1.0 mg/g for largefish, such as swordfish and tuna, are not based on a detailed risk assessment,
but reflect methylmercury concentrations prevalent in seafood. A healthy diet that includes
two fish dinners per week must be based on seafood that averages a mercury concentration of
no more than 0.1 mg/g to avoid exceeding the reference dose (RfD) of 0.1 mg/kg body weight
per day defined by the U.S. Environmental Protection Agency (2001). Popular and nutritious
fish with low mercury concentrations include salmon, sardine, and flounder.
Exposure limits for methylmercury have been revised downward to protect sensitive life
stages (National Research Council, 2002; JECFA, 2003). The major limits are the reference
dose (RfD) of 0.1 mg/kg body weight per day (National Research Council, 2002) and the
provisional tolerable weekly intake (PTWI) of 1.6 mg/kg body weight per week (JECFA,
2003) correspond to hair mercury concentrations of approximately 1–2 mg/g. Data on blood
mercury concentrations in the U.S. general population suggest that between 5 and 10% of
different population sections exceed the RfD.