MeHg chloride, which was produced as a by-product in the acetaldehyde plant of the
chemical factory located there, was detected in the wastewater from the acetaldehyde plant,
but not from its vinyl chloride plant. Initially, this contaminant had been released into
Minamata Bay for more than a decade causing severe acute poisoning cases in the vicinity.
Acetaldehyde is an intermediate by-product in the manufacture of plastics and its production
had expanded steadily to meet the growing demands for plastics at that time. In order to
increase production, the factory had expanded the manufacturing plants and changed the
drainage site from Minamata Bay to the mouth of the Minamata River [3]. This resulted in
further dissemination of the pollution into the surrounding waters of the Shiranui Sea. The
factory continued the production of acetaldehyde and the release of MeHg into the sea waters
until 1968. Because fishing in that part of the Shiranui Sea was never restricted, people living
in coastal areas–many of whom depended on the sea for a large part of their food supply–were
exposed to MeHg by ingestion of polluted fish [4] resulting in chronic MeHg poisoning