Assessing mercury contamiantion in the Amazon

3. Discussion

3.6.3. Human health studies in the Amazon

Mean mercury concentrations in fish are typically 0.36 µg/g from the Madeira River and 0.30 µg/g from the Tapajos River (Table 2). Over 90% of the mercury present in Amazon fish is methylmercury (Akagi et al., 1994). Furthermore, riverine human populations, ribeirinhos, in the Amazon rely heavily on fish as a food source and observed mean fish meals per day ranged between 1.8 and 1.9 (Boischio and Henshel, 2000). Field research with 607 individuals showed that daily fish consumption rates resulted in a log normal distribution with a median of 200g (Boischio and Henshel, 2000). As a result of fish ingestion riverside human hair methylmercury concentrations typically vary between 10 and 20 µg/g (Lebel et al., 1996). This is concerning as the Faroe study has shown neuropsychological dysfunctions in children prenatally exposed to methylmercury at maternal hair mercury concentrations below 10 µg/g during pregnancy. Unfortunately, prenatal studies similar the Faroe and the Seychelles Study have yet to be done for the Amazon region. However, studies relating mercury exposure to neuropsychological and motor performance in adults and children are present.
An adult study was carried out in a village on the Tapajos River on 91 inhabitants (15-81 years), whose hair mercury levels were inferior to 50 µg/g (Lebel et al., 1998). Mean total hair mercury were 23.9 ±9.3 µg/g for fishermen, 14.3 ±9.4 µg/g for other men and 12.6 ±7.0 µg/g for women. Performance on a neurofunctional test battery and clinical manifestations of nervous system dysfunction were examined in relation to hair mercury concentrations. Near visual contrast sensitivity and manual dexterity, adjusted for age, decreased significantly with hair mercury levels (P < 0.05), while there was a tendency for muscular fatigue to increase and muscular strength to decrease in women. For the most part, clinical examinations were normal, however, hair mercury levels were significantly higher (P < 0.05) for persons who presented disorganized movements on an alternating movement task and for persons with restricted visual fields.
Another adult study from the same river system showed similar results to that of Lebel et al. (1998). Dolbec et al. (2000) examined 84 individuals between 15 and 79 years of age. Median hair mercury concentration was 9 ppm. Methylmercury accounted for more than 90% of the total mercury. Psychomotor performance was evaluated using the Santa Ana manual dexterity test, the grooved pegboard fine motor test and the fingertapping motor speed test. The Santa Ana manual dexterity test was similar to the manual dexterity test used by Lebel et al. (1998). Diminished performance on the Santa Ana manual dexterity test, the grooved pegboard test and the fingertapping test was associated with increasing hair mercury levels. Interestingly, the grooved pegboard test was chosen, because a similar test was done on methylmercury exposed non-human primates in a laboratory by Rice (1989). After 6 years of exposure, from birth, to 50 µg methylmercury/kg/day, the monkeys showed diminished performance in retrieving raisins from a recessed grid.
Grandjean et al. (1999) examined 351 of 420 eligible children between 7 and 12 years of age in four comparable Amazonian villages. In three Tapajos villages with the highest exposures, more than 80% of 246 children had hair mercury concentrations above 10 µg/g. Neuropsychological tests of motor function, attention, and visuospatial performance showed decrements associated with the hair mercury concentrations. Especially on the Santa Ana form board and the Stanford-Binet copying tests, similar associations were also apparent in the 105 children from the village with the lowest exposures, where all but two children had hair mercury concentrations below 10 µg/g. Although average exposure levels may not have changed during recent years, prenatal exposure levels are unknown, and exact dose relationships cannot be generated from this cross-sectional study. However, the current mercury pollution seems sufficiently severe to cause adverse effects on brain development.
In summery, there is still some uncertainty surrounding the 10 µg/g safety limit for pregnant women. Many of the uncertainties arise from differences in test procedures from study to study. However, the Faroese study suggests that the present safety limit for pregnant women should be adjusted to a lower value. Studies from the Amazon show that hair mercury concentrations, between 10 and 20 µg/g, are sufficient to cause adverse neurological performance.

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