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

MERCURY 239
2. HEALTH EFFECTS
None of the tests indicated an adverse effect of methylmercury exposure. As evaluated through regression
analyses, there was no reduction in performance with increasing maternal hair mercury levels for the
neurobehavioral parameters examined. In contrast, scores were better for four of the six tests in the highest
MeHg-exposed groups, compared with lower exposure groups for both prenatal and postnatal exposure (the
four test were the (1) General Cognitive Index (GCI) of the McCarthy Scales of Children's `Abilities (to
estimate cognitive ability); (2) the Preschool Language Scale (PLS) total score (to measure both expressive
and receptive language ability); (3) the Letter and Word Recognition and (4) Applied Problems subtests of
the Woodcock-Johnson (W-J) Tests of Achievement (to measure reading and arithmetic achievement).
While the positive outcomes are not considered to indicate any beneficial effect of methylmercury on
neurological development or behavior, they might be more appropriately attributed to the beneficial effects
of omega-3 fatty acids or other constituents present in fish tissue, since the methylmercury levels in hair are
known to correlate closely with fish intake. The slight decreases in the subjectively reported activity level
of boys reported in the 29-month observations were not seen during the 66-month tests. The mean maternal
hair level of 15.3 ppm in the highest exposed group in the 66-month test cohort is, therefore, considered a
NOAEL for SCDS and is used by ATSDR as the basis for derivation of a chronic oral MRL for
methylmercury. A related study (Myers et al. 1997) by some members of the same team of researchers
from the University of Rochester examined the Seychellois children for attainment of the same
developmental milestones reported to have been delayed in the Iraqi poisoning incident in the early 1970s
(Cox et al. 1989); however, unlike the Iraqi study, no delays in the age of first walking and talking was seen
in the Seychellois children exposed in utero.
Sensitivity of Neurobehavioral Measures /Reliability of Tests
The neurobehavioral test battery used in the 66-month Seychelles study was designed to assess multiple
developmental domains (Davidson et al. 1998). The tests were considered to be sufficiently sensitive and
accurate to detect neurotoxicity in the presence of a number of statistical covariates. On-site test administration
reliability was assessed by an independent scorer, and mean interclass correlations for interscorer
reliability were 0.96–0.97 (Davidson et al. 1998). The sample size was determined to be sufficient to detect
a 5.7-point difference on any test with a mean (SD) of 100 (16) between low (0–3 ppm) and high (>12 ppm)
hair mercury concentration groups for a 2-sided test (A = 0.05 at 80% power).