current EPA RfD for MeHg is 0.1 µg/kg-day. The RfD is an important risk-characterization tool that is broadly used as a measure of the “acceptability” of population exposure levels. It is used to guide risk-management decisions and regulatory policies ranging from fish-consumption advisories to air-emission permits. This section provides an overview of the development of the MeHg RfD.
Neurotoxicity in children exposed in utero is the health outcome selected by EPA for the current MeHg RfD. The RfD is based on data from the Iraqi poisoning episode, where the population consumed high levels of MeHg from treated seed grain. The critical study for the RfD conducted by Marsh et al. (1987) identified 81 children who had been in utero during the episode and examined their neurodevelopmental outcomes. Maternal-child pairs were selected from one of five Hg-hair-concentration groups, and the combined incidence of developmental effects (late walking, late talking, mental symptoms, seizures, or increased neurological score) was determined for each group. Exposure levels measured by maternal-hair concentration and combined developmental effects were used to estimate a benchmark dose. The benchmark dose of 11 ppm of Hg in hair was calculated as the 95% lower confidence limit on the maternal-hair concentration corresponding to a 10% extra risk level (Crump et al. 1995). In this report, the lower confidence limit is referred to as the BMDL. The following section describes how EPA derived the current RfD from that value.
A ratio of 250:1 was used to convert hair Hg concentration (mg of Hg/kg of hair) to blood Hg concentration (mg of Hg/L of blood) to derive the RfD critical dose (EPA 1997c):
11 mg/kg of hair would correspond to 11/250 = 44 µg/L of blood.
The following equation was used to obtain a daily dietary intake of MeHg that results in a blood Hg concentration of 44 µg/L:
d = daily dietary intake (micrograms of MeHg per kilogram of body weight per day),
C = concentration in blood (44 µg/L),
b = elimination constant (0.014 days-1),