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

MERCURY 237
C
2. HEALTH EFFECTS
levels (r=0.6–0.8). Concentrations in maternal blood correlated with infant blood levels (r=0.65); and infant
blood levels correlated to infant brain levels (r=0.4–0.8).
Accordingly, ATSDR used maternal hair mercury levels as the exposure measurement to derive a chronic
MRL for methylmercury. While hair analysis can be confounded by outside sources of contamination (e.g.,
as might occur in certain occupational settings) (Hac and Krechniak 1993), the study population used as the
basis of the chronic oral MRL for methylmercury is far removed from external or industrial sources of
mercury, effectively eliminating this as a consideration for the following analysis.
An MRL of 0.0003 mg Hg/kg/day has been derived for chronic-duration oral exposure (365 days
or longer) to methylmercury.
The chronic oral MRL for methylmercury is based upon the Seychelles Child Development Study (SCDS),
in which over 700 mother-infant pairs have, to date, been followed and tested from parturition through
66 months of age (Davidson et al. 1998). The SCDS was conducted as a double-blind study and used
maternal hair mercury as the index of fetal exposure. Enrollees were recruited by the head nurse/hospital
midwife by asking the mothers if they wished to participate in the study when they arrived at the hospital
for delivery. The first 779 who did not decline participation became the mothers in the study cohort. Of the
initial 779 mothers enrolled in the study at parturition, 740 remained at the pre-determined child testing age
of 6.5 months, 738 remained in the 19-month cohort, 736 remained at 29 months, and 711 remained for the
66-month neurobehavioral and developmental examinations.
The Seychellois were chosen as a study population for a number of reasons.
C (1) All fish contain some level of methylmercury (Davidson et al. 1998); and the Seychellois
regularly consume a high quantity and variety of ocean fish, with 12 fish meals per week representing
a typical methylmercury exposure.
C (2) The median total mercury concentration in 350 fish sampled from 25 species consumed by the
Seychellois was