How to Determine the Relief Target for Minamata Disease

Abstract

Minamata disease that occurred in southern Kyushu and Niigata prefecture in Japan was caused by ingesting seafood that is contaminated by methylmercury discharged from factories. This is a world-renowned pollution disease. Minamata disease is judged from the symptoms peculiar to mercury poisoning. However, it is known that mercury poisoning causes other disorders even at low concentrations of exposure, especially in fetal exposure. Those who are not certified as Minamata disease patients is widely rescued if they have eaten seafood in the area and have some kind of disability. In this chapter, we will discuss the concept of the precautionary principle and its scope of application through the case of Minamata disease. The key concepts are benchmark dose (BMD) and its lower limit (BMDL). When a person’s symptoms occur at a certain frequency, say 5%, without mercury intake, with a higher probability of a certain exposure of mercury intake, say 10%, this exposure level is called BMD. BMD is used for tolerable intake. The relationship between this exposure and the incidence of symptoms is estimated by a dose–response relationship, but the estimation includes uncertainty. Based on the precautionary principle, the fifth percentile of BMD is calculated. This is called BMDL. It is used as a recommendation based on the precautionary principle as a guide to refrain from mercury intake. In Japan, this is used as the standard for people who are eligible for relief from Minamata disease. In that case, 5% of people who are not actually exposed to mercury will be eligible for relief, and there will be less than 10% of true relief recipients due to mercury exposure.