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Methylmercury (MeHg) is formed by saprophyte microorganisms from inorganic mercury compounds in the aquatic environment.1) It is accumulated in fish and shellfish through the marine food web. Since the MeHg accumulation increases with the food web, carnivorous fish such as tuna, swordfish and shark often exhibit high levels of mercury. Furthermore, due to the long biological half-life of MeHg, the chemical tends to accumulate throughout the life of fish.2) Marine mammals such as whales and dolphins also show high concentrations of mercury. Accordingly, the major route of human exposure to MeHg is the ordinary consumption of fish and shellfish. MeHg is readily absorbed from the gastrointestinal tract and distributed among various tissues including the brain. The permeability of the chemical at the blood-brain barrier is responsible for its hazardousness neurotoxic effect.A WHO report 3) concluded that the no observed adversary effect level (NOAEL) for adults is 50 µg/ g of the hair mercury level based on the analytical data of MeHg pollution in the past. Since the developing nervous system of the fetus has been considered highly susceptible to the effect of MeHg, 4) the report also mentioned a possible association with an increased risk to the neurodevelopment of the fetus when maternal hair levels rise above 10 µg/g. Accordingly, recent studies on the health effects of MeHg have focused on the exposure risk to pregnant women and the neuropsychological outcomes in newborns.In Japan, the provisional regulatory standards of mercury and MeHg in fish and shellfish were determined in 1973 to be 0.4 and 0.3 µg/g, respectively, based on the assumption of a safe intake limit of (Received December 3, 2003) Methylmercury (MeHg) is an environmental pollutant with neurotoxic effects on the central nervous system. The major exposure route of MeHg to humans is via consumption of fish and shellfish which accumulate the chemical through the food web in an aquatic environment. We have been conducting a survey on hair mercury contents among general populations from different districts to estimate the current Japanese MeHg exposure level. In Japan, a provisional regulatory standard of mercury and MeHg in fish and shellfish was determined in 1973 based on the assumption of a safe intake limit of 0.17 mg/person/week (0.48 µg/kg/day). On the other hand, the US EPA issued a revised reference dose based on a cohort study conducted in the Faroe Islands. Recently, a provisional tolerable weekly intake (PTWI) of MeHg was revised to 1.6 µg/kg/week in 61st Joint FAO/WHO Expert Committee on Food Additives (JECFA), which was about half that of the Japanese standard. The distribution of hair mercury levels in Japanese populations currently obtained from 10 districts indicated that 25% of the Japanese females of childbearing age were estimated to be exposed to MeHg over the PTWI level. This would reflect the high Japanese consumption of marine products. Not only mercury contamination, but also the nutritional benefit may have to be considere...

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