Lead gains access to the human body principally through the air we breathe and the substances we ingest. Residents of industrialized nations acquire about half of their "body burden" of lead from polluted respired air. Healthy adults only absorb about 10 percent of ingested lead, but children may absorb as much as half of the lead they eat or drink. Lead absorption is enhanced by a low level of calcium in the diet. Lead may also be absorbed through the skin. Prolonged applications of lead-containing substances such as poultices or cosmetics may result in health-threatening absorption of lead.

Absorbed lead is distributed throughout the body by the blood. The human body's ability to excrete absorbed lead is so limited, however, that normal life activities in Western countries will produce lead absorption in excess of the body's excretory capacity. About 5 percent of unexcretable lead is deposited in the liver, brain, and other viscera, where its resi dence time is only a matter of a few weeks. The other 95 percent is stored in bone for an extraordinarily long time, measurable in decades in the adult. If absorption ceases, such lead will be leached slowly from the skeleton over a period of many years, and is then excreted through the kidneys. Lead may be transferred to the fetus via the placenta.

Methods of exposure to lead as well as the nature of toxic episodes are discussed under History and Geography.

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