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Frequent Questions

Responses to frequent questions were compiled from inquiries made either by telephone to the Bioavailability Committee hotline (toll-free 1-866-282-8622) or via mailto:BAhelp@epa.gov (please refer to the Technical Assistance Page for further information).


What is the purpose of the bioavailability guidance?

The guidance addresses three issues:
  1. a recommended process for deciding when to collect site-specific information on the oral bioavailability of metals in soils for use in human health risk assessments
  2. a recommended process for documenting the data collection, analysis, and implementation of a validated method that would support site-specific estimates of oral bioavailability
  3. general criteria that EPA will use to evaluate whether a specific bioavailability method has been validated for regulatory risk assessment purposes.
This guidance is focused on media-specific relative bioavailability and does not address adjustments to default absolute bioavailability values. Also, this guidance applies only to human health risk assessment and is not necessarily applicable to ecological receptors. Finally, the guidance document provides information on methodologies for directly assessing bioavailability and does not pertain to indirect methods for predicting bioavailability (e.g., speciation).

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Where can I find the Standard Operating Procedures for the in vitro lead bioaccessibility (IVBA) assay?

The Standard Operating Procedures for the in vitro lead bioaccessibility assay may be found in Section 3 of the Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods (PDF)(386 pp, 4.9 MB). A separate SOP is being developed and will be posted when finalized.

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Now that I have a relative bioavailability (RBA) value for lead in soil/dust, what do I do?

The RBA value for lead in soil/dust that is reported by IVBA assays typically needs to be converted to a predicted relative bioavailability (the TSD for lead provides the necessary equation). The IEUBK model accepts absolute bioavailability (ABA) inputs. To convert RBA to ABA, multiply the RBA by 0.5 (ABA for lead acetate in diet and water) and use that value. For example if IVBA results reported that lead RBA in the soil/dust sample was 52%, the ABA of the sample would be 26%. The value 26 would be entered into the IEUBK model in the soil and dust fields of the GI/Bio Data entry window.

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Is guidance available concerning sampling (number and location) to ensure adequate site coverage?

The requirements for sampling will depend on the intended objectives of the RBA assessment and site characteristics (i.e., geography, geology, and history of lead contamination of soil). Guidance documents specifically directed at sampling for RBA assessments have not been developed by US EPA; however, the existing Agency guidance on soil sampling can be found at the Soil Sampling Quality Assurance User's Guide (PDF)(267 pp., 3.61 MB). In addition, the Bioavailability Committee can be consulted for advice on sampling objectives and strategies to address factors that can impact bioavailability (e.g., minderal composition, particle size, etc.). Inquires can be directed to the Bioavailability Committee Co-Chairs BAhelp@epa.gov.

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Can the IVBA method be used to assess RBA of lead in any soil?

The IVBA method has been found to reliably predict RBA of lead mineral types in a variety of soil types, in samples collected from lead mining and milling sites. Detailed descriptions of the composition of these samples can be found in Appendix F of the Lead TSD: Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods (PDF)(386 pp, 4.9 MB). Hence, the IVBA method may be used without in vivo analyses for these lead mineral types and soil compositions. Soil speciation of a representative portion of the samples is generally recommended to characterize the lead mineral types and soil compositions at a site.

Although, these evaluations described in the TSD suggest that the IVBA method would be applicable to a wide range of lead mineral types and soil compositions, the reliability of the method for predicting RBA of other lead mineral types contained in other soil composition, not described in the Lead TSD, is unknown. This uncertainty should be considered (and documented) if the method is applied to soils dissimilar from those described in the Lead TSD (including mixtures with other mineral or soil types), or for which there is no in vivo verification of the IVBA test results. In the future, as additional samples with a variety of new and different lead forms and soil types are tested by both in vivo and in vitro methods, the verified applicability range of the IVBA method may be expanded.

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Can the IVBA method for lead be used to assess bioavailability of phosphate amended soils?

