revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 246
2. HEALTH EFFECTS
the most appropriate and reliable database currently available for calculation of a chronic oral MRL from a
population exposed only to methylmercury by a relevant route of exposure for the overall U.S. population.
Again, ATSDR would like to strongly emphasize that both the Seychelles study and the Faroese study
represent credible scientific contributions by widely respected research teams. Similarly, both studies
extend our knowledge base well beyond that provided by the Iraqi study and make significant contributions
to our understanding of the effects of low-level exposure to methylmercury by an exposure route and
vehicle (i.e., food) relevant to U.S. populations. The continuing monitoring and evaluation of the
Seychellois and Faroese populations with more comparable neurobehavioral indices should help strengthen
our understanding of the effects of low-level chronic methylmercury exposure and should reduce the
uncertainty regarding the public health implications of exposure.
Other Key Studies Reviewed by ATSDR
Other epidemiology studies were also considered by ATSDR in evaluating the database on human exposure
to methylmercury. Lebel et al. (1996) evaluated a fish-eating populations in the Amazon River Basin with a
neurofunctional test battery and clinical manifestations of nervous system dysfunction in relation to hair
mercury concentrations. The villagers examined live along the Tapajos River, a tributary of the Amazon.
The study population consisted of 91 adult inhabitants 15–31 years of age. Hair mercury levels were below
50 µg/g (ppm). Clinical examinations were essentially normal, although persons displaying disorganized
movements on an alternating movement task and those with restricted visual fields generally had higher hair
mercury levels. Near visual contrast, sensitivity, and manual dexterity (adjusted for age) were found to
decrease significantly with increasing mercury levels, while a tendency for muscular fatigue and decreasing
strength were observed in women. The authors suggested that dose-dependent nervous system alterations
might be associated with hair mercury levels below 50 ppm. This study, however, also had a number of
potentially confounding factors. The impact of parasitic and other diseases endemic to the study area is of
primary concern in the interpretation of the Lebel et al. (1996) results. In addition, the overall nutritional
status of the study population was not known or reported, and the use of neuroactive drugs (from local
herbs, plants, roots, or mushrooms) was not considered as a potential confounder or covariate. The
previous mercury exposure history of the study cohort is also unclear. This is of particular importance
because gold mining procedures that use metallic mercury have been commonly practiced along the
Amazon Basin for decades. Finally, the end points of the Lebel et al. (1977) study evaluated adult toxicity
and not effects in the developing fetus or the newborn (i.e., the most sensitive human population).