Transition in conditions of pollution
In the area around Minamata Bay and the Agano River basin, until now various types of investigations about water quality, bottom sediment, fish and shellfish, hair sample, etc., have been carried out with respect to environmental pollution As a result, it is thought that continuous methylmercury exposure at the level which could cause Minamata Disease existed until no later than 1968 in the area around Minamata Bay, and until no later than 1965 in the Agano River basin, and that, from that time, there has not been such exposure that could lead to the occurrence of Minamata Disease.
Monitoring of methylmercury concentration of fish and shellfish continues even today in the area around Minamata Bay and in the Agano River basin.