Methyl mercury is
a compound worthy of scientific and societal concern. It is clear that MeHg is
a widespread environmental contaminant and a potent neurotoxicant that
adversely affects the developing nervous system. Mercury continues to be
released into the environment by both natural and human-generated sources. It is readily
converted to MeHg and accumulates in the food supply, primarily in fish and marine
mammals. MeHg is readily absorbed and
distributed throughout the body, including the brain and the fetus. Fetal
exposure appears to be at a level that is greater than maternal blood levels. Studies of humans
exposed to elevated levels of MeHg clearly demonstrate its neurotoxic
potential. Animal studies using rodents
and nonhuman primates have confirmed the neurotoxic potential of MeHg. However, research
into cellular and molecular mechanisms has yet to produce an understanding of
MeHg sufficient to allow accurate prediction of its neurotoxicity. Furthermore, human and animal studies on the
neurobehavioral effects of developmental MeHg exposure have not determined a
level of exposure that is convincingly harmless to the developing fetus.
In many ways, our
understanding of the neurotoxic potential of MeHg is similar to that of lead 20
years ago; MeHg is a known neurotoxicant at high levels of exposure but there
is little understanding of its effects at lower levels of exposure. The failure
to adequately characterize the functional effects of low-level MeHg exposure
has compromised the formulation of a sound policy regarding the safe levels of
MeHg exposure, particularly for pregnant women or women of child-bearing age.
Examination of the results
of human studies on the effects of MeHg indicate that maternal hair levels of
10 to 20 ppm may result in adverse effects on fetal outcome. Making the
appropriate assumptions and calculations, a level of exposure not expected to
be hazardous (RfD) would be 0.06 μg/kg/day. Evaluation of
results from animal studies on the developmental effects of MeHg provided an
estimated RfD of 0.025 μg/kg/day. The human and animal RfDs are in
very good agreement.
Given the current state of
knowledge with regard to MeHg exposure, the following recommendations are
offered:
• reduce environmental
release of all forms of mercury;
• consider restricting the
global production and sale of mercury;
• strongly advise pregnant
women and women of child beating age to limit their exposure to sources of
MeHg;
• establish an RfD
(reference dose) for MeHg of 0.025 to 0.06 μg/kg/day;
• continue research to
determine a level of MeHg exposure that would not harm the developing nervous
system;
• continue research to
understand the underlying molecular mechanisms of action of MeHg;
• assess the long-term
neurodegenerative effects of developmental MeHg exposure.
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