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Soil Bioavailability at Superfund Sites: Human Health

Information for the general public

In most cases, the toxicity of contaminant depends on how much of it is absorbed into the body. For soil contaminants where the exposure is by ingestion, toxicity depends upon absorption from the gut. Information on how well a contaminant is absorbed from the gut is important to determining how much of a contaminant humans can be exposed to before health effects occur.

Information for site assessors

Because typical health effect dose-response assessments (and resulting oral reference doses (RfDs) and cancer slope factors (CSFs)) are generally expressed in terms of ingested dose (rather than absorbed dose), accounting for potential differences in absorption between different exposure media can be important to site risk assessment.

While this is true for all chemicals, it is of special importance for ingested metals. This is because metals can exist in a variety of chemical and physical forms, and not all forms of a given metal are absorbed to the same extent. For example, ingestion of a metal in contaminated soil may be absorbed from the gut to a lesser extent than when ingested in drinking water or food. Thus, if the oral RfD or CSF for a metal is based on studies using the metal administered in water or food, risks from ingestion of the metal in soil might be overestimated. Even a relatively small adjustment in oral bioavailability can have significant impacts on estimated risks and potentially for cleanup goals at hazardous waste sites.

The Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation (Part A) (RAGS) (EPA, 1989) supports the consideration of bioavailability in the determination of site-specific human health risks. This guidance has been used to support bioavailability adjustments across different routes of exposure at contaminated sites. However, the use of bioavailability information in site-specific risk assessment has not been widespread (due to limited data, uncertain methodologies and lack of method validation). The primary impediment to the broad use of bioavailability data in risk assessment and decision-making is the absence of rapid and inexpensive tools that can generate reliable relative bioavailability estimates in the receptors of concern.

Over the past several years, considerable effort has been directed at developing validated laboratory methods for determining relative bioavailability of soil-borne lead, arsenic and other metals, including the development of rapid screening tools (e.g., in vitro bioaccessibility tests). The availability of new methods has reinforced the need for additional guidance on evaluating relative bioavailability data and incorporating this information into site-specific risk assessments. In mid-2002, EPA created a work group to respond to the need for additional guidance. A bioavailability workshop in April 2003 brought together a diverse group of experts from academia, industry and government to discuss and provide input to EPA on bioavailability issues. The information shared at the workshop informed the development of EPA’s Risk Assessment Guidance for Superfund.