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 hile the US Environmental Protection Agency's guideline
for protecting the public from a toxic form of mercury is justifiable,
based on the latest scientific evidence, some children of women
who consume large amounts of fish and seafood during pregnancy
may be at special risk of neurological problems, says a new report
from the National Academies' National Research Council. Congress
requested that the Research Council provide independent, scientific
advice in establishing appropriate exposure limits for methylmercury.
 Fish
and other seafood products are the main source of methylmercury
in the human diet. Fetuses are particularly vulnerable to methylmercury
because of their rapid brain development, and some may currently
be receiving exposures at levels that cause observable adverse
neurological effects.
"Although
we believe EPA's guideline on methylmercury is generally adequate
to protect most people, more must be done to gain a better understanding
of various risk factors for the US population," said Robert
A. Goyer, chair of the committee that wrote the report and professor
emeritus at the University of Western Ontario, who now resides
in Chapel Hill, N.C. "Trends in methylmercury exposure,
including regional differences, should be analyzed, as should
subpopulations whose diets are high in fish and seafood. And
we need to better understand how this chemical affects brain
development in fetuses and children."
Based on an
analysis of available data that included exposure levels to methylmercury
and food-consumption surveys, the committee said the majority
of Americans are at low risk of adverse health effects. However,
the committee estimated that each year about 60,000 children
may be born in the United States with neurological problems that
could lead to poor school performance because of exposure to
methylmercury in utero.
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EPA's
current reference dose for methylmercury is 0.1 micrograms per
kilogram of body weight per day - the amount of methylmercury
to which an individual can be exposed on a daily basis without
adverse health consequences. EPA's reference dose is used to
guide risk-management decisions and regulatory policies ranging
from fish-consumption advisories to air-emission permits. According
to EPA, the typical American consumer eats less than a third
of an ounce of fish per day, and would be exposed to considerably
less than its current guideline.
To draw its
conclusions, the committee evaluated the range of data on which
risk assessments conducted by EPA and other regulatory agencies
are based. It also reviewed new findings that have emerged since
the development of EPA's current reference dose in 1995, and
met with researchers of major ongoing population studies. The
overall weight of the evidence from this comprehensive review
led the committee to conclude that EPA's reference dose is scientifically
justifiable for protecting the health of the vast majority of
Americans.
When the agency
first developed its guideline five years ago, EPA judged data
from a 1971 Iraqi poisoning incident to be the most relevant.
To provide EPA with more appropriate data in formulating its
reference dose, the committee analyzed population studies in
the Faroe Islands, Seychelles Islands, and New Zealand. It concluded
that the Faroe Islands analysis should be used by EPA as the
critical study for deriving the reference dose, the report says.
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Neurodevelopmental
problems are the most appropriate basis for setting an exposure
limit, the committee found. Strong scientific evidence exists
from human and animal studies to link certain levels of methylmercury
exposure and neurological problems, including poor performance
on tests that measure attention and motor function. However,
researchers still need to understand if there is a precise time
during development when the brain is most sensitive to methylmercury
and exactly how the chemical exerts its effects. Evidence also
indicates that the cardiovascular and immune systems could be
affected by methylmercury, the report notes. Information on whether
methylmercury causes cancer in humans is still inconclusive.
Scientists
do not agree on how to account for some uncertainties, such as
varying individual responses to methylmercury exposure and emerging
health concerns. Better data are needed to decrease the uncertainties,
the report says. For example, further investigation is needed
on low-dose exposure to methylmercury throughout the life-span
of humans and animals, and on carcinogenic, neurologic, reproductive,
and immunologic effects, including the emergence of delayed neurological
effects later in life. More research on factors that might influence
responses, such as genetics, age, sex, health status, and nutrition,
also is needed.
Likewise,
research should be conducted to gather data on methylmercury
exposure in different regions of the United States and in specific
populations with high consumption of fish, the committee noted.
In addition to methylmercury, research on exposure to other forms
of mercury, including mercury from dental fillings, is needed
to see if they affect the human body's response to methylmercury.
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Mercury
exists naturally in the environment and finds its way into the
air through both natural processes and human activities. Power
plants that burn fossil fuels, particularly coal, generate the
greatest amount of mercury emissions. Once mercury is deposited
in lakes, rivers, and oceans, it is converted to methylmercury
by aquatic organisms. Humans are exposed to the chemical when
they eat fish.
In the United
States, responsibility for regulating mercury is shared by two
federal agencies: EPA and the Food and Drug Administration (FDA).
FDA is charged with regulating commercially sold fish and seafood.
EPA monitors concentrations in the environment and regulates
industrial releases of mercury to surface water and air. 
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The
study was sponsored by the US Environmental Protection Agency.
The National Research Council is the principal operating arm
of the National Academy of Sciences and the National Academy
of Engineering. It is a private, nonprofit institution that provides
independent advice on science and technology issues under a congressional
charter.
Copies
of Toxicological Effects of Methylmercury will be available at
the end of August from the National Academy Press, (202) 334-3313
or (800) 624-6242. The cost of the report is $54.00 (prepaid)
plus shipping charges of $4.50 for the first copy and $.95 for
each additional copy
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