The IVBA method for lead has not been validated for phosphate-amended soils. Thus, EPA does not recommend that the IVBA method in the Lead TSD be used for assessing the RBA of lead in phosphate-amended soils.

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What is the validation status of other assays of bioaccessibility?

A variety of in vitro lead bioaaccessibility assays have been reported; however, at this time only the assay described in the Lead TSD: Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods (PDF)(386 pp, 4.9 MB) has been validated for applications in US EPA risk assessments. As described in the bioavailability guidance, other methods and associated validation data may be submitted to the Bioavailability Committee for consideration as valid test methods.

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What is the validation status of the bioavailability and bioaccessibility methods for other metals, including arsenic?

At this time, bioavailability and bioaccessibility assays have only been validated for lead. EPA is currently in the process of evaluating the in vitro bioaccessibility assay for predicting RBA of arsenic in soil.

EPA encourages further development of new methods for lead and other metals; however, as discussed in the guidance only the information provided by validated models is recommended to be used for quantitative human health risk assessments in the Superfund program.

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Can the lead IVBA assay be used to predict in vivo RBA of arsenic in soil?

EPA has conducted preliminary studies exploring application of the lead IVBA assay for predicting in vivo RBA of arsenic in soil. At this time, the data are inadequate to demonstrate that RBA values predicted from IVBA data would be quantitatively reliable across a broad range of soil characteristics. Therefore, the lead IVBA assay does not satisfy method validation criteria for use in arsenic risk assessment applications established by EPA (Guidance for Evaluating the Bioavailability of Metals in Soils for Use in Human Health Risk Assessment). The IVBA assay protocol has not been optimized for predicting arsenic RBA. It is possible that further efforts to optimize the IVBA procedure may result in an in vitro technique for arsenic with higher predictive capability that would satisfy validation criteria and would offer a faster and less expensive alternative to in vivo assays for estimating site-specific RBA of arsenic.

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Are other IVBA methods available for predicting in vivo RBA of arsenic in soil?

Other IVBA methods for arsenic have been published which have been evaluated to varying degrees for predicting in vivo RBA. All of these methods attempt to measure the dissolution of arsenic into an extractant that simulates, to varying degrees, the environment of the gastrointestinal tract. EPA has not made a determination about the validity of these other methods for use in arsenic risk assessment applications.

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What are the standard reference materials for IVBA of lead?

Currently, the National Institute of Standards and Technology (NIST) Standard Reference Materials (SRM) 2710 and 2711 are used for quality assurance purposes when performing the IVBA method. Note that the original reference material (not the backup, for example 2710a) should be used as a control material. The backup material is from the same source and has been analyzed for concentration of contaminants; however, the bioaccessibility of lead in the backup material may differ from SRM 2710.

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Are the values shown in the IEUBK (e.g., 30% for soil) absolute bioavailability values or relative bioavailability values?

The values shown in the model are absolute bioavailability values. It is important to be certain that data from experimental assays are clearly expressed as either absolute bioavailability estimates or relative bioavailability estimates.

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Are all of the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) criteria presented in the guidance equally important in the evaluation of new methods?

No. Certain criteria are considered more important than others. For example, the criterion to demonstrate a linkage between the new test and an existing test (i.e., correlation of an in vitro method with an in vivo method) may weigh more heavily than the other criteria when evaluating new methods. Correlation does not imply that a 1:1 correlation between in vitro and in vivo assays is required, only that the in vitro bioaccessibility method be predictive of in vivo bioavailability.

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In terms of bioavailability research, how is the STOP after Step 2 (“Has EPA identified a validated method available for estimating site-specific BA?”) to be interpreted if it is determined no acceptable approach is available for obtaining reliable data?

The recommended decision framework is intended for the collection of data to inform site-specific risk-based decisions. The framework is not intended to address the collection of data for research purposes that may serve to expand scientific knowledge for future assessments. Hence, the STOP is only meant to prevent use of an unvalidated model for site-specific risk assessment. EPA encourages research and development of alternative bioavailability methods to fill this data gap.

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