Acute mercury exposure has given rise to psychotic reactions such as delirium, hallucinations, and suicidal tendency. Occupational exposure has resulted in erethism, with irritability, excitability, excessive shyness, and insomnia as the principal features of a broad-ranging functional disturbance. With continuing exposure, a fine tremor develops, initially involving the hands and later spreading to the eyelids, lips, and tongue, causing violent muscular spasms in the most severe cases. The tremor is reflected in the handwriting which has a characteristic appearance. In milder cases, erethism and tremor regress slowly over a period of years following removal from exposure. Decreased nerve conduction velocity in mercury-exposed workers has been demonstrated. Long-term, low-level exposure has been found to be associated with less pronounced symptoms of erethism, characterized by fatigue, irritability, loss of memory, vivid dreams, and depression (WHO, 1976).

Effects of chronic occupational exposure to mercury, such as that commonly experienced by affected hatters, include mental confusion, emotional disturbances, and muscular weakness. Severe neurological damage and kidney damage can also occur. Neurological effects include Korsakoff's dementia and erethism (the set of neurological symptoms characteristically associated with mercury poisoning). Signs and symptoms can include red fingers, red toes, red cheeks, sweating, loss of hearing, bleeding from the ears and mouth, loss of appendages such as teeth, hair, and nails, lack of coordination, poor memory, shyness, insomnia, nervousness, tremors, and dizziness. A survey of exposed U.S. hatters revealed predominantly neurological symptomatology, including intention tremor. After chronic exposure to the mercury vapours, hatters tended to develop characteristic psychological traits, such as pathological shyness and marked irritability (box). Such manifestations among hatters prompted several popular names for erethism, including "mad hatter disease", "mad hatter syndrome", "hatter's shakes" and "Danbury shakes".

Erethism or erethism mercurialis is a neurological disorder which affects the whole central nervous system, as well as a symptom complex derived from mercury poisoning. This is also sometimes known as the mad hatter disease. Historically, this was common among old England felt-hatmakers who used mercury to stabilize the wool in a process called felting, where hair was cut from a pelt of an animal such as a rabbit. The industrial workers were exposed to the mercury vapors, giving rise to the expression “mad as a hatter.” Some believe that the character the Mad Hatter in Lewis Carroll's Alice in Wonderland is an example of someone suffering from erethism, but the origin of this account is unclear. The character was almost certainly based on Theophilus Carter, an eccentric furniture dealer who was well known to Carroll.

Mad hatter disease, or mad hatter syndrome, was an occupational disease among hatmakers, caused by chronic mercury poisoning. It affected those whose felting work involved prolonged exposure to mercury vapors. The neurotoxic effects included tremor and the pathological shyness and irritability characteristic of erethism.

Erethism is due to mercury poisoning. Mercury is an element that is found all over the earth in soil, rocks, and water. People who get erethism are usually exposed to jobs that have something to do with these elements, such as construction. People who work in factory jobs tend to have a higher chance of getting erethism. The problem with mercury is that if humans are exposed to any of the forms of mercury, depending on the amount (dose), route (ingestion, skin contact, inhalation), duration (time) of exposure, it can be toxic. Some elemental and chemical forms of mercury (vapor, methylmercury, inorganic mercury) are more toxic than other forms. The human fetus and medically compromised people (for example, patients with lung or kidney problems) are the most susceptible to the toxic effects of mercury.

It is commonly characterized through behavioral changes such as irritability, low self-confidence, depression, apathy, shyness and timidity, and in some extreme cases with prolonged exposure to mercury vapors, delirium, personality changes and memory loss occur as a result. People with erethism find it difficult to interact socially with others, with behaviors similar to that of a social phobia. Although most of the effects of erethism are neurological, some physical problems arise as well, including a decrease in physical strength, “headaches, general pain, and tremors after exposure to metallic mercury” as well as irregular heartbeat. It has been documented that “the tremor in the hands can be so severe that the victim is unable to hold a glass of water without spilling its contents.”

