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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