The biological half-time of heavy metals
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- Sugita, M. Int. Arch Occup Environ Heath (1978) 41: 25. doi:10.1007/BF00377797
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Concentrations of Cd (475 samples), Pb (271), and total Hg (166) were determined in the organs and tissues during autopsies of inhabitants of the Tokyo metropolitan area who had experienced no known exposure to an abnormally high level of heavy metals and had died sudden deaths by accident. The results of this study do not differ greatly from those of other reports. Based on the intraorganic accumulation of the heavy metals according to age when they were not experimentally administered, the biological half-time (BHT) was estimated using a mathematical model with differential equations. It was hypothesized that the input of heavy metals into organs is proportional to the amount of food intake according to age (assuming little or no chronological change of heavy metals concentrations in food over several decades), and that the output is proportional to the intraorganic accumulation. The resulting BHT was very long, 10 to 100 times that computed in a number of studies from observation of the attenuation curve for a relatively short period after the experimental adminstration of heavy metals to humans or animals.
The author devised a model consisting of two series compartments in one organ: the superficial, where heavy metals enter directly and are weakly bound with protein, and the profound, where they enter only via the superficial compartment to be strongly bound with the constituents. It was elucidated theoretically that the short BHT obtained by heavy metal administration is associated only with the superficial compartment of the organ, and that the long BHT obtained without experimental administration of heavy metals is due to the detour circuit from the superficial to the profound compartments. The ratio of the short BHT to the long BHT is the proportion of the content of a heavy metal in the superficial compartment to the total content in the whole organ. In order to prove the existence of the two compartments, superficial and profound, and to compute their ratios, further studies should be performed. The attenuation curve of the concentration, or of the amount after a single administration of a heavy metal, consists of the rapid component (first) and the slow component (second). The latter has been generally used for computation of BHTs. The slowest component is frequently present several years after the first two. There is a fair chance that the BHT based on the slowest component agrees with the BHT found in the present study.