Kinetics and Metabolism
Absorption of lead depends on the chemical and physical state of the metal and is influenced by the genetic factors, nutritional status, physiological status, and age of a person. The most common routes of lead absorption in children and adults are ingestion and inhalation, correspondingly. The absorption of lead in children may be 5-10 times greater than in adults [18-20]. Children, malnourished individuals, and pregnant women can absorb 40%-70% of ingested lead. About 5%-15% of ingested lead is absorbed in the gut of adults, which at fasting conditions may rise up to 45% [21]. Dietary deficiencies of zinc, calcium, iron, selenium or phosphate, copper, and ascorbic acid can cause the increase of gastrointestinal lead absorption, and currently, it is recommended that the supplementation is only applicable when a specific deficiency is proven. Lead competes with calcium for binding proteins involved with gastrointestinal absorption as they have a similar ionic size. Certain groups of people such as children and nursing mothers more efficiently absorb calcium than general people. It is hypothesized that these groups of people also more efficiently absorb lead due to their increased calcium absorption efficiency (particularly if an individual has calcium deficiency) [14]. Most inhaled lead is absorbed directly [21].
The absorption route of lead has a negligible effect on its distribution throughout the body. After the absorption, lead is mostly transported to the blood where it binds with erythrocyte proteins. Lead is distributed to soft tissues (e.g., brain, kidneys, liver) and bone with the circulation of blood, and gradually this metallic compound aggregates in those tissues and bone. The half-life of lead in the soft tissue and blood is 20-40 days whereas in bone is 10-30years [9]. However, the half-life of lead in the blood differs contingent upon the exposure duration [22,23]. Several years of lead half- life in serum have been found in adults with occupational exposure [24,25]. The amount of lead deposited in the teeth and bone increases with the increase of age; roughly 70% of lead is deposited in children’s bones and more than 95% of lead is deposited in the teeth and bones of adults [26]. The absorption and elimination of lead into bones are similar to those of calcium [27]. Accumulated lead can re-enter the blood and can re-distributed at times of stress or increased calcium use for instance menopause, pregnancy, chronic disease, lactation, chelation, and broken bones [28,29].
Adults excrete most of the lead within several weeks after an acute exposure, while children likely retain more lead for a longer time [20].
