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A 3D molecular model of Methylmercury. It is an organometallic cation with the formula CH3Hg+. The "X" stands for any anion.


Minamata disease (Japanese: 水俣病 Hepburn: Minamata-byō), sometimes referred to as Chisso-Minamata disease (チッソ水俣病 Chisso-Minamata-byō), is a mercury induced 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 bio-accumulated 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.

Methylmercury (sometimes methyl mercury) is an organometallic cation with the formula CH3Hg+. The " X" stands for any anion. It is the major source of organic mercury for all humans. It is a bioaccumulative environmental toxicant



The Chisso Corporation first opened a chemical factory in Minamata in 1908. Initially producing fertilisers, the factory followed the nationwide expansion of Japan's chemical industry, branching out into production of acetylene, acetaldehyde, acetic acid, vinyl chloride, and octanol, among others. The Minamata factory became the most advanced in all of Japan, both before and after World War II.[citation needed] The waste products resulting from the manufacture of these chemicals were released into Minamata Bay through the factory wastewater. These pollutants had an environmental impact. Fisheries were damaged in terms of reduced catches, and in response, Chisso reached two separate compensation agreements with the fishery cooperative in 1926 and 1943.

The rapid expansion of the Minamata factory spurred on the local economy and as Chisso prospered, so did Minamata. This fact, combined with the lack of other industry, meant that Chisso had great influence in Minamata. At one point, over half of the tax revenue of Minamata City authority came from Chisso and its employees, and the company and its subsidiaries were responsible for creating a quarter of all jobs in Minamata. Minamata was even dubbed Chisso's "castle town", in reference to the capital cities of feudal lords who ruled Japan during the Edo period.

The Chisso Minamata factory first started acetaldehyde production in 1932, producing 210 tons that year. By 1951, production had jumped to 6,000 tons per year and reached a peak of 45,245 tons in 1960. Throughout, the Chisso factory's output amounted to between a quarter and a third of Japan's total acetaldehyde production. The chemical reaction used to produce the acetaldehyde used mercury sulfate as a catalyst. Starting August 1951, the co-catalyst was changed from manganese dioxide to ferric sulfide. A side reaction of this catalytic cycle led to the production of a small amount of an organic mercury compound, namely methylmercury. This highly toxic compound was released into Minamata Bay from the change of the co-catalyst in 1951 until 1968, when this production method was discontinued.


On April 21, 1956, a five-year-old girl was examined at the Chisso Corporation's factory hospital in Minamata, Kumamoto, a town on the west coast of the southern island of Kyūshū. The physicians were puzzled by her symptoms: difficulty walking, difficulty speaking, and convulsions. Two days later, her younger sister also began to exhibit the same symptoms and she, too, was hospitalised. The girls' mother informed doctors that her neighbour's daughter was also experiencing similar problems. After a house-to-house investigation, eight further patients were discovered and hospitalised. On May 1, the hospital director reported to the local public health office the discovery of an "epidemic of an unknown disease of the central nervous system", marking the official discovery of Minamata disease.

To investigate the epidemic, the city government and various medical practitioners formed the Strange Disease Countermeasures Committee (奇病対策委員会 Kibyō Taisaku Iinkai) at the end of May 1956. Owing to the localised nature of the disease, it was suspected to be contagious and as a precaution patients were isolated and their homes disinfected. Although contagion was later disproved, this initial response contributed to the stigmatisation and discrimination experienced by Minamata victims from the local community. During its investigations, the committee uncovered surprising anecdotal evidence of the strange behaviour of cats and other wildlife in the areas surrounding patients' homes. From around 1950 onward, cats had been seen to have convulsions, go mad, and die. Locals called it the "cat dancing disease" (猫踊り病 neko odori byō), owing to their erratic movement. Crows had fallen from the sky, seaweed no longer grew on the sea bed, and fish floated dead on the surface of the sea. As the extent of the outbreak was understood, the committee invited researchers from Kumamoto University to help in the research effort.

The Kumamoto University Research Group was formed on August 24, 1956. Researchers from the School of Medicine began visiting Minamata regularly and admitted patients to the university hospital for detailed examinations. A more complete picture of the symptoms exhibited by patients was gradually uncovered. The disease developed without any prior warning, with patients complaining of a loss of sensation and numbness in their hands and feet. They became unable to grasp small objects or fasten buttons. They could not run or walk without stumbling, their voices changed in pitch, and many patients complained of difficulties seeing, hearing, and swallowing. In general, these symptoms deteriorated and were followed by severe convulsions, coma, and eventually death. By October 1956, 40 patients had been discovered, 14 of whom had died, an alarming mortality rate of 35%.

