Assessing mercury contamiantion in the Amazon

3. Discussion

3.7. Relations between mercury in fish and mercury in humans in the Amazon

Human exposure to methylmercury is through fish ingestion in the Amazon region. This gives us an opportunity to evaluate a predictive model, which relates methylmercury concentrations edible fish to methylmercury concentrations in human hair (WHO, 1990).
A few aspects of the single-compartment model will be given first. Then data acquired from the literature will be used to calculate average mercury concentrations in hair and fish samples. Finally, a comparison between predicted average methylmercury concentrations and observed values will be given. The areas of interest will be the Madeira and the Tapajos River.

3.7.1 The single compartment model

Relationships between mercury concentration in fish and human hair are based on a single-compartment model summarized by the International Programme on Chemical Safety for methylmercury (WHO, 1990). The elimination of methylmercury generally follows first order ki-netics since excretion is directly proportional to body burden (Nielsen and Andersen, 1991). Duration of exposure may affect the excretion process of methylmercury. A two-compartment model was established by Rice et al. (1989) for a single oral dose study in monkeys. In this study an initial rapid elimination phase was followed by a slower elimination phase. However, following continu-ous dosing for 2 years, a single-compartment model was considered a more reasonable fit for the data. The single compartment model suggests that continuous exposure of methylmercury results in a steady state, where intake equals excretion after approximately 5 half times. The half time for whole-body methylmercury is estimated to be 70 days, thus steady state is attained approximately after 1 year (WHO, 1990). An important prediction of the single-compartment model is that constant dietary exposure to methylmercury for a period of several years should not result in any greater accumulation than after one year of exposure. Calculations based on the single-compartment model assume that all of the mercury ingested is methylmercury; 95% of the mercury intake is absorbed through the intestines; 5% of the absorbed mercury goes to the blood compartment; the blood volume is 5 liters; the ratio of methylmercury between blood and hair is 1:250; and the elimination constant for methylmercury is 0.01 days^-1 (Who, 1990). The validity of the single-compartment model is supported by the reasonable agreement between predicted and observed blood concentrations of methylmercury in single-dose tracer studies, single-dose fish intake ex-periments, and studies involving the extended controlled intake of methylmercury from fish (WHO, 1990).

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