revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 258
2. HEALTH EFFECTS
data for the benchmark analysis.) Consequently, Clewell et al. (1998) concluded that using a NOAEL of
7 ppm (21 ppm / 3 (UF) provides additional protection against the possibility that effects could occur at lower
concentrations in some populations. Based upon this reasoning, Clewell and his colleagues recommended a
health guidance value (i.e., an RfD) of 0.0004 mg/kg/day. If a modifying factor of 1.5 is used to further
address the domain-specific findings in the Faroe study, a final MRL of 0.3 µg/kg/day results.
The above benchmark analysis of 29-month data from the Seychelles Child Development Study strongly
supports the MRL of 0.0003 mg/kg/day calculated by ATSDR in this profile. Similarly, addressing the
Seychellois 66-month data from the perspective of using the mean value (15.3 ppm) of the highest exposure
group in the study, a method prescribed in ATSDR's published guidance for MRL development (Chou et al.
1998), also results in an identical MRL. ATSDR therefore has high confidence that this level is protective of
the health of all potentially exposed human populations.
Employment of the Chronic Oral MRL for Methylmercury
It should be emphasized that the MRL is considered by ATSDR to be a level of exposure to a single
chemical/substance which is considered “safe” for all potentially exposed populations for a specified
duration of time (acute, intermediate, or chronic). It is not considered be a threshold for adverse effects, and
not address the likelihood of adversity at any incremental level above the MRL value. ATSDR notes that the
0.3 µg/kg/day chronic oral MRL for methylmercury is in close agreement with the tolerable daily intake
(ADI) levels of 0.47 and 0.48 µg/kg/day established by the FDA and WHO, respectively.
MRLs are, by definition (Chou et al. 1998), substance-specific and do not include effects attributable
to interaction (whether additive, synergistic, or antagonistic) with other chemicals or environmental
substances. Their relevance to the mission of ATSDR is to assist public health officials in the
identification of chemicals/elements of potential health concern at hazardous waste sites. The ATSDR
MRL is not intended to be used in the regulatory or site clean-up process, but is instead intended to
serve as a basis of comparison with actual measured levels of environmental exposure. Further, the role of
informed biomedical judgment is crucial in the application of any MRL, or the media-specific health
guidance values (HGVs) derived from them, in any given exposure scenario (Risher and De Rosa 1997).
MRLs for a particular substance are based upon the most sensitive effect/endpoint in that portion of the
human population considered to be most susceptible to injury from exposure to that substance. Thus, the
MRL has never been intended as a one-size-fits-all tool for all hazardous waste site exposure scenarios;
rather, it is merely a starting point for further examination of potential health risk. Therefore, at sites