Finding the causeEdit

Researchers from Kumamoto University also began to focus on the cause of the strange disease. They found that the victims, often members of the same family, were clustered in fishing hamlets along the shore of Minamata Bay. The staple food of victims was invariably fish and shellfish from Minamata Bay. The cats in the local area, which tended to eat scraps from the family table, had died with symptoms similar to those now discovered in humans. This led the researchers to believe that the outbreak was caused by some kind of food poisoning, with contaminated fish and shellfish being the prime suspects.

On November 4, the research group announced its initial findings: "Minamata disease is rather considered to be poisoning by a heavy metal, presumably it enters the human body mainly through fish and shellfish."

Identification of mercuryEdit


Methylmercury, an Organic compound of Mercury compound released in factory wastewater and the cause of Minamata disease. The uploader was Benjah-bmm27. He is not listed as the author, but probably is since he made the visually and meta-technically similar File:Butane-3D-space-filling.png image.

As soon as the investigation identified a heavy metal as the causal substance, the wastewater from the Chisso plant was immediately suspected as the origin. The company's own tests revealed that its wastewater contained many heavy metals in concentrations sufficiently high to bring about serious environmental degradation, including lead, mercury, manganese, arsenic, thallium, and copper, plus the chalcogen selenium. Identifying which particular poison was responsible for the disease proved to be extremely difficult and time-consuming. During 1957 and 1958, many different theories were proposed by different researchers. At first, manganese was thought to be the causal substance due to the high concentrations found in fish and the organs of the deceased. Thallium, selenium, and a multiple contaminant theory were also proposed, but in March 1958, visiting British neurologist Douglas McAlpine suggested that Minamata symptoms resembled those of organic mercury poisoning, so the focus of the investigation centered on mercury. Methylmercury, an organic mercury compound released in factory wastewater was predicted to be the and was the cause of Minamata disease.

In February 1959, the mercury distribution in Minamata Bay was investigated. The results shocked the researchers involved. Large quantities of mercury were detected in fish, shellfish, and sludge from the bay. The highest concentrations centred around the Chisso factory wastewater canal in Hyakken Harbour and decreased going out to sea, clearly identifying the plant as the source of contamination. Pollution was so heavy at the mouth of the wastewater canal, a figure of 2 kg of mercury per ton of sediment was measured: a level that would be economically viable to mine. Indeed, Chisso did later set up a subsidiary to reclaim and sell the mercury recovered from the sludge.

Hair samples were taken from the victims of the disease and also from the Minamata population in general. In patients, the maximum mercury level recorded was 705 parts per million (ppm), indicating very heavy exposure and in nonsymptomatic Minamata residents, the level was 191 ppm. This compared to an average level of 4 ppm for people living outside the Minamata area.

On November 12, 1959, the Ministry of Health and Welfare's Minamata Food Poisoning Subcommittee published its results: "Minamata disease is a poisoning disease that affects mainly the central nervous system and is caused by the consumption of large quantities of fish and shellfish living in Minamata Bay and its surroundings, the major causative agent being some sort of organic mercury compound."


Minamata map illustrating Chisso factory effluent routes2

The Chisso factory and its wastewater routes. Author: User:Bobo12345.

The Chisso factory and its wastewater routesEdit

During the investigation by researchers at Kumamoto University, the causal substance had been identified as a heavy metal and it was widely presumed that the Chisso plant was the source of the contamination. Chisso was coming under closer scrutiny and to deflect criticism, the wastewater output route was changed. Chisso knew of the environmental damage caused by its wastewater and was well aware that it was the prime suspect in the Minamata disease investigation. Despite this, from September 1958, instead of discharging its waste into Hyakken Harbour (the focus of investigation and source of original contamination), it discharged wastewater directly into Minamata River. The immediate effect was the death of fish at the mouth of the river, and from that point on, new Minamata disease victims began to appear in other fishing villages up and down the coast of the Shiranui Sea, as the pollution spread over an even greater area.

Chisso failed to co-operate with the investigation team from Kumamoto University. It withheld information on its industrial processes, leaving researchers to speculate what products the factory was producing and by what methods. The Chisso factory's hospital director, Hajime Hosokawa, established a laboratory in the research division of the plant to carry out his own experiments into Minamata disease in July 1959. Food to which factory wastewater had been added was fed to healthy cats. Seventy-eight days into the experiment, cat 400 exhibited symptoms of Minamata disease and pathological examinations confirmed a diagnosis of organic mercury poisoning. The company did not reveal these significant results to the investigators and ordered Hosokawa to stop his research.

