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

MERCURY 142
2. HEALTH EFFECTS
level of 14 ppm, was estimated for the Iraqi women using a simple, one-compartment pharmacokinetic
model (see Section 2.4).
Davidson et al. (1995b) studied the effects of prenatal methylmercury exposure from a diet high in fish on
developmental milestones in children living in the Republic of Seychelles (i.e., the Seychelles Child
Development Study (SCDS). In this double blind study, children were evaluated with the Bayley Scales of
Infant Development (BSID) at 19 months of age (n=738). The 19-month cohort represented 94% of the
initially enrolled pairs. The cohort was evaluated again at 29 months (n=736) with the BSID and the
Bayley Infant Behavior Record. Mercury exposure was determined by cold vapor atomic absorption
analysis of maternal hair segments during pregnancy. The 29-month cohort represented approximately 50%
of all live births in the year 1989. This particular study population was carefully selected based on the
following reasons: (1) they regularly consume a high quantity and variety of ocean fish; (2) pre-study
mercury concentration in maternal hair was in the appropriate range (45 ppm) to study low-level
exposure; (3) there is no local industry for pollution, and the Seychelles location is 1,000 miles from any
continent or large population center; (4) the Seychellois population is highly literate, cooperative, and has
minimal immigration; and (5) the Seychellois constitute a generally healthy population, with low maternal
alcohol and tobacco use. The association between maternal hair mercury concentrations and
neurodevelopmental outcomes at 19 and 29 months of age was examined by multiple regression analysis
with adjustment for confounding variables. Testing was performed by a team of Seychellois nurses
extensively trained in administration of the BSID.
Maternal hair concentrations measured in hair segments that corresponded to pregnancy ranged from 0.5 to
26.7 ppm, with a median exposure of 5.9 ppm for the entire study group. The mean BSID Mental Scale
Indexes at both 19 and 29 months were comparable to the mean performance of U.S. children. The mean
BSID Psychomotor Scale Indexes at 19 and 29 months were 2 standard deviation units above U.S. norms,
but consistent with previous findings of motoric precocity in children reared in African countries. No effect
of mercury was detected on BSID scores at either age. On the Bayley Infant Behavior Record, activity level
in boys, but not girls, decreased with increasing mercury exposure. The only subjective observation
correlated with prenatal mercury exposure was a slight decrease in activity level in boys (but not girls) as
determined by the Bayley Infant Behavior Record.
The overall study cohort was broken down into sub-groups based upon maternal hair mercury concentration
as follows: $3 ppm (n=164), 4–6 ppm (n=215), 7–9 ppm (n=161), 10–12 ppm (n=97), and