The primary risk factor for erethism is long-term exposure to mercury vapors and gasses at high levels. One group at risk for mercury poisoning is industrial workers and those exposed to high levels of mercury residing naturally in the environment. Erethism is not as serious an issue as it was back before acceptable working condition regulations were enforced. Preventing mercury levels from getting too high limits the amount available for inhalation.

There is a risk of mercury poisoning in the home in some cases. Exposure to mercury vapor may stem from cultural and religious reasons where mercury is sprinkled on the floor of a home or car, burned in a candle, or mixed with perfume. Due to widespread use and popular concern, the risk of toxicity from dental amalgam has been exhaustively investigated. Many studies have not revealed convincing evidence of toxicity . However, in 2015 research showed that an increased mercury release from dental amalgam restorations after exposure to electromagnetic fields is a potential hazard for hypersensitive people and pregnant women.

, sometimes referred to as , is a neurological syndrome caused by severe mercury poisoning. Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, loss of peripheral vision, and damage to hearing and speech. In extreme cases, insanity, paralysis, coma, and death follow within weeks of the onset of symptoms. A congenital form of the disease can also affect fetuses in the womb.

Minamata disease was first discovered in Minamata city in Kumamoto prefecture, Japan, in 1956. It was caused by the release of methylmercury in the industrial wastewater from the Chisso Corporation's chemical factory, which continued from 1932 to 1968. This highly toxic chemical bioaccumulated in shellfish and fish in Minamata Bay and the Shiranui Sea, which, when eaten by the local populace, resulted in mercury poisoning. While cat, dog, pig, and human deaths continued for 36 years, the government and company did little to prevent the pollution. The animal effects were severe enough in cats that they came to be named as having "dancing cat fever".

As of March 2001, 2,265 victims had been officially recognised as having Minamata disease (1,784 of whom had died) and over 10,000 had received financial compensation from Chisso. By 2004, Chisso Corporation had paid $86 million in compensation, and in the same year was ordered to clean up its contamination. On March 29, 2010, a settlement was reached to compensate as-yet uncertified victims.

A second outbreak of Minamata disease occurred in Niigata Prefecture in 1965. The original Minamata disease and Niigata Minamata disease are considered two of the four big pollution diseases of Japan.

Photographic documentation of Minamata started in the early 1960s. One photographer who arrived in 1960 was Shisei Kuwabara, straight from university and photo school, who had his photographs published in "Weekly Asahi" as early as May 1960. The first exhibition of his photographs of Minamata was held in the Fuji Photo Salon in Tokyo in 1962, and the first of his book-length anthologies, "Minamata Disease", was published in Japan in 1965. He has returned to Minamata many times since.

However, a dramatic photographic essay by W. Eugene Smith brought world attention to Minamata disease. His Japanese wife and he lived in Minamata from 1971 to 1973. The most famous and striking photo of the essay, "Tomoko Uemura in Her Bath" (1972) shows Ryoko Uemura, holding her severely deformed daughter, Tomoko, in a Japanese bath chamber. Tomoko was poisoned by methylmercury while still in the womb. The photo was very widely published. It was posed by Smith with the co-operation of Ryoko and Tomoko to dramatically illustrate the consequences of the disease. It has subsequently been withdrawn from circulation at the request of Tomoko's family, so does not appear in recent anthologies of Smith's works. Smith and his wife were extremely dedicated to the cause of the victims of Minamata disease, closely documenting their struggle for recognition and right to compensation. Smith was himself attacked and seriously injured by Chisso employees in an incident in Goi, Ichihara city, near Tokyo on January 7, 1972, in an attempt to stop the photographer from further revealing the issue to the world. The 54-year-old Smith survived the attack, but his sight in one eye deteriorated and his health never fully recovered before his death in 1978.

Japanese photographer Takeshi Ishikawa, who assisted Smith in Minamata, has since exhibited his own photographs documenting the disease. His photographs cover the years 1971 to the present, with Minamata victims as his subjects.

The prominent Japanese documentary filmmaker Noriaki Tsuchimoto made a series of films, starting with "" (1971) and including "The Shiranui Sea" (1975), documenting the incident and siding with the victims in their struggle against Chisso and the government.