(1) Conditions and 7tends of Methylmercury Compound Pollution

Mercury Poisoning and Control Responses

Methylmercury compound, inorganic mercury consisting of methyl and mercury, is a highly toxic substance. It is a by-product produced from mercury sulfate, which had been used at Chisso Minamata Plant as a catalyst for manufacturing acetaldehyde which is processed into acetic acid and other chemicals (See Figure 3-3).
From the acetaldehyde manufacturing process, effluents containing methylmercury compound had been released into Minamata Bay and the mouth of the Minamata River since the plant operation started in 1932. Mercury discharge peaked in 1959 and then gradually declined year by year as a result of various preventive measures taken by Chisso at later stages, and finally shut off by the total circulation system adopted in 1966.
The temporal variations in the acetaldehyde production, mercury discharge and annual increment of officially-recognized patients are shown in Figure 3-4.
Water, bottom sludge, fish, shellfish, and human hair were sampled in the vicinity of Minamata Bay and methylmercury concentration was measured. The results of the above shows that continuous methylmercury exposure at a certain level of intensity that may cause Minamata Disease has not been observed since 1968 (See Figure 3-5).
The measurement of mercury concentration in fish and shellfish continues up to the present in the area around Minamata Bay.