Advances in evaluating the oral bioavailability of inorganics in soil for use in human health risk assessment

Cleanup goals for sites affected by inorganic contaminants often are establ ished on the basis of risk assessments, and these assessments rely an the e stimated oral toxicity of the substances of concern. These toxicity estimat es typically are based on historical studies in which a soluble salt of the metal was dissolved in water or mixed in food and then ingested by an anim al or human. However, these toxicity studies do not account for the charact eristics of a metal in soil or the limitations that these characteristics p lace on enteric absorption of that metal. Therefore, a more accurate risk a ssessment must account for the bioavailability of the metal in site-specifi c soil, relative to the bioavailability of the metal in the form administer ed in the toxicity study (i.e., the relative bioavailability of the element in soil). Historically, relative bioavailability estimates for metals in s oil have been based on in vivo studies in laboratory animals. Given the cos ts and time constraints associated with such studies, it is clear that a mo re efficient alternative is desirable. The most promising option involves t he development and validation of in vitro extraction tests that are predict ive of oral metals bioavailability from soil. Such tests would provide a ra pid and inexpensive method for developing more accurate exposure estimates for use in human health risk assessments. This paper reviews the site-speci fic in vivo studies that have been conducted to estimate the relative bioav ailability of arsenic and lead in soil, discusses the soil and mineralogica l factors that influence the bioavailability of these elements, and reviews the research to date on the development of bioavailability-predictive extr action tests for metals in soil. Finally, this paper outlines an ongoing co llaborative research project to formally validate an in vitro extraction te st for use in estimating the oral bioavailability of arsenic and lead in so il.