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The Policy Forum “Balancing fish consumption benefits with mercury exposure” by Grace M. Egeland and John P. Middaugh (12 Dec., [p. 1904][1]) questions the wisdom of the Environmental Protection Agency's (EPA's) using our Iraqi data ([1][2]) to calculate their reference dose (Rfd), arguing that

The long half-time for methylmercury in the neonatal rats is explained by the neonatal liver's inability to secrete the toxin into bile, which in adults is the main route of elimination. The ability to secrete mercury into bile develops between 2 and 4 weeks of age and is correlated with the increasing ability of the developing liver to secrete glutathione into bile.

Phenylmercury absorbed through the skin from contaminated diapers affected urinary excretion in infants in Buenos Aires. The effects were reversible and quantitatively related to the concentration of urinary mercury. Excretion of gamma-glutamyl transpeptidase, an enzyme in the brush borders of renal tubular cells, increased in a dose-dependent manner when mercury excretion exceeded a "threshold" value. Urine volume also increased but at a higher threshold with respect to mercury. The results support the threshold concept of the systemic toxicity of metals. gamma-Glutamyl transpeptidase is a useful and sensitive marker for preclinical effects of toxic metals.