In an attempt to undermine Kumamoto University researchers' organic mercury theory, Chisso and other parties with a vested interest that the factory remain open (including the Ministry of International Trade and Industry and the Japan Chemical Industry Association) funded research into alternative causes of the disease, other than its own waste.

Compensation of fishermen and patients, 1959Edit

For more details on this topic, see Minamata disease compensation agreements of 1959. Polluting wastewater had damaged the fisheries around Minamata ever since the opening of the Chisso factory in 1908. The Minamata Fishing Cooperative had managed to win small payments of "sympathy money" (見舞い金 mimaikin) from the company in 1926 and again in 1943, but after the outbreak of Minamata disease, the fishing situation was becoming critical. Fishing catches had declined by 91% between 1953 and 1957. The Kumamoto prefectural government issued a partial ban on the sale of fish caught in the heavily polluted Minamata Bay, but not an all-out ban, which would have legally obliged it to compensate the fishermen. The fishing cooperative protested against Chisso and angrily forced their way into the factory on 6 August and 12 August, demanding compensation. A committee was set up by Minamata Mayor Todomu Nakamura to mediate between the two sides, but this committee was stacked heavily in the company's favour. On 29 August, the fishing cooperative agreed to the mediation committee's proposal, stating: "In order to end the anxiety of the citizens, we swallow our tears and accept". The company paid the cooperative ¥20 million (US$183,477) and set up a ¥15 million ($137,608) fund to promote the recovery of fishing.

Since the change of route of wastewater output in 1958, pollution had spread up and down the Shiranui Sea, damaging fisheries there, too. Emboldened by the success of the small Minamata cooperative, the Kumamoto Prefectural Alliance of Fishing Cooperatives also decided to seek compensation from Chisso. On 17 October, 1,500 fishermen from the alliance descended on the factory to demand negotiations. When this produced no results, the alliance members took their campaign to Tokyo, securing an official visit to Minamata by members of the Japanese Diet. During the visit on 2 November, alliance members forced their way into the factory and rioted, causing many injuries and ¥10 million ($91,738) worth of damage. The violence was covered widely in the media, bringing the nation's attention to the Minamata issue for the first time since the outbreak began. Another mediation committee was set up, and an agreement was hammered out and signed on 17 December. Some ¥25 million of "sympathy money" was paid to the alliance and a ¥65 million fishing recovery fund was established.

In 1959, the victims of Minamata disease were in a much weaker position than the fishermen. The recently formed Minamata Disease Patients Families Mutual Aid Society was much more divided than the fishing cooperatives. Patients' families were the victim of discrimination and ostracism from the local community. Local people felt that the company (and their city that depended upon it) was facing economic ruin. To some patients, this ostracism by the community represented a greater fear than the disease itself. After beginning a sit-in at the factory gates in November 1959, the patients asked Kumamoto Prefecture Governor Hirosaku Teramoto to include the patients' request for compensation with the mediation that was ongoing with the prefectural fishing alliance. Chisso agreed and after a few weeks' further negotiation, another "sympathy money" agreement was signed. Patients who were certified by a Ministry of Health and Welfare committee would be compensated: adult patients received ¥100,000 ($917) per year; children ¥30,000 ($275) per year, and families of dead patients would receive a one-off ¥320,000 ($2935) payment.

Wastewater treatmentEdit

On October 21, 1959, Chisso was ordered by the Ministry of International Trade and Industry to switch back its wastewater drainage from the Minamata River to Hyakken Harbour and to speed up the installation of wastewater treatment systems at the factory. Chisso installed a Cyclator purification system on December 19, 1959, and opened it with a special ceremony. Chisso's president Kiichi Yoshioka drank a glass of water supposedly treated through the Cyclator to demonstrate that it was safe. In fact, the wastewater from the acetaldehyde plant, which the company knew still contained mercury and led to Minamata disease when fed to cats, was not treated through the Cyclator at the time. Testimony at a later Niigata Minamata disease trial proved that Chisso knew the Cyclator to be completely ineffective: "The purification tank was installed as a social solution and did nothing to remove organic mercury."

The deception was successful and almost all parties involved in Minamata disease were duped into believing that the factory's wastewater had been made safe from December 1959 onward. This widespread assumption meant that doctors were not expecting new patients to appear, resulting in numerous problems in the years to follow, as the pollution continued. In most people's minds, the issue of Minamata disease had been resolved.


