Bioavailability of soilborne lead in adults, by stable isotope dilution

Using stable isotope dilution, we determined the bioavailability of soilbor ne lead (Pb) in human adult volunteers, Soil from a residential yard at a m ining-impacted federal Superfund site that had negligible amounts of other priority pollutants was dried and screened through a 25-mu m mesh sieve. Th e <250-mu m fraction, which likely represents that ingested via hand-to-mou th activity, was then sterilized by exposure to radiation. Ten replicate sa mples yielded a mean (SD) soil Pb concentration of 2924 +/- 36 ppm, and a m ean Pb-206/Pb-207 ratio of 1.1083 +/- 0.0002, indicating remarkable soil ho mogeneity. Six adults with Pb-206/Pb-207 ratios of >1.190 were admitted to the clinical research center and fasted overnight prior to dosing with 250 mu g Pb/70 kg bw (i.e., 85.5 mg soil/70 kg) in a gelatin capsule. Blood for Pb and Pb-206/Pb-207 ratios was obtained at 14 time points through 30 hr. Results oi the isotopic analyses from these subjects indicate that on avera ge 26.2% +/- 8.1 of the administered dose was absorbed. Six additional subj ects were subsequently studied but ingested soil immediately after a standa rdized breakfast. Bioavailability in this group was only 2.52% +/- 1.7. Col lectively, this study provides the first experimental estimates of soil Pb absorption in humans, and should allow for more precise estimates of health risks due to Pb-contaminated soil.