Cleanup goals for sites affected by inorganic contaminants
often are established on the basis of risk assessments,
and these assessments rely on the estimated oral toxicity
of the substances of concern. These toxicity estimates
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 animal or human. However,
these toxicity studies do not account for the characteristics
of a metal in soil or the limitations that these characteristics
place on enteric absorption of that metal. Therefore, a
more accurate risk assessment must account for the
bioavailability of the metal in site-specific soil, relative to
the bioavailability of the metal in the form administered in the
toxicity study (i.e., the relative bioavailability of the
element in soil). Historically, relative bioavailability estimates
for metals in soil have been based on in vivo studies in
laboratory animals. Given the costs and time constraints
associated with such studies, it is clear that a more efficient
alternative is desirable. The most promising option
involves the development and validation of in vitro extraction
tests that are predictive of oral metals bioavailability
from soil. Such tests would provide a rapid and inexpensive
method for developing more accurate exposure estimates
for use in human health risk assessments. This paper
reviews the site-specific in vivo studies that have been
conducted to estimate the relative bioavailability of arsenic
and lead in soil, discusses the soil and mineralogical
factors that influence the bioavailability of these elements,
and reviews the research to date on the development of
bioavailability-predictive extraction tests for metals in soil.
Finally, this paper outlines an ongoing collaborative
research project to formally validate an in vitro extraction
test for use in estimating the oral bioavailability of
arsenic and lead in soil.
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