The years between the first set of "sympathy money" agreements in 1959 and the start of the first legal action to be taken against Chisso in 1969 are often called the "ten years of silence". In fact, much activity on the part of the patients and fishermen took place during this period, but nothing had a significant impact on the actions of the company or the coverage of Minamata in the national media.

Continued pollutionEdit

Despite the almost universal assumption to the contrary, the wastewater treatment facilities installed in December 1959 had no effect on the level of organic mercury being released into the Shiranui Sea. The pollution and the disease it caused continued to spread. The Kumamoto and Kagoshima prefectural governments conducted a joint survey in late 1960 and early 1961 into the level of mercury in the hair of people living around the Shiranui Sea. The results confirmed that organic mercury had spread all around the inland sea and that people were still being poisoned by contaminated fish. Hundreds of people were discovered to have levels greater than 50 ppm of mercury in their hair, the level at which people are likely to experience nerve damage. The highest result recorded was that of a woman from Goshonoura island who had 920 ppm in her sample.

The prefectural governments did not publish the results and did nothing in response to these surveys. The participants who had donated hair samples were not informed of their result, even when they requested it. A follow-up study ten years later discovered that many had died from "unknown causes".

Congenital Minamata diseaseEdit

Local doctors and medical officials had noticed for a long time an abnormally high frequency of cerebral palsy and other infantile disorders in the Minamata area. In 1961, a number of medical professionals including Masazumi Harada (later to receive an honour from the United Nations for his body of work on Minamata disease) set about re-examining children diagnosed with cerebral palsy. The symptoms of the children closely mirrored those of adult Minamata disease patients, but many of their mothers did not exhibit symptoms. The fact that these children had been born after the initial outbreak and had never been fed contaminated fish also led their mothers to believe they were not victims. At the time the medical establishment believed the placenta would protect the foetus from toxins in the bloodstream, which is indeed the case with most chemicals. What was not known at the time was that exactly the opposite is the case with methylmercury: the placenta removes it from the mother's bloodstream and concentrates the chemical in the foetus.

After several years of study and the autopsies of two children, the doctors announced that these children were suffering from an as yet unrecognised congenital form of Minamata disease. The certification committee convened on 29 November 1962 and agreed that the two dead children and the 16 children still alive should be certified as patients, and therefore liable for "sympathy" payments from Chisso, in line with the 1959 agreement.

Outbreak of Niigata Minamata diseaseEdit

Minamata disease broke out again in 1965, this time along the banks of the Agano River in Niigata Prefecture. The polluting factory (owned by Showa Denko) employed a chemical process using a mercury catalyst very similar to that used by Chisso in Minamata. As in Minamata, from the autumn of 1964 to the spring of 1965, cats living along the banks of the Agano River had been seen to go mad and die. Before long, patients appeared with identical symptoms to patients living on the Shiranui Sea, and the outbreak was made public on 12 June 1965. Researchers from the Kumamoto University Research Group and Hajime Hosokawa (who had retired from Chisso in 1962) used their experience from Minamata and applied it to the Niigata outbreak. In September 1966, a report was issued proving Showa Denko's pollution to be the cause of this second Minamata disease.

Unlike the patients in Minamata, the victims of Showa Denko's pollution lived a considerable distance from the factory and had no particular link to the company. As a result, the local community was much more supportive of patients' groups and a lawsuit was filed against the company in March 1968, only three years after discovery.

The events in Niigata catalysed a change in response to the original Minamata incident. The scientific research carried out in Niigata forced a re-examination of that done in Minamata and the decision of Niigata patients to sue the polluting company allowed the same response to be considered in Minamata. Masazumi Harada has said that, "It may sound strange, but if this second Minamata disease had not broken out, the medical and social progress achieved by now in Kumamoto... would have been impossible."

Around this time, two other pollution-related diseases were also grabbing headlines in Japan. Victims of Yokkaichi asthma and Itai-itai disease were forming citizens' groups and filed lawsuits against the polluting companies in September 1967 and March 1968, respectively. As a group, these diseases came to be known as the four big pollution diseases of Japan.

Slowly but surely, the mood in Minamata and Japan as a whole was shifting. Minamata patients found the public gradually becoming more receptive and sympathetic as the decade wore on. This culminated in 1968 with the establishment in Minamata of the Citizens' Council for Minamata Disease Countermeasures, which was to become the chief citizens' support group to the Minamata patients. A founding member of the citizens' council was Michiko Ishimure, a local housewife and poet who later that year published Pure Land, Poisoned Sea: Our Minamata disease (苦海浄土―わが水俣病 Kugai Jōdo: Waga Minamatabyō) a book of poetic essays that received national acclaim.



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