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

MERCURY 240
Supporting Studies
2. HEALTH EFFECTS
Crump et al. (1998) conducted benchmark dose (BMD) calculations and additional regression analyses of
data collected in a study in which a series of scholastic and psychological tests were administered to
children whose mothers had been exposed to methylmercury during pregnancy. Hair samples were
collected from 10,970 new mothers in New Zealand in 1977 and 1978. High hair mercury levels were
considered to be those over 6 ppm, which was the hair level predicted to result at steady state from
consumption of mercury at the WHO/FAO Provisional Tolerable Weekly Intake of 0.3 mg total
mercury/week and 0.2 mg methylmercury/week. By this criterion, 73 of approximately 1,000 mothers who
had consumed fish more than 3 times/week during pregnancy were determined to have high hair mercury
levels. In 1985, when the children were 6 to 7 years of age, 61 children (1 set of twins) of the 73 mothers in
the high hair mercury group were located; these children constituted the high exposure group, which was
matched with three control groups (one with 3–6 ppm maternal hair mercury levels, one with 0–3 ppm
whose mothers had been high fish consumers, and one with 0–3 ppm whose mothers had not been high fish
consumers). The entire study cohort consisted of 237 children. A battery of 26 psychological and
scholastic tests were administered to the children at school during the year 1985. Mothers were interviewed
at the time of test administration to obtain additional data on social and environmental factors. In the high
exposure group of children, one boy’s mother had a hair mercury level of 86 ppm, which was more than
four times higher than the next highest hair mercury level of 20 ppm. BMDs (10% response rate) calculated
from five tests ranged from 32 to 73 ppm, when the 86 ppm mother’s child was included. This
corresponded to a BMDL range of 17 to 24 ppm. Although none of the 86 ppm child’s test scores was an
outlier according to the definition used in the analyses, his scores were significantly influential in the
analyses. When this child was omitted from the analyses, BMDs ranged from 13 to 21, with corresponding
BMDLs of 7.4 to 10 ppm.
Developing fetuses in the SCDS were exposed through maternal fish ingestion before and during
pregnancy. Each child was evaluated at 19 months and again at 29 months (±2 weeks) for infant
intelligence (Bayley Scales of Infant Development [BSID] Mental and Psychomotor Scales), with a
modified version of the BSID Infant Behavior Record to measure adaptive behaviors at 29 months
(Davidson et al. 1995b). 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 Indices determined at both 19 and 29 months were found to be comparable