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

MERCURY 307
2. HEALTH EFFECTS
More recently, Grandjean et al. (1997b, 1998) evaluated a cohort of 1,022 consecutive singleton births
generated during 1986–1987 in the Faroe Islands. Increased methylmercury exposure from maternal
consumption of pilot whale meat was indicated by mercury concentrations in cord blood and maternal hair.
Neurophysiological tests emphasized motor coordination, perceptual-motor performance, and visual acuity;
pattern reversal visual evoked potentials (VEP) with binocular full-field stimulation, brain stem auditory
evoked potentials (BAEP), postural sway, and the coefficient of variation for R-R interpeak intervals
(CVRR) on the electrocardiogram were measured. Clinical examination and neurophysiological testing did
not reveal any clear-cut mercury-related abnormalities. However, mercury-related neuropsychological
dysfunctions were most pronounced in the domains of language, attention, and memory, and to a lesser
extent in visuospatial and motor functions. These associations remained after adjustment for covariates and
after exclusion of children of mothers with maternal hair mercury concentrations above 10 µg/g (50 nmol/g).
The effects on brain function associated with prenatal methylmercury exposure appear widespread, and early
dysfunction is detectable at exposure levels currently considered safe.
There are differences in the outcomes of these epidemiology studies on low level chronic exposures to
methylmercury in foods. Davidson et al. (1998) report no adverse developmental effects associated with
prenatal and postnatal exposure to methylmercury in fish in a Seychelles Island cohort of children at age
66 months (n=708). The exposure levels are reflected in maternal hair levels of 6.8 ppm for the prenatal
exposure (SD=4.5, n=711) and children’s hair levels of 6.5 ppm (SD=3.3, n=708) for both the prenatal and
subsequent postnatal exposure. The age-appropriate main outcome measures included: (1) the McCarthy
Scales of Children’s Abilities, (2) the Preschool Language Scale, (3) the Woodcock-Johnson Tests of
Achievement - Letter and Word Recognition, (4) Woodcock-Johnson Tests of Achievement - Applied
Problems and, (5) the Bender Gestalt test, and (6) the Child Behavior Checklist. The test results were
similar to what would be expected from a healthy, well-developing U.S. population. No test indicated a
deleterious effect of methylmercury from the exposure levels received in this population. Four of the six
measures showed better scores in the highest MeHg groups compared with lower groups for both prenatal
and postnatal exposure. This result is likely due to the benefits of increased levels of fish in the diet,
possibly because of increased consumption of omega-3-fatty acids. Serum from a subset of 49 of the
children was sampled for polychlorinated biphenyl levels (PCBs). None of the samples had detectable
levels (detection limit 0.2 ng/mL) for any of the 28 congeners assayed (from congener 28 to 206) indicating
that was no concurrent (i.e, potentially confounding) exposure to PCBs in this population. The median
level of total mercury for each of 25 species sampled was 0.004–0.75 ppm, with most medians in the range
of 0.05–0.25 ppm, levels that are comparable to fish in the U.S. market. The authors conclude that this