Mercury poisoning is illness caused by exposure to mercury or its compounds. Mercury is a naturally occurring element found in air, water and soil, and its toxicity depends strongly on its chemical form, the dose, the route of exposure and the age of the person exposed. The World Health Organization lists mercury among its top ten chemicals of major public health concern, and the nervous system and the developing fetus are the most sensitive targets. Public health research treats the subject as three overlapping problems: poisoning from organic methylmercury in food, poisoning from inhaled mercury vapour, and poisoning from inorganic mercury salts.
Chemical forms and their toxicity
Toxicologists usually divide mercury into three groups, and each behaves differently in the body. Elemental mercury is a heavy, silvery liquid that evaporates slowly at room temperature. Very little of the liquid is absorbed if it is swallowed, so it poses little risk of severe toxicity after a single ingestion. Its vapour, however, is readily absorbed through the lungs, distributes throughout the body, crosses into the central nervous system and the placenta, and can cause kidney and nervous system damage. Inorganic mercury compounds, such as mercuric salts, are corrosive when swallowed and mainly damage the gastrointestinal tract and kidneys. Organic mercury, of which methylmercury is the most important example, is the most neurotoxic form and is the cause of Minamata disease.
Routes of exposure
The main pathway for most of the population is diet. methylmercury forms in sediments and water through microbial activity and then accumulates in fish and shellfish. Because of bioaccumulation in aquatic food webs, large predatory fish such as shark, swordfish and some tuna contain the highest concentrations. Cooking does not remove the compound from fish. Inhalation is the principal route for workers in industries that heat or process mercury, and for people working in artisanal and small-scale gold mining, where mercury is used to form an amalgam with gold that is then burned off. Other sources include broken household items such as thermometers, fluorescent lamps and older barometers; certain traditional remedies, cosmetics such as skin-lightening creams, and dental amalgam, which releases small amounts of elemental mercury. Coal combustion and waste incineration release mercury into the wider environment, which then returns to people through food.
Mechanisms of toxicity
Methylmercury is lipid-soluble and easily crosses the blood-brain-barrier, which explains its effect on the brain. Its biological half-life in humans is roughly 65 days, and most of it is excreted in feces following enterohepatic circulation. Laboratory studies indicate that mercury binds strongly to thiol and selenol groups in proteins, disrupting enzyme function and cellular redox balance. In neurons, reported effects include oxidative stress, mitochondrial dysfunction, disturbed calcium and glutamate handling, endoplasmic reticulum stress and apoptosis. Mercury can also disturb the immune system and kidneys. Most adverse outcomes appear to involve several mechanisms at once rather than a single pathway.
Clinical features
Symptoms depend on the form and dose. Acute inhalation of high concentrations of mercury vapour can injure the airways and lungs, causing chest tightness, cough and, at high exposures, severe respiratory distress. Chronic low-level vapour exposure is associated with tremor, mood changes such as irritability and shyness, insomnia, gum disease and a metallic taste. Ingestion of inorganic mercury produces vomiting, bloody diarrhoea and, when absorbed, kidney injury that can progress to kidney failure. Methylmercury poisoning mainly affects the nervous system and may cause numbness or pain in the extremities, impaired coordination, speech and hearing disturbances, constriction of the visual field, blindness, muscle weakness, memory problems and seizures. At high doses, it can lead to coma, paralysis and death. Many of its effects on the brain are permanent.
Vulnerable populations
The fetus is the most susceptible group. Methylmercury passes readily through the placenta, and exposure in the womb can impair brain development, causing lasting problems with cognition, memory, attention, language, fine motor skills and visual-spatial ability. Severe cases can present as cerebral palsy, microcephaly, deafness and blindness. Maternal exposure can also lead to pregnancy loss. Infants and children are vulnerable because of their developing nervous systems and higher intake relative to body weight. In adults, the risk is greatest for people who eat large amounts of predatory fish, subsistence fishing communities, and workers exposed occupationally.
Diagnosis and management
Clinicians assess exposure through laboratory measurement of mercury in blood, urine or hair, with the chemical form and the timing of exposure guiding interpretation. Whole blood mercury is normally low in unexposed people, but elevated values can reflect diet or occupational exposure. Treatment focuses on removing the source of exposure and supporting affected organs. Chelation with thiol-containing agents is used in some cases of significant inorganic or elemental mercury poisoning, but it is generally considered less effective for methylmercury, which is largely bound within the brain and nervous tissue. Damage to the nervous system from methylmercury is often irreversible, so prevention is the primary strategy.
