Mercury - Toxicology of Metals - page 699

concentration in the critical organ (the brain). Each
approach is used below for each effect whenever data
are available.

out that in these epidemiological studies of the Iraqi epi-
demics, the recorded and observed signs were signs of
severe neurological impairment and retardation result-
ing from high level of exposure for a limited period. So
far, no such information is available for the Iraqi popu-
lation concerning the level of methylmercury exposure
during pregnancy in which period more subtle changes
may occur, changes detectable only in psychomotor
tests on a group basis. A study in New Zealand (Kjell-
strom and Kennedy, 1985) indicates that more subtle
effects may occur at hair-mercury levels exceeding 6
mg/kg (ppm), as demonstrated in a population of 31
mother-child pairs, in which the fi sh consumption dur-
ing pregnancy resulted in hair-mercury levels within
the range of 5-20 ppm. The 31 children were tested with
the Denver test at 4 years of age, and the test results
were compared with those of matched controls. A sig-
nifi cant dose-related difference was found (Kjellstrom
and Kennedy, 1985). Forty-six children were tested at
6 years of age and showed exposure-associated defi -
cits in Wechsler intelligence scale for children-revised
or in language development test compared with refer-
ents at maternal hair levels of 13-15 ppm (Crump et al .,
1998). However, in both studies, there were differences
between exposed and referents not fully controlled for.
A large cohort of 917 children, 7 years old, with moth-
ers exposed to Me Hg and PCBs, mainly from consump-
tion of Pilot whale, was studied in the Faroe Islands. The
geometric mean maternal MeHg hair level was 4.5 ppm
(range, 0.6 - 39.1 ppm). A comprehensive test battery
including neurophysiological and neuropsychological
tests showed pronounced mercury exposure-related
defi cient performance in attention, memory, and lan-
guage in children of mothers with <10 ppm MeHg in
hair and also an exposure associated delay in peak III
of the brain stem auditory evoked potentials. (Grand-
jean et al ., 1997) This increased latency has also been
seen in a cross-sectional study of 149 approximately
7-year-old children in Madiera when maternal MeHg in
hair from fi sh consumption exceeded 10 ppm (Murata
et al ., 1999) A follow-up of the Faroe cohort at 14 years
of age showed that the fi ndings at 7 years of age were
persistent (Debes et al ., 2006).
Attempts to control for PCB levels in cord blood did
not support a contribution of PCB exposure to the effect
of mercury in the Faroe Island cohort (Budtz-Jörgensen
et al ., 1999). In another study by the Faroe Group of 182
neonatals with prenatal exposure to MeHg and PCBs,
similar to the Faroe cohort described previously, the
neurological optimality score (NOS) was assessed.
A signifi cant negative correlation between cord blood
Hg and NOS was found (Steuerwald et al ., 2000).
A prospective longitudinal cohort study of 711 chil-
dren followed to 9 years of age in the Seychelles did

7.1.5.1 Incidence of Signs and Symptoms of Prenatal
MeHg Poisoning Related to Concentration
of MeHg in the Hair of the Mother
In the Minamata epidemic of MeHg poisoning, 23
cases of prenatal intoxication were diagnosed. Among
the Japanese cases, the mothers of the affected children
had commonly shown no clinical signs of poisoning.
Scanty data available on mercury levels in biological
materials like blood and hair (Swedish Expert Group,
1971) indicate that the difference in sensitivity between
the fetus and the adult organism is less than a factor
of 5 and closer to a factor of 2. In a follow-up study of
children in the Minamata area 20 years later, a strong
association between prevalence of mental retardation
and mercury concentration in the umbilical cord, cus-
tomarily saved in Japanese families, was encountered
(Harada et al ., 1977). Marsh et al . (1980) studied, in an
epidemiological investigation in Iraq, the relationship
between mercury concentration in the mothers' hair
during pregnancy and prevalence of signs and symp-
toms of neurological damage or mental retardation in
the children; neurological signs in the mothers them-
selves were also examined. The mercury exposure dur-
ing pregnancy was assessed by scanning hair samples
from the mothers, approximately 9 cm of hair, repre-
senting the entire pregnancy period. The children were
followed up for 4-5 years, and milestones like onset
of walking or speech as well as prevalence of mental
retardation and seizures were recorded. All the chil-
dren were neurologically examined by specialists.
Among children of mothers who, during any month
of the pregnancy, had mercury concentrations in hair
exceeding 50 mg/kg, there was a signifi cant increase
in the prevalence of severe psychomotor retardation
involving delayed onset of walking and speech, as
well as the occurrence of seizures. In two children with
severe psychomotor retardation, the highest mercury
concentration in the hair of the mother during 1 month
was approximately 50 mg/kg. This material, so far
comprising 29 mothers and infants, does not preclude
that in the interval—10-70 mg/kg MeHg in hair dur-
ing 1 month of pregnancy—there may be an increased
risk of psychomotor retardation for the children.
It is, at present, unclear whether the effect of MeHg on
the CNS of the fetus is related to the mercury exposure
during the entire pregnancy or whether adverse effects
can appear as a result of high MeHg exposure during a
short period of the pregnancy. Neither is any informa-
tion available about what period of the pregnancy is the
most sensitive to MeHg exposure. It should be pointed