A toxicokinetic model for predicting the tissue distribution and elimination of organic and inorganic mercury following exposure to methyl mercury in animals and humans. II. Application and validation of the model in humans.

The objective of this study was to develop a biologically based dynamical model describing the disposition kinetics of methyl mercury and its inorganic mercury metabolites in humans following different methyl mercury exposure scenarios. The model conceptual and functional representation was similar to that used for rats but relevant data on humans served to determine the critical parameters of the kinetic behavior. It was found that the metabolic rate of methyl mercury was on average 3 to 3.5 times slower in humans than in rats. Also, excretion rates of organic mercury from the whole body into feces and hair were 100 and 40 times smaller in humans, respectively, and urinary excretion of organic mercury in humans was found to be negligible. The human transfer rate of inorganic mercury from blood to hair was found to be 5 times lower than that of rats. On the other hand, retention of inorganic mercury in the kidney appeared more important in humans than in rats: the transfer rate of inorganic mercury from blood to kidney was 19 times higher than in rats and that from kidney to blood 19 times smaller. The excretion rate of inorganic mercury from the kidney to urine in humans was found to be twice that of rats. With these model parameters, simulations accurately predicted human kinetic data available in the published literature for different exposure scenarios. The model relates quantitatively mercury species in biological matrices (blood, hair, and urine) to the absorbed dose and tissue burden at any point in time. Thus, accessible measurements on these matrices allow inferences of past, present, and future burdens. This could prove to be a useful tool in assessing the health risks associated with various circumstances of methyl mercury exposure.