Academic journal article Environmental Health Perspectives

Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using Young Swine

Academic journal article Environmental Health Perspectives

Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using Young Swine

Article excerpt

In this article we summarize the results of a series of studies that measured the relative bioavailability (RBA) of lead in a variety of soil and soil-like test materials. Reference material (Pb acetate) or Pb-contaminated soils were administered orally to juvenile swine twice a day for 15 days. Blood samples were collected from each animal at multiple times during the course of the study, and samples of liver, kidney, and bone were collected at sacrifice. All samples were analyzed for Pb. We estimated the RBA of a test material by fitting mathematical models to the dose-response curves for each measurement end point and finding the ratio of doses that gave equal responses. The final RBA for a test material is the simple average of the four end point-specific RBA values. Results from 19 different test materials reveal a wide range of RBA values across different exposure materials, ranging from 6 to 105%. This variability in RBA between different samples highlights the importance of reliable RBA data to help improve risk assessments for Pb in soil. Although the RBA value for a sample depends on the relative amounts of the different chemical and physical forms of Pb present, data are not yet adequate to allow reliable quantitative predictions of RBA from chemical speciation data alone. Key words: lead, RBA, relative bioavailability, swine. Environ Health Perspect 114:1162-1171 (2006). doi:10.1289/ehp.8852 available via http://dx.doi.org/[Online 4 April 2006]

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Reliable evaluation of the potential hazard to children from ingestion of lead in the environment depends in part on accurate information on the rate and extent of Pb absorption ("bioavailability") from each exposure medium. This is especially true for soil because Pb in soil can exist in a variety of different mineral forms and particle types, some of which tend to have low absorbability. Thus, equal ingested doses of different forms of Pb in soil may not be of equal health concern.

Oral bioavailability of Pb in a particular medium may be expressed either in absolute terms [absolute bioavailability (ABA)] or in relative terms [relative bioavailability (RBA)]. ABA is the fraction of Pb that reaches the systemic circulation after oral ingestion. Typically, ABA is measured by comparing the time course of absorption after both oral and intravenous (iv) doses and comparing the area under the curve (AUC) of blood Pb concentration versus time:

ABA = [AU[C.sub.oral]/dos[e.sub.oral]]/[AU[C.sub.iv]/dos[e.sub.iv]]. [1]

This ratio is also referred to as the oral absorption fraction. RBA is the ratio of the ABA of Pb present in some test material compared with the ABA of Pb in some appropriate reference material:

RBA = [AB[A.sub.test]]/[AB[A.sub.reference]]. [2]

Usually, the form of Pb used as a reference material is a soluble compound, such as Pb acetate, that is expected to completely dissolve in gastrointestinal fluids when ingested.

We have been engaged in a multiyear investigation of Pb absorption in juvenile swine after oral exposure to a variety of different environmental media, especially soils and solid wastes associated with mining, milling, and smelting sites. Initial studies in the program (referred to as "Phase 0" and "Phase I") were performed by R. Poppenga and B. Thacker at Michigan State University (Weis et al. 1995). The study designs and protocols developed during the early studies were refined and standardized (by S. Casteel, at the University of Missouri-Columbia) and applied to a number of different test materials collected from various Superfund sites. This series of measurements is collectively referred to as "Phase II," and the results are presented in this article. A more detailed presentation of the Phase II work, including raw data from all studies, is available from the U.S. Environmental Protection Agency (U.S. EPA 2006). Drexler and Brattin (in press) have compared the results of the Phase II in vivo studies with the results of an in vitro technique for estimating Pb RBA in soil samples. …

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