Minamata disease (Japanese: 水俣病, Hepburn: Minamata-byō) is a neurological disease 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 form of the disease in babies, with microcephaly, extensive cerebral damage, and symptoms similar to those seen in cerebral palsy also occurred. Minamata disease was first discovered in the city of Minamata, Kumamoto Prefecture, Japan, in 1956. It was caused by the release of methylmercury in the industrial wastewater from a chemical factory owned by the Chisso Corporation, which continued from 1932 to 1968. It has also been suggested that some of the mercury sulfate in the wastewater was also metabolized to methylmercury by bacteria in the sediment. This highly toxic chemical bioaccumulated and biomagnified in shellfish and fish in Minamata Bay and the Shiranui Sea, which, when eaten by the local population, resulted in mercury poisoning. The poisoning and resulting deaths of both humans and animals continued for 36 years, while Chisso and the Kumamoto prefectural government did little to prevent the epidemic. 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 people had been officially recognized as having Minamata disease and over 10,000 had received financial compensation from Chisso. By 2004, Chisso 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.
1908–1955
In 1908, the Chisso Corporation first opened a chemical factory in Minamata, Kumamoto Prefecture, located on the west coast of the southern island of Kyūshū. 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. The waste products resulting from the manufacture of these chemicals were released into Minamata Bay through the factory wastewater. These pollutants had a deleterious environmental impact. Fisheries were damaged in terms of reduced catches; in response, Chisso reached two separate compensation agreements with the fishery cooperative in 1926 and 1943. The rapid expansion of the Chisso factory spurred on the local economy, and as the company prospered so did Minamata. This fact, combined with the lack of other industry, meant that Chisso had great influence in the city. 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. The city 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 factory first started acetaldehyde production in 1932, with 210 tons that year. In 1951, production had jumped to 6,000 tons and eventually peaked at 45,245 tons in 1960. The factory's output historically amounted to between a quarter and a third of Japan's total acetaldehyde production. The chemical reaction used to produce the acetaldehyde employed mercury sulfate as a catalyst. Starting in 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 significant amount (about 5% of the outflow) of the organic mercury compound methylmercury. As a result of the catalyst change, this highly toxic compound was released into Minamata Bay regularly between 1951 and 1968, when this production method was finally discontinued.
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