Mercury, the only liquid metal at room temperature, has long been known for its dual nature. While elemental mercury found use in medical therapy, mercury toxicity has also been appreciated from the Middle-Ages. Mercury toxicity results when this heavy metal interferes with critical cellular enzymes and functional proteins. All three forms of mercury, elemental, inorganic and organic, have unique patterns of toxicity.

Elemental mercury, used in traditional remedies, dental amalgam, barometers, thermometers and photography, can cause toxicity when absorbed as vapors through the lung. Even a small accidental spill of elemental mercury can result in acute toxicity. Effects of elemental mercury toxicity are mainly seen in the lung and the brain. The airways and alveoli of the lung become irritated, leading to bronchitis and pneumonia. In severe cases respiratory failure presents with cough, breathlessness and low oxygen levels in blood. Involvement of the central nervous system in mercury toxicity causes convulsions and headache. Ingestion of elemental mercury does not cause mercury toxicity as long as the intestinal mucosa is normal. However, people with gut problems like gastrointestinal disorders like diverticulosis and fistula may be at risk of mercury toxicity after ingestion.

Besides the tremors, irritation of the oral cavity, excessive salivation and sweating, chronic elemental mercury toxicity causes some unique symptoms labeled as erethism. It includes extreme shyness, emotional instability, nervousness, insomnia, memory problems, and an inability to concentrate. Signs of chronic toxicity may even mimic Parkinsons disease with the characteristic shuffling gait and hand tremors.

Inorganic mercury (e.g. mercuric dichloride) is very toxic and may even be fatal on ingestion. It presents with vomiting of blood (hematemesis), abdominal pain and kidney failure. Death usually occurs due to shock and cardiovascular collapse. Chronic inorganic mercury toxicity, often seen in the hatting and furring industry, are similar to those of elemental mercury and include erethism. Lewis Carroll's mad hatter in Alice in Wonderland seems to have been inspired by the symptoms of erethism. The mad hatter presumably suffered from chronic mercury toxicity due to the now abandoned process of carroting felt in hat manufacture.

Long-chain organic mercury causes toxicity similar to that seen in contamination with chronic inorganic mercury. Short-chain inorganic mercury compounds are readily absorbed on ingestion and affect the central nervous system. Erethism, gait disturbance (ataxia), uncoordinated speech, a narrow visual field, and abnormal sensations (paresthesias) in the limbs result from organic mercury activity.

These short-chain organic compounds, like methyl mercury, readily cross the placenta to inhibit brain development in the fetus. Congenital organic mercury poisoning was perhaps most poignantly illustrated by the severe outbreak in Minamata, Japan, in the1950s. Various congenital defects including microcephaly (small brain), mental retardation, blindness, motor defects, and difficulty in swallowing result from congenital organic mercury poisoning.

During the 1930s and 1940s, 500 children died in the UK due to the Pink disease, which is caused by mercury containing calomel teething powder. Pink disease is characterized by pink, swollen painful limbs with a peeling skin. Other associated symptoms include fear of light (photophobia), low muscle tone, insomnia, and apathy alternating with irritability. Nowadays, Pink disease is occasionally reported due to absorption of mercury from diapers contaminated with phenyl mercury. It is not known whether pink disease reflects mercury toxicity or a hypersensitivity reaction to it. Mercury Toxicity is diagnosed by testing blood, scalp hair and 24-hour urine samples for mercury levels. Mercury levels in hair and urine are raised in both acute and chronic exposure whereas blood levels only reflect recent toxicity. However, some forms of mercury poisoning may not show up in the urine.

The Minimata exposure was estimated on the basis of 16 blood and hair samples, while urine excretion was minimal. Therefore, scalp hair testing may be a good way to test for chronic mercury toxicity, subject to the reliability of the commercial kit used. Mercury levels in scalp hair above 20 ppm may result in nerve system damage. Pubic hair or nails testing may not be as sensitive to mercury exposure as scalp hair. Oral chelation therapy, along with decontamination, forms the corner stone of medical treatment of mercury toxicity.