New directions in the toxicokinetics of human lead exposure.

An important determinant of body lead (Pb) burden and Pb toxicity in exposed humans is Pb metabolism, or more correctly, Pb toxicokinetics. It affects the former through the quantitative processes of uptake, distribution and retention/excretion and the latter via delivery of toxic doses to cellular/molecular sites of action. Pb toxicokinetics has useful application in understanding Pb's behavior in populations. Several of these applications have been studied and results are presented for the toxicokinetic basis of dose-neurotoxic effect relationships in selected longitudinal studies and the use of toxicokinetic modeling for estimation of body lead burden in early populations. Three well-known, ongoing longitudinal studies of developmental neurotoxicity--in Boston, Cincinnati, and Port Pirie, Australia--involve cohorts who differ markedly as to their pre- and postnatal lead exposure profiles. Toxicokinetic examination of these exposure differences helps to explain the temporal variability seen in blood Pb-toxic effect relationships and supports a causal role for lead. Toxicokinetic models of Pb uptake and in-vivo behavior are increasingly being considered for estimating Pb-B levels in lieu of direct measurement. A linear biokinetic model, using reliable input data for natural/prehistoric levels of Pb in sources, was applied to estimation of prehistoric/preindustrial children's blood lead. A range of 0.06 to 0.12 microgram/dl was estimated for two lead intakes. These estimates are still two orders of magnitude (85 to 165-fold) lower than the newly issued CDC toxicity guideline for children of 10 micrograms/dl. Lastly, the toxicokinetics of lead in bone, particularly its resorption with metabolic stimuli, is of concern, particularly for "baby boom" women who are either of childbearing age or approaching menopause and who had greatly elevated environmental lead exposures in the 1940s to 1970s.