Half lives, biological tissue lead burdens

Table 1 Estimated biological half-life values for age-dependent tissue lead burdens"...

Kinetic studies in man show that the lead body burden consists essentially of three compartments (1) a rapid exchange pool in blood and soft tissues (2) an intermediate exchange pool in muscles, skin and bone marrow (3) a slow exchange pool in dense bones and teeth (biological half-life about 20 years). The amount of lead stored in the latter compartment increases throughout life (Steenhout, 1982 Steenhout and Pourtois, 1981). 

Most chemicals have longer residence times in the body than carbon monoxide. The term used to denote the length of time that 50 percent of the body burden of a substance is excreted is the biological half-life. The biological half-life of a compound can vary from one tissue to another. Thus, the biological half-life of lead in bone is between ten to twenty years, but in blood and soft tissues it is in the 30-day range. 

The earliest models of lead toxicokinetics are typified by that of Rabinowitz et al. (1976, 1977), using stable lead isotope in human volunteers, and which indicate that there are at least three kinetically distinct body compartments for lead disposition in vivo. These compartments consist of a central blood compartment, a second lead depository in peripheral soft tissues, and, finally, the large bone compartment for lead. Lead in blood is the most kinetically labile, whereas lead in soft tissues has a somewhat larger biological half-life. The bone compartment retains lead for the longest time. Blood and soft tissues contain relatively small burdens of lead, ca. 1.9 and 0.6 mg respectively, while the vast majority of the body burden of lead is sequestered in a kinetically slow compartment of bone, with levels that can exceed 200 mg of the toxicant. 

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