Table 5.

Key Features of NRC Recommendation and EPA RfD

Scientific Bases for Risk Assessment
	NRC Recommendation and EPA RfD
Toxicological studies	29 toxicological studies covering acute, sub-chronic and chronic exposures and in young animals of 3 species (monkey, rat and mouse); no NOAEL detected
Epidemiological studies	16 published reports on 9 observational studies of developmental neurotoxicity in infants and children in 9 countries; studies in the Faroes Islands, Seychelles and New Zealand considered strongest basis for quantitative risk assessment because they were large, well-conducted and in the dose-range of exposed US populations
Toxicokinetics	One-compartment (IPCS 1990; EPA 1997) and PBPK (Clewell et al. 1999) models
Sensitive population/lifestage	Developing fetus to in utero exposure (reproductive age women)
Principal study	Faroes 7-year old cohort (Grandjean et al. 1997)
RfD Derivation
	NRC Recommendation	EPA RfD
Critical health effect	Developmental neurotoxicity - score on Boston Naming Test	Developmental neurotoxicity -scores on several neuropsychological test scores
BDML05	58 ppb blood	46–79 ppb blood
Uncertainty factor (biological variability and database insufficiency) and rationale	Factor of 2–3 for intra-species variability, another factor for database uncertainty with a total of “at least 10”	3-fold for toxicokinetic variability (estimating ingested MeHg dose from cord blood) and 3-fold for a total of 10
RfD	0.1 ug/kg/d	0.1 ug/kg/d

Sources: NRC 2000; EPA 2001.