Minamata Disease, methylmercury poisoning, was recognized late in 1953 among the inhabitants living around Minamata Bay. In a chemical factory situated near Minamata Bay, acetaldehyde had been synthesized by the hydration of acetylene till 1968; in the reaction mercury oxide dissolved in sulfuric acid had been used as a catalyst. Inorganic mercury in waste water from the acetaldehyde plant had been discharged into the bay and it had accumulated into bottom sediment. It was proved that a part of inorganic mercury used as the catalyst had changed into methylmercury by a sidereaction in the plant and waste water containing methylmercury from the plant had discharged into the bay and methylmercury had accumulated into the fishes. The mercury concentrations in the muds were very high: in 1963, 29~713 ppm (dry weight); in 1969, 19~908 ppm (dry weight); in 1970, 8~253 ppm (dry weight) and in 1971, 14~586 ppm (dry weight). Since 1977, dredging work had been carried out to remove mercury-contaminated mud and all of the work had finished at March 1990. The concentration of mercury in fishes from the bay was very high in 1959: shellfishes 108~178 ppm (dry weight) and fish 15 ppm (wet weight). Mercury concentration in fishes has decreased markedly since 1966. Total mercury concentration in fishes (87 species) were 0.01~1.74 ppm (wet weight) and fishes containing over 0.4 ppm of total mercury were 16 species in 1989. The hair of patients contained a high concentration of mercury, the highest being 705 ppm. In 1968, the average mercury concentration in patients was 10.6 ppm, for fishermen, the average was 9.2 ppm, and for general inhabitants, the average was 8.1 ppm. In 1982, the average methylmercury concentration in fishermen was 6.15 ppm and for general inhabitants, the averge was 3.78 ppm. Thus, the mercury content in hair decreased gradually with time. After treatment of mercury in waste water was initiated, the mercury content in fishes from Minamta Bay was gradually reduced. It is necessary to supervise strictly to avoid mercury pollution of the environment.

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