Minamata disease is a neurological syndrome caused by severe mercury poisoning. Signs and 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 perfecture, 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. The highly toxic chemical bio-accumulated in shellfish and fish in Minamata Bay and the Shiranui Sea, which when eaten by the local population resulted in mercury poisoning. While cat, dog, pig and human deaths continued for 36 years.
The Chisso corporation first opened a chemical factory in Minamata in 1908. Initially producing fertilizers, the factory allowed the nationwide expansion of Japan’s chemical industry, branching out into production of acettylene, acetaldehyde, acetic acid, vinyl chloride and octanol. The waste products resulting from the manufacture of these chemicals were released into Minamata Bay through the factory waste water. These pollutants had an environmental impact and Fisheries were damaged and in response, Chisso reached two separate compensation agreements with the fishery cooperative in 1926 and 1943. In 1951 Chisso changed the co-catalyst from manganese dioxide to ferric sulfide to produce acetaldehyde using 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, namely methylmercury. This highly toxic compound was released into Minamata Bay from the change of the co-catalyst in 1951 until 1968.
On April 21, 1956 a five year old girl was examined at the Chisso Corporation factory hospital, 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. After a house to house investigation, eight further patients were discovered and hospitalized. To investigate the epidemic, the city government and various medical practitioners formed the Strange Disease Countermeasures Committee and they uncovered surprising anecdotal evidence of the strange behavior of cats and other wildlife in the areas surrounding patients’ homes. Crows had fallen from the sky, seaweed no longer grew on the sea bed and fish floated dead on the surface of the sea.
Researchers from Kumamoto University were invited 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, presumable it enters the human body mainly through fish and shellfish.
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 high concentration such as lead, mercury, manganese, arsenic, thallium, copper and selenium. In February 1959, the mercury distribution in Minamata Bay was investigated. Large quantities of mercury were detected in fish, shellfish and sludge from the bay. The highest concentrations centered around the Chisso factory wastewater canal and decreased going out to sea, clearly identifying the plant as the source of contamination. 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 ppm indicating very heavy exposure and in non-symptomatic Minamata residents the level was 191 ppm. This compared to an average level of 4 ppm for people living outside Minamata area.
After the confirmation of cause and source of the heavy metal, Chisso was coming under closer scrutiny and to deflect criticism the wastewater output route was changed. Chisso knew the environmental damage caused by its wastewater and was well aware that it was prime suspect in the Minamata disease invetigation. Instead of discharging its waste into Hyakken Harbour, it discharged wastewater into Minamata River. The immediate effect was the death of fish at mouth of the river and from that point on, new Minamata disease victims began to appear in other fishing villages uo and down the coast as the pollution spread over an even greater area.
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 and opened it with a special ceremony where Chisso’s president Kiichi Yoshioka drank a glass of water supposedly treated through the Cyclator to demonstrate that it was safe. Testimony at a later 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. 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 of the people’s mind, the issue of Minamata disease had been resolved.
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 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. The 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.
Japan has learnt lots of lessons from the Minamata Disease and several Measures were implemented for mercury management.
Establishment of environmental standards and emission reduction for public water bodies, ground water and soil.
Establishment of health risk evaluation guideline value for ambient air and emission reduction for atmosphere.
Establishment of special treatment standards for waste containing mercury above standard values.
Mercury free dry cell batteries achieved in early 1990s.
Reduction of mercury volume encapsulated in fluorescent lamps, promotion of LED lamps.
Closure of all primary mercury mines in Japan by 1974.
Discontinuation of the use of mercury in manufacturing processes.
As a result of the measures and actions, domestic mercury demand declined from its highest value of 2500 tons to approximately 9 tons.