Mercury is present in the environment in elemental, inorganic and organic (methylated) forms. As methylmercury bioaccumulates up the marine food chain, large predatory species such as tuna, shark and swordfish have high concentrations of methylmercury in their tissue. Almost all of the mercury in marine fish is methylmercury. As small cetaceans such as toothed whale and dolphin are long-lived and occupy the top of the marine food chain, they contain more mercury in their tissue than large predatory fish. Small cetaceans demethylate methylmercury into inorganic mercury and store it in the liver as mercury selenide. Marine seafood is one of the major sources of mercury in the human food chain.
The provisional tolerable weekly intake (PTWI) for methylmercury of 3.3 μg/kg-body weight for the general population was established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 1972. In 1973, the Japanese Ministry of Health and Welfare set the provisional permitted levels of total mercury and methylmercury in marine food at 0.4 and 0.3 μg/wet-g, respectively. These levels were based on the PTWI of JECFA. In 2003, from the outcomes of two cohort studies in the Faroe Islands and Seychelles Islands regarding the effect of fetal methylmercury exposure on children’s development, JECFA set the PTWI of methylmercury at 1.6 μg/kg-body weight for pregnant women. In 2005, the Japanese Food Safety Commission announced a tolerable weekly intake (TWI) for methylmercury of 2.0 μg/kg-body weight for women who are or may be pregnant. The Japanese average daily intake of mercury from foods is about 60% of the TWI.