4.4 Minimata Methylmercury
The Chisso Corporation first opened a chemical factory in Minamata, Japan, in 1908. Initially producing fertilizers, the factory followed the nationwide expansion of Japan's chemical industry, branching out into production of acetylene, acetaldehyde, acetic acid, vinyl chloride, and other chemicals. The Minamata factory became the most advanced chemical company in Japan in the 1930s and 1940s. The waste products resulting from the manufacture of these chemicals were released right into Minamata Bay. As with any chemical put into the environment, these pollutants had an impact. Owing to the chemicals in the environment, the fisheries were damaged. The Chisso company negotiated two separate compensation agreements with the fishery cooperative in the years of 1926 and 1943 (13).
The Chisso Minamata factory was very successful and it had a very positive effect on the local economy (14). The area lacked other industry and Chisso had great influence in Minamata. Over half of the tax revenue of Minamata City authority came from Chisso and its employees. Also, the company and its subsidiaries were responsible for creating a quarter of all jobs in Minamata.
The Chisso Minamata factory first started acetaldehyde production in 1932. In the first year, they produced 210 tons. Acetaldehyde is used as a chemical intermediary in the production of numerous products, for instance, vinyl. By 1951, production had jumped to 6000 tons/year and reached a peak of 45,245 tons in 1960 (13). 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. A side reaction of the catalytic cycle led to the production of a small amount of an organic mercury compound. This compound was methylmercury (15). This very toxic compound was discharged into Minamata Bay from the start of production in 1932 until 1968. Interesting enough, elemental mercury is poorly absorbed through the skin or through ingestion. However, the vapors of elemental mercury are much more hazardous. Methylmercury is very hazardous, as compared with elemental mercury. In 1968, the production process was modified and mercury was no longer used.
On April 21, 1956, a 5-year-old girl was examined at the Chisso Corporation's factory hospital in Minamata, Japan, a town on the west coast of the southern island of Kyushu. The symptoms the child presented baffled the physicians. The child had difficulty walking, difficulty speaking, and suffered convulsions. Two days later, her younger sister also began to exhibit the same symptoms. This child was also hospitalized. The girls' mother informed doctors that her neighbor's daughter was also experiencing similar problems. After a house-to-house investigation, eight other patients were found to be experiencing the same symptoms and were hospitalized. 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 (16).
Medical researchers from the Kumamoto University began to research the disorder. 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 fish and shellfish from the bay. Local cats, which were fed scraps from the family table, had similar symptoms and had died. This led the researchers to believe that the outbreak was caused by some kind of food poisoning. Contaminated fish and shellfish were the prime suspects in the investigation.
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” (13).
The wastewater from the Chisso plant was immediately suspected as the origin when the investigation identified a heavy metal as the causal agent. The company's own tests revealed that its wastewater contained many heavy metals in high concentrations. These levels were sufficiently high to bring about serious environmental degradation. The metals included lead, mercury, manganese, arsenic, selenium, thallium, and copper. All of these can be extremely toxic if released into the environment. Identifying which particular poison was responsible for the disease could be extremely difficult. During the years of 1957 and 1958, many different theories were proposed for the cause of the ailments. Initially, manganese was thought to be the causal agent because it was found in large concentrations in fish and in the organs of the deceased victims. Thallium, selenium, and a multiple contaminant theory were also thought to be the toxic agent. In March 1958, visiting British neurologist Douglas McAlpine suggested that the symptoms shown by victims in Minamata resembled those of organic mercury poisoning. From that point forward, the focus of the investigation centered on mercury.
In February 1959, the distribution of mercury in Minamata Bay was investigated. The results showed that large quantities of mercury were detected in the aquatic environment: fish, shellfish, and sludge from the bay. At the entrance of the wastewater canal, there was approximately 4.4 lb (2 kg) of mercury per ton of sediment. This level would be economically viable to mine. The Chisso company did set up a subsidiary to reclaim and sell the mercury recovered from the sludge (13). Fifty years later, the legacy of Minamata Bay lives on. Victims still receive payment for their injuries and methylmercury still persists in the environment.