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

MERCURY 143
2. HEALTH EFFECTS
>12 ppm (n=99). No significant or remarkable effect on the activity of the respective groups of children
was observed outside the highest concentration group (i.e., maternal hair concentrations >12 ppm). When
boys were examined separately, there appeared to be a trend toward decreased activity with increasing
mercury concentrations, most visible above the group median value of 5.9 ppm. The mercury effect was
highly significant in males (p=0.0004), but it was not statistically significant (p=0.87) in females. The
activity level scores for males decreases 1/10 point (on a 9-point scale) for every ppm of mercury in
maternal hair. While the activity score for the overall cohort was comparable to the mode of 5 for U.S.
children, those children born to mothers with hair mercury levels of 20 ppm, males scored >1 point below
the U.S. mode value. Scores of females remained at the comparable value for U.S. children, regardless of
the magnitude of maternal hair mercury level. When the subjective activity scores for male and female
children are evaluated collectively, no significant/remarkable decrease in activity is apparent outside the
>12 ppm maternal hair concentration group. The affect on activity level in boys is not considered an
adverse effect, and the 5.9 ppm level is categorized as a NOAEL. Since the children had been exposed in
utero, they represent the most sensitive subpopulation.
Myers et al. (1997) evaluated the population of the SCDS for developmental milestones similar to those
determined in Iraq. As part of this ongoing study, cohort children were evaluated at 6.5, 19, 29, and
66 months of age. At 19 months care-givers were asked at what age the child walked (n=720 out of 738)
and talked (n=680). Prenatal mercury exposure was determined by atomic absorption analysis of maternal
hair segments corresponding to hair growth during the pregnancy.
The median mercury level in maternal hair for the cohort in this analysis was 5.8 ppm with a range of
0.5–26.7 ppm. The mean age (in months) at walking was 10.7 (SD=1.9) for females and 10.6 (SD=2.0) for
males. The mean age for talking (in months) was 10.5 (SD=2.6) for females, and 11.0 (SD=2.9) for males.
After adjusting for covariates and statistical outliers, no association was found between the age at which
Seychellois children walked or talked and prenatal exposure to mercury. The ages for achievement of the
developmental milestones were normal for walking and talking in the Seychellois toddlers following
prenatal exposure to methylmercury from a maternal fish diet. The 5.8 ppm NOAEL of this study is thus
considerably below the one derived from the dose-response analysis of the data for the Iraqi methymercury
poisonings (10 ppm).
The SCDS cohort continues to be monitored and evaluated for developmental effects. In an analysis of the
latest round of outcome measures for children at age 66 months (n=708), Davidson et al. (1998) report no