History
Mercury has been used and recognized as toxic for centuries, but its health effects became a major public concern in the twentieth century. The best-known episode occurred in Japan. From 1932 to 1968, a chemical factory in Minamata discharged wastewater containing methylmercury into the bay, which accumulated in local fish and then in the people who ate them. The first human case was recorded in 1956, and the disease was named Minamata disease in 1957. Congenital cases among children born to exposed mothers were identified in 1962. The Japanese government did not officially recognize the disease until 1968. Estimates of victims vary between sources, but they include thousands of people affected and roughly two thousand deaths. A smaller methylmercury incident occurred in Niigata Prefecture in the 1960s, caused by a different factory. A notable episode outside Japan took place in Iraq in 1971–1972, when many thousands were poisoned after eating bread made from grain treated with a mercury-based fungicide. Estimates of the death toll differ across accounts.
Regulation and international response
The Minamata Convention on Mercury was adopted in 2013 at a diplomatic conference in Japan and entered into force in August 2017. It is the first legally binding instrument dedicated to mercury and covers its entire life cycle, including mining, trade, products, emissions, waste and the protection of human health. Its measures include phasing out certain mercury-added products such as some batteries and switches, reducing emissions from coal-fired power plants and industrial boilers, and regulating artisanal gold mining. The convention is administered under the United Nations Environment Programme, and more than 140 states had become parties by the mid-2020s. National and international guidelines set limits for mercury in drinking water and tolerable intake of methylmercury in food. The World Health Organization recommends measures to reduce exposure, including substitution of mercury-containing thermometers and blood pressure devices in health care.
Debates and open questions
Public health guidance involves a trade-off. Fish is a valuable source of protein and omega-3 fatty acids, so advice to limit fish consumption must balance mercury exposure against nutritional benefits. Official guidance generally emphasizes avoiding high-mercury species during pregnancy and in early childhood, while considering low-mercury fish as a healthy option. Evidence on the effects of low-level, long-term exposure in adults remains debated, with some studies reporting cardiovascular and other associations. The carcinogenicity of mercury is not established in humans: some studies report associations, but a 2005 United States evaluation concluded that environmental exposure to inorganic and methylmercury is not likely to cause cancer in humans. Ethylmercury, a different compound used as a preservative in some vaccines, is considered by health agencies not to pose a comparable health risk to methylmercury. Artisanal gold mining remains a major source of atmospheric mercury, and experts continue to debate whether economic pressures and the cost of alternatives will allow regulations to reduce use in that sector.
References
- Health Effects of Exposures to Mercuryepa.gov
- Mercury Poisoninghhs.iowa.gov
- Methylmercury poisoningucsfhealth.org
- Health Impacts and Biomarkers of Prenatal Exposure to Methylmercury: Lessons from Minamata, Japanncbi.nlm.nih.gov
- Mercury Toxicity: Background, Etiology, Epidemiologyemedicine.medscape.com
- Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation - ScienceDirectsciencedirect.com
- Mercury poisoning: MedlinePlus Medical Encyclopediamedlineplus.gov
- Mercury in Foodfda.gov
- Minamata Convention on Mercuryenb.iisd.org
- Minamata Convention, History, Area Covered, Featuresvajiramandravi.com
- The Minamata Convention on Mercury: A First Step toward Protecting Future Generationsncbi.nlm.nih.gov
- Homepageminamataconvention.org
- Mercury and Minamata Disease: a Lesson from Japanunep.org
- Minamata Bay mercury poisoningebsco.com
- Minamata Convention on Mercury - DCCEEWdcceew.gov.au
- How the Minamata Convention is aiming to end mercury’s millennia-long toxic rununep.org
- Minamata Disease and the Legacy of Mercury Poisoning - ScienceInsightsscienceinsights.org
- Mercurywho.int
- 1 Mercury Compounds Hazard Summaryepa.gov
- Fact Sheet: Mercury and Health - Pure Earthpureearth.org
- exposure to mercury: a major public health concernwho.int
- Mercury (CH0104)undrr.org
- Mercury - PAHO/WHOpaho.org