Minamata Disease (1956): Industrial Mercury Poisoning in Japan

Minamata disease is a severe form of mercury poisoning that was officially identified in the fishing town of Minamata, in southern Japan, on 1 May 1956. It was caused by methylmercury released in waste water from the Chisso Corporation’s chemical factory, which contaminated the fish and shellfish eaten by local families. The disaster became one of the world’s best-known cases of industrial pollution and gave its name to a global treaty on mercury.

Background: Chisso and Minamata

Minamata lies on the Shiranui Sea on the west coast of Kyushu, in Kumamoto Prefecture. In the early twentieth century it became a company town for the Chisso Corporation (originally Nippon Chisso Hiryo), which produced fertilisers and later chemicals. Chisso was one of Japan’s leading chemical firms and the town’s largest employer.

From 1932 the Minamata factory produced acetaldehyde, a chemical used to make plastics and other products, using mercury sulphate as a catalyst. A by-product of the process was methylmercury, an organic form of mercury that is far more toxic than the metal itself. Waste water containing it flowed into Minamata Bay and later into the wider Shiranui Sea. Production expanded rapidly during Japan’s post-war economic recovery in the 1950s.

The outbreak

Strange signs appeared in the early 1950s. Fish floated dead in the bay, seabirds fell from the sky, and cats in fishing villages behaved erratically, convulsing and sometimes falling into the sea. Local people called it the “dancing cat disease”.

Then people became ill. In April 1956 a young girl from a fishing family was brought to the Chisso factory hospital, unable to walk or speak properly; her younger sister soon showed the same symptoms. On 1 May 1956 the hospital director, Dr Hajime Hosokawa, reported to the Minamata public health office that an unknown disease of the central nervous system had broken out. This date is regarded as the official discovery of Minamata disease.

Patients suffered numbness, tremors, difficulty walking, narrowed vision, slurred speech and hearing loss. Many died within months. Some babies were born with severe disabilities because their mothers had eaten contaminated fish during pregnancy; methylmercury passed through the placenta even when the mothers themselves showed few symptoms. This became known as congenital Minamata disease.

Most of the early victims came from poor fishing families in small villages around the bay, who ate fish and shellfish every day. The disease brought not only illness but also discrimination: neighbours at first feared it was contagious, and fishermen lost their livelihoods when buyers refused fish from the area. Some families hid their sick relatives rather than face suspicion and hostility from the community.

Finding the cause, and denying it

A research group from Kumamoto University quickly concluded that the illness was a form of poisoning from eating seafood from Minamata Bay. In 1959 it identified organic mercury as the cause and pointed to the Chisso factory. The same year Dr Hosokawa fed factory waste water to cats in secret experiments; one cat, known as Cat 400, developed the disease in October 1959. Chisso ordered him to stop and the result was not made public at the time.

The company disputed the university’s findings, and the chemical industry promoted other explanations, such as explosives dumped after the war. In December 1959 Chisso installed a waste-water treatment device, the “Cyclator”, which was presented as a solution but did not remove methylmercury. In the same month the company signed a “sympathy money” agreement with victims’ groups, giving small payments in exchange for a promise not to seek further compensation, even if the factory were later proved responsible.

Chisso kept discharging mercury waste until it ended acetaldehyde production in May 1968. Only on 26 September 1968 did the Japanese government officially state that methylmercury from the Chisso factory was the cause of the disease.

A second outbreak in Niigata

In 1965 a second outbreak was identified along the Agano River in Niigata Prefecture, on the other side of Japan. It was caused by methylmercury from a Showa Denko acetaldehyde plant. This “Niigata Minamata disease” showed that the danger was not unique to one company and added to public pressure for action.

The struggle for justice

From the late 1960s victims and their families took their case to court and confronted Chisso directly, including sit-ins at the company’s Tokyo headquarters. On 20 March 1973 the Kumamoto District Court ruled that Chisso had been negligent and ordered it to pay compensation. Together with three other pollution trials of the period, the case helped push Japan to adopt strict anti-pollution laws in the early 1970s.

The American photographer W. Eugene Smith and his wife Aileen documented the victims between 1971 and 1973. Their photographs, published in Life magazine and in the book Minamata (1975), brought the disaster to an international audience. In 1972 Smith was seriously injured when he was attacked by men employed by Chisso at a company plant in Goi, near Tokyo. The writer Michiko Ishimure’s book Kugai Jodo (Paradise in the Sea of Sorrow, 1969) gave voice to the victims in Japan.

The question of who counted as a victim remained bitterly disputed. The official certification criteria were strict, and many people with milder symptoms were refused. Political settlements in 1995 and under a special law of 2009 provided payments to tens of thousands of further claimants, though lawsuits continued afterwards.

By the numbers

Legacy and the Minamata Convention

Between 1977 and 1990 the most contaminated sediments in Minamata Bay were dredged and part of the bay was reclaimed; a memorial park now stands on the reclaimed land. A net that had closed off the bay to stop contaminated fish from leaving was removed in 1997 after fish were declared safe.

Minamata became a symbol of the human cost of rapid industrial growth, alongside later disasters such as the Bhopal gas tragedy of 1984. It also influenced the environmental movements that grew in the 1960s after works such as Silent Spring.

In October 2013 delegates from around the world met in Kumamoto Prefecture to adopt the Minamata Convention on Mercury, named in memory of the victims. The treaty, which entered into force on 16 August 2017, aims to reduce mercury use and emissions worldwide.

Timeline

• 1932: Chisso’s Minamata factory begins producing acetaldehyde using a mercury catalyst.
• Early 1950s: Fish, birds and cats around Minamata Bay die or behave abnormally.
• 1956, 1 May: Dr Hajime Hosokawa reports an unknown nervous disease; Minamata disease is officially identified.
• 1959: Kumamoto University identifies organic mercury as the cause; the “sympathy money” agreement is signed in December.
• 1965: A second outbreak is identified in Niigata Prefecture.
• 1968, May: Chisso stops acetaldehyde production at Minamata.
• 1968, 26 September: The Japanese government officially recognises the cause.
• 1973, 20 March: The Kumamoto District Court finds Chisso liable.
• 2013, October: The Minamata Convention on Mercury is adopted in Japan.
• 2017, 16 August: The convention enters into force.

Why it matters

Minamata disease showed how industrial waste could poison an entire community through its food, and how corporate and government denial could prolong the harm for years. Its victims’ long fight helped shape modern pollution law in Japan and inspired a worldwide treaty on mercury.