Lead Pb

Human exposure to lead has declined as awareness of the toxic effects of lead has increased and use of leaded gasoline and lead-based paints has declined. The median blood lead concentration in children dropped 7-fold between 1978 and 1999. However, many environmental sources in certain geographical areas still contribute to lead poisoning. Following inhalation or absorption, lead is deposited in all tissues and is excreted in the urine and bile. The half-life of lead in blood is 1-2 months while the half-life of lead in other tissues such as bone is as much as 10-20 years. There are several mechanisms of lead toxicity. For example, lead inhibits the enzyme amino levulinic acid dehydratase that promotes the production of heme from porphyrin and leads to anemia. There is also a decreased incorporation of iron into protoporphyrin IX resulting in a proportional increase in dysfunctional zinc protoporphyrin. Lead binds readily to sulfhydryl groups, leading to protein dysfunction in a wide variety of tissues, including the central nervous system (5,37).

The critical populations to monitor for lead poisoning are children under 6 years of age and adults at risk of occupational exposure. Guidelines for testing and laboratory performance expectations have been published by several organizations worldwide. In the United States, guidelines published by the CDC for children and by OSHA for adults are observed (44). Children are particularly vulnerable to lead poisoning because they absorb a relatively high percentage of lead (up to 50% more than adults) and exhibit behaviors such as hand-mouth exploration of dirt, paint chips, toys, and other likely lead sources. Central nervous system damage in cases of lead toxicity results in intellectual impairment in children. Inverse correlations between IQ and blood lead concentrations in children have been demonstrated at relatively low levels. Whole blood (generally capillary blood for screening and venous for confirmation) is the specimen of choice for the diagnosis and monitoring of lead poisoning, although X-ray fluorescence is gaining popularity as a non-invasive tool for assessing body lead burden (45-47).

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