{Dental amalgams and ethylmercury injected by physicians and nurses
remain major sources of mercury exposure and should be considered with
the findings described in the NIEHS summary. -Teresa}
Methylmercury and Children's Heart Function
John Tibbetts
http://ehp.niehs.nih.gov/docs/2004/112-15/forum.html?section=children#meth
Pregnant women who consume significant amounts of seafood may have a new
reason to take precautions against methylmercury, the most hazardous
form of mercury: a recent study suggests that when expectant women
consume fish containing high levels of the toxicant, their children's
future cardiovascular health may be jeopardized.
Fish and shellfish are the main sources of exposure to methylmercury for
most Americans. Methylmercury tends to accumulate the most in large
predatory species such as yellowfin tuna, shark, swordfish, and marlin.
Other commonly eaten species can accumulate intermediate levels of
methylmercury. Fish with the lowest mercury content include cod,
flounder, salmon, herring, and smaller tuna species that Americans buy
canned.
In 1986, researchers led by Harvard environmental epidemiologist
Philippe Grandjean and Faroese Hospital System chief physician Pal Weihe
began a long-term study of mothers in the Faroe Islands and their
children. The Faroese are among the world's leading seafood consumers
per capita, with the average islander eating 2.4 ounces of fish per day.
This diet exposes them to increased amounts of methylmercury.
Over a 21-month period, the researchers gathered a cohort of 1,022 women
giving birth in the Faroe Islands. They tested mercury concentrations in
the children by analyzing cord blood samples at birth and blood and hair
samples taken at ages 7 and 14 years. They also measured the mercury in
each woman's hair by taking a sample at the time of parturition.
In one of the latest papers to come from this study, published in the
February 2004 issue of the Journal of Pediatrics, Grandjean and his
colleagues report that mercury which passed from mother to child
inutero, first measured in cord blood, produced long-lasting harm to the
child's neurologic mechanism that regulates heart function, as measured
by heart rate variability. At higher mercury exposures, children were
less capable of maintaining normal heart rate variability, which is a
risk factor for development of heart disease. The decrease in heart rate
variability at increasing mercury exposures was the steepest in the low
range of mercury exposures, around the U.S. Environmental Protection
Agency exposure limit. When the exposures increased above twice that
limit, the effect was not as clear.
Very little is known about the impact of heart rate variability in
children, except that children with congenital heart disease also have
lower heart rate variability. Grandjean says, "The mercury-associated
changes in the Faroe Islands study persisted at least to age fourteen,
and it's possible that they are permanent. In adults, decreased heart
rate variability is a known risk factor for heart disease mortality."
Alan Stern, an adjunct associate professor of public health at the
University of Medicine and Dentistry of New Jersey, points out that
because this effect is likely the result of developmental changes in the
children's neurologic systems, it may be a sentinel for other
neurophysiological disturbances. The developing brain is particularly
vulnerable to methylmercury, and brain damage incurred during
development is likely to be permanent.
However, Gary Myers, a pediatric neurologist who studies mercury
exposure at the University of Rochester in New York, says that the
Faroese are unusual in their diet of whale meat, which is especially
high in concentrations of mercury and other toxicants. Therefore, he
says, this study cannot be generalized to the United States and other
countries with populations that do not consume whale meat.
But many Faroese do not eat whale, says Grandjean, and its availability
varies seasonally and among communities. He says mercury associations
found at low-exposure levels are more likely to be related to other
kinds of seafood with high mercury concentrations. Still, he cautions
that scientific conclusions should not be based on a single study.
Moreover, consumers should not be scared away from eating seafood, but
should instead be wary of fish with elevated mercury concentrations,
particularly large predatory species.
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