Half-life of lead

Another important storage depot for toxic compounds is the skeleton. In particular, cadmium and lead bind and accumulate in the bone tissue from which they are released very slowly. The half-life of elimination of cadmium is several years, the half-life of lead is several months. 

A striking aspect of lead in the body is its very rapid transport to bone and storage there. Lead tends to undergo bioaccumulation in bone throughout life, and about 90% of the body burden of lead is in bone after long-term exposure. The half-life of lead in human bones is estimated to be around 20 years. Some workers exposed to lead in an industrial setting have as much as 500 mg of lead in their bones. Of the soft tissues, the liver and kidney tend to have somewhat elevated lead levels. 

Cumulative absorption of lead over time is a more reliable predictor of adverse effects of lead than a single blood lead measurement. The blood concentration tends to fall markedly within weeks of removal from exposure. The biologic half-life of lead in blood... 

Children absorb lead from the diet with greater efficiency than adults (WHO, 2000). After absorption and distribution in blood, where most lead is found in erythrocytes, it is initially distributed to soft tissues throughout the body. Subsequently, lead is deposited in the bone, where it eventual accumulates. The half-life of lead in blood and other soft tissues is 28-36 days. Lead that is deposited in physiologically inactive cortical bones may persist for decades without substantially influencing the concentrations of lead in blood and other tissues. On the other hand, lead that is accumulated early in life may be released later when bone resorption is increased, e.g., as result of calcium deficiency or osteoporosis. Lead that is deposited in physiologically active trabecular bones is in equilibrium with blood. The accumulation of high concentrations of lead in blood when exposure is reduced may be due to the ability of bones to store and release lead. 

The half-life of lead in humans is estimated to be about 6 years for the whole body burden and from 15 to 20 years for skeleton. Thus, an excretion from the skeleton is very slow. Lead, like mercury, is a cumulative poison. The skeletal burdens of lead increase almost linearly with age. This suggests that the Pb steady state is not... 

Since lead in bone has a biologic half-life measmed in decades, compared to a biologic half-life of lead in blood of only 2-4 weeks [72], the bone more closely reflects cumulative body lead stores. Chelatable lead correlates well with bone lead [4, 31]. The decrease in bone lead stores can be monitored by in vivo tibial K x-... 

Owing to the short half-life of lead in the blood (only several weeks—after that it moves into tissues and bone), the period during which lead can be detected in the blood can be far shorter than the duration of its toxic actions in the brain. 

Lead is another element of interest in a number of applications. Most of the ingested lead is stored in bone, and is difficult to remove once it is incorporated into the mineral phase. The half-life of lead in bone can be up to 20 years (Anderson and Danylchuck 1977, Drasch 1982). Lead is not distributed equally among the bones of the skeleton, although there seems to be a relationship among anatomical units within one skeleton that would allow an estimation of total skeletal lead burden (Wittmers et al. 1988). In a modern population of humans (n = 240) that had not been exposed to lead occupationally, the mean lead content in the femur was 3.86 mg/kg bone wet weight as compared to the temporal bone (5.59 mg/kg) and the pelvic bone (1.65 mg/kg) (Drasch et al. 1987). An occupationally exposed population had approximately ten to twenty times the amount of lead in bone compared to the unexposed population (Brito et al. 2000). 

Lead is believed to directly incorporate into the calcium sites in hydroxyapatite in bone. Bone lead levels provide a useful biomarker of long term, cumulative exposure to lead, because the half-life of lead in bone is >10 years... 

FIGURE 13.5 The uranium-238 decay series. Radium (Ra) and polonium (Po), the two elements discovered by Marie Curie, are part of this series. Radon (Rn), the radioactive gas of environmental concern, is generated as shown here wherever rocks contain uranium. Lead-206 is not radioactive. The half-lives of the isotopes in this decay series vary considerably. Uranium-238 has a half-life of 4.5 billion years, whereas the half-life of lead-210 is 22 years. Some radioactive elements, like thallium-210, have half-lives of only a few minutes. 

Example 2-9. The half-life of lead-214 is 26.8 min. Assuming that the sample is initially 100% Pb, use a spreadsheet to calculate the percentage of Pb remaining as a function of time every 10 min for a total of 100 min. Then graph these data as % Pb versus time. 

The lead that remains in the bloodstream is lost steadily with a short-term half-life of ca. 20 days (adult males, normal blood lead concentration). The half-life for excretion of the lead from the body is, however, somewhat longer at ca. 28 days [10]. This latter figure is consistent with the half-life of lead in the bloodstream found in the long-term studies of Rabinowitz [8], which suggests that some of the lead initially lost to storage re-enters the bloodstream before being excreted. 

There is now considerable evidence to show that the half-life for excretion of lead from the body is not constant, but is a function of the blood lead concentration (henceforth, blood lead concentration will be called simply blood lead or PbB). As the blood lead increases, so the half-life of lead in the body decreases. There is thus a certain natural protective mechanism against exposure to increased amounts of lead. The evidence for this takes the form of an increased rate of urinary excretion of lead, the major pathway for removal of absorbed lead, per unit of blood lead, as PbB increases. This observation is also consistent with the numerous epidemiological studies which approximate the relationship of blood lead levels to exposure by a log-linear or log-log plot of a slope <1, i.e. one in which the incremental increase in blood lead per unit of intake decreases as the intake increases. It may also be that the fractional uptake of lead into the bloodstream decreases as the body is exposed to higher concentrations. 

However, lead inputs to a more localized atmosphere can occur by reentrainment of lead from depositional sites such as roadway soils and dusts to the contiguous atmosphere. Other sources of reentrained dust lead are fugitive dusts mobilized from point source waste storage or persisting surface contamination. The long half-life of lead in these media assures that resuspended lead levels in the atmosphere will pose exposure problems for future decades. 

The half-life of lead-202 is 53 000 years. A sample of lead contains only of the expected amount of lead-202. How old is the lead sample ... 

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 relative contribution of current uptake versus bone content of lead to PbB in young children, unlike the case for adults, is not well understood. The probability exists that lead resorption from bone to blood in children would be a more dynamic process than in adults, given the biological half-life of lead in bone of young children as estimated by Harley and Kneip (1984) and shown in Table 1 as being ca. 30% that of teenagers. 

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