 and Pakistan.39 In other countries methyl- turbances, respiratory tract irritation, and blistering 19 mercury has been used as a fungicide in paints, and of the skin. poisoning has resulted.40

Mercury in the environment Medicine and mercury Mercury is naturally present in the earth’s crust. It The mercury sphygmomanometer invented by is also present in the atmosphere either derived nat- Scipione Riva-Rocci has been the mainstay for blood urally from the degassing of the earth’s crust, emis- pressure for the last century.41 Only sions from volcanoes, evaporation from the world’s within the last decade have automated mechanical , or from industrial pollution, which has greatly devices of sufficient accuracy become available.42 increased our exposure. Suggestions that the mercury sphygmomanometer Elemental mercury and its inorganic salts have a may be responsible for mercury poisoning have wide range of uses within industry. In the -min- rarely been discussed with no records in the litera- ing areas of the Amazon, where mercury is used in ture of it directly being responsible for the deaths of the extraction of gold, there is extensive human and patient or operator. However concerns arise from the environmental owing to the methods fact the each mercury sphygmomanometer contains used.24–26 In the Western world, mercury plays a role approximately 64–85 g of elemental mercury. The Toxicity of mercury NJ Langford and RE Ferner 654 actual amount has been found to vary considerably, 17.5 ␮g/day.58 (The General Public Threshold as mercury is slowly lost either by direct spillage, (NOAEL)—no observed level ie, the vaporisation or secondary to oxidation. Studies have level at which adverse effect have never been indicated that between 62–87% of mercury man- observed—is 5 ␮g/m3.) Owing to the prevalence of ometers are affected in this way.43 Where the mer- dental fillings the US government commissioned a cury has been lost to remains debatable. Environ- review of the literature published in 1993 which mental lobbyists and journalists44 have been keen to concluded: ‘. . . Current scientific evidence does not speculate, claiming the subsequent vaporisation of show that exposure to mercury from resto- mercury was endangering health staff’s well being rations poses a serious health risk in except with chronic mercury exposure. Fortunately though for an exceedingly small number of allergic reac- there is little evidence for this, though reports do tions.’59 exist on both sides of the Atlantic recounting tox- Despite this and other follow-up reports,60 the icity to medical technicians charged with repairing issue remains in dispute with ‘The Times’ reporting and servicing the mercury manometers.45 Such a case of a woman requiring admission to a psychi- problems usually arise owing to mercury spills in atric unit claiming that mercury fillings ‘drove small, enclosed areas with poor ventilation, often in woman to attack mother’.61 In Lorscheider’s review a well-heated room where the spillage has been of dental amalgams, he concluded that there was inadequately disposed of. (For information on deal- insufficient evidence to prove that amalgams were ing with mercury spillages see appendix 1.) safe.62 A US study showed that despite low levels Although these problems exist, it was concluded in of mercury some evidence of subtle preclinical an American study46 that ‘the prevention of mercury behavioural effects are present,63 further substantiat- exposure in the occupational settings . . . should be ing this view. The debate will no doubt continue readily achievable’. The importance of mercury in though present evidence shows there is little room sphygmomanometers is also diminished by the rela- for concern (see Eley’s series of articles on the dental tively small amounts of mercury involved compared amalgam in the British Dental Journal).64 with industry. (In 1992 the United States, according to the USEPA’s Toxic Release Inventory, released nearly 16000 pounds (approximately 7000 kg) of Conclusion elemental mercury to , surface , and the Overt mercury poisoning is rare in the modern atmosphere.) world, although it is widely spread in the environ- The sphygmomanometer is not alone in medicine ment both naturally and as a result of pollution. Its as an instrument relying on mercury for its proper- toxicity is well known, though the presentation of ties. The mercury thermometer will also be affected its symptoms depends largely on the route of admin- by the Community’s legislation. No reports exist of istration and its form. Modern clinical use of mer- intact thermometers causing mercury poisonings, cury appears to be safe, owing to the very limited although once broken thermometers may cause exposure in our daily , thought caution is problems. The most significant poisoning was due necessary if handling large amounts of mercury or to inhalation of mercury vapour occurring in a neo- repairing mercury-containing instruments. The nate in an isolette (incubator), where a mercury- symptoms purportedly caused by chronic low-dose operated had broken, spilling mercury into a mercury poisonings as with dental amalgam remains confined area that was maintained at 25°C.47 How- unproven, though research continues in this area. ever the majority of incidents involving mercury thermometers are directly related to trauma, as the Appendix 1: Dealing with spilt mercury65 thermometer is broken. The sites of injury include the floor of the mouth,48 rectal injury,49 eye injury,50 Spilled mercury can shatter into tiny droplets. This and injuries to hands and fingers,51 and the lower increases the surface area and also the rate of evap- limb.52 In almost all of these cases the extent of oration and represents a hazard. Therefore a mer- injury is related to local damage rather than cury spillage should be treated seriously. The rec- systemic effects. Despite this, since January 1992, ommended action is to ensure that the area is mercury thermometers have been banned from segregated to prevent the contamination from import, manufacture, or sale in Sweden.53 spreading, and to maximise ventilation by opening The other main medicinal use of mercury has windows. Wearing protective gloves and mask join been as a dental amalgam. Its use within dental sur- up the globules of mercury to form a large pool using geries has in the past been somewhat chequered a wooden spatula or equivalent before aspirating with at least one death attributed to the poor into a syringe. The aspirate is then placed in a speci- environmental controls present within the surgery.54 ally designated container, or if that is unavailable, However, the amount of mercury adsorbed into the in a jar of water prior to sealing it. Spreading a paste body from a dental amalgam remains largely dis- containing equal parts of sulphur and puted. All parties agree that some mercury from the over the surface further decontaminates amalgam is vaporised and absorbed by the body, it, by forming black insoluble mercury (II) sulphide particularly after chewing and brushing teeth.55–57 therefore rendering it non-volatile, preventing the However the amount of mercury released and the further evaporation of mercury.66 The paste is mixed quantity required to have an effects within the body with the remains of the spilt mercury for a minimum is yet to be ascertained. A review of seven studies of 2 to 3 min, and then disposed of via the spatula reported the daily dose ranging from 1.7 ␮g/day to into an appropriate waste container. The area is Toxicity of mercury NJ Langford and RE Ferner 655 wiped with a damp cloth, which is then placed in 22 Bakir F et al. 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