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Heavy metals are commonly defined as those elements with a high (>5.0) relative density. There is, however, an ongoing debate regarding the exact definition and, in terms of health, it is better to think of these as metals or semi-metal compounds which have the potential to cause environmental or human toxicity. Heavy metal poisoning can be acute or chronic and may be caused by the following:
- Arsenic (technically not a true metal but a semi-metal - ie non-metal with some metallic properties).
The metals may enter the body by:
- Absorption through the skin or mucous membranes.
They are then stored in the soft tissues of the body. The heavy metals once absorbed, compete with other ions and bind to proteins, leading to impaired enzymatic activity resulting in damage to many organs throughout the body.
Heavy metal poisoning is rare in the UK, even in industries where there is an increased risk.
- The most common cause of heavy metal poisoning is lead. The incidence of lead poisoning has been falling steadily in affluent countries, due to removal of lead from paint, petrol and food cans.
- Lead poisoning remains a problem, however, in older housing where lead water pipes and lead paint may still be present. In housing such as this, there is a particular risk to children. 'Lead law' legislation appears to provide protection against increased lead exposure in children.
- Other sources of lead poisoning are occupations (eg, smelting, battery manufacture), traditional remedies or occasionally foreign bodies (lead weights).
- Mercury can be found in the elemental state (dental amalgam, thermometers), inorganic (industrial processes) and organic compounds (pesticides, wood preservatives, some medicines and contaminated fish).
- Ingestion of disc batteries by children can also lead to heavy metal poisoning amongst other problems. These batteries can contain varying amounts of metals, including mercury, manganese and cadmium.
- Poisoning from other heavy metals most often occurs in individuals regularly exposed to the metals in their work environment.
- Criminal poisoning with lead has been reported.
The presentation will depend on the age of the individual, the metal absorbed and whether this was the result of acute exposure - eg, vapour inhalation - or exposure over a more prolonged period of time.
The differential diagnosis will depend on the symptoms and signs displayed but may include causes of encephalopathy, dementia, substance abuse and causes of vomiting and diarrhoea.
Investigation of any person in whom a diagnosis of heavy metal poisoning is a possibility may include:
- Full history - including occupational history, age of house and water supply if known.
- FBC and film - basophilic stippling with lead and arsenic poisoning, normochromic or microcytic anaemia with lead toxicity.
- Blood levels for lead and mercury.
- 24-hour urine collection - mercury and arsenic levels.
- Long bone X-ray in children - horizontal metaphyseal lines ('lead bands' caused by failure of the bones to remodel; not seen in adults).
- CXR - may show radiodense pulmonary emboli following injection of mercury.
See separate Acute Poisoning - General Measures article.
Current advice on the management of any form of heavy metal poisoning may be obtained from:
The toxic dose for mercury compounds is about 10-50 mg/kg although this varies with the form of mercury, the dose and rate of exposure - such as whether it is in the elemental state, in inorganic (mercurous and mercuric) compounds, or in organic compounds (especially methyl mercury). The target organ for inhaled mercury vapour is primarily the brain. Mercurous and mercuric salts chiefly damage the gut lining and kidney, while methyl mercury is widely distributed throughout the body. Toxicity varies with dosage; large acute exposures to elemental mercury vapour induce severe pneumonitis, which can be fatal. Low-grade chronic exposure to elemental or other forms of mercury induces subtler symptoms and clinical findings.
Inorganic and elemental mercury are poorly absorbed from the gut and ingestion is usually harmless (unless aspiration occurs). A single dose (eg, a broken thermometer) usually causes no problems. It is only slowly absorbed through the skin but can cause contact dermatitis.
In the mid-1950s to 1968 the people of Minamata in Japan were affected by chronic organic mercury poisoning from pollution from a local factory (Minamata disease). 'Mad as a hatter' is derived from poisoning among hatmakers who used mercuric nitrate to soften the hair of animal hides.
- An acute pneumonitis (± adult respiratory distress syndrome (ARDS)).
- Flu-like symptoms.
- Gastrointestinal upset.
- Subsequent peripheral neuropathy, hepatic dysfunction or renal failure may develop.
- Personality changes.
- Peripheral neuropathy.
- Memory problems.
- Respiratory problems (pneumonitis and ARDS).
- Gastrointestinal upset (abdominal pain, gingivitis and stomatitis, nausea, vomiting).
- Renal problems include acute renal failure, nephrotic syndrome and acute tubular necrosis.
Management of inhalation of mercury should be treated with intravenous (IV) hydrocortisone to minimise pulmonary complications. Acute intakes of inorganic mercury should be treated with chelating agents such as d-penicillamine.
Iron poisoning can present with:
- Nausea ± vomiting.
- Abdominal pain and diarrhoea.
- Possible gastrointestinal bleeds.
- Serious overdose, which causes hepatocellular necrosis (jaundice, hepatic failure).
- Gastric outflow obstruction, which may be a late complication.
Iron poisoning needs urgent treatment with emergency admission and desferrioxamine (IV) as an antidote. Chronic iron excess is dealt with in the separate Iron Overload article.
Toxic by inhalation and ingestion and also, although occurring rarely, absorption through the skin. Symptoms develop 12-36 hours after inhalation (cadmium fume fever is an occupational illness due to exposure to fumes during welding).
It presents with:
- Metallic taste and increased salivation.
- Nausea, vomiting and diarrhoea.
- Impaired sensation.
- Difficulty breathing, cough, chest pain.
Complications include pneumonitis and pulmonary oedema.
Chronic exposure may cause anaemia, emphysema or renal failure and cadmium may be a risk factor in the development of prostate or lung cancer.
At present there is no effective therapy for cadmium poisoning; treatment is supportive and symptomatic. It is hoped that some of the newer chelating agents may help in reducing cadmium levels in the body.
Has a few industrial uses in pigments, ceramics and low-melting alloys.
It can cause:
- Kidney damage and the development of a nephrotic syndrome.
- Encephalopathy - ie it may also cause the development of a reversible encephalopathy.
A chelating agent can be used in the management; however, treatment is not always necessary.
Further reading and references
Metals as toxins; Electronic Medical Curriculum, Edinburgh University Faculty of Medicine
Tchounwou PB, Yedjou CG, Patlolla AK, et al; Heavy metal toxicity and the environment. EXS. 2012101:133-64. doi: 10.1007/978-3-7643-8340-4_6.
Meyer PA, Brown MJ, Falk H; Global approach to reducing lead exposure and poisoning. Mutat Res. 2008 Jul-Aug659(1-2):166-75. Epub 2008 Mar 20.
Kennedy C, Lordo R, Sucosky MS, et al; Primary prevention of lead poisoning in children: a cross-sectional study to evaluate state specific lead-based paint risk reduction laws in preventing lead poisoning in children. Environ Health. 2014 Nov 713:93. doi: 10.1186/1476-069X-13-93.
Berlin M, Zalups RK, Fowler BA. Mercury. In: Nordberg GF, Fowler BA, Nordberg M, Friberg LT, editors; Handbook on the Toxicology of Metals. 3rd edition. Chapter 33. New York. Elsevier 2007.