Jump to main content.

National Information
Region 8 Risk Assessment
Human Health Risk Assessment pages

Bioavailability

On This Page

You will need the free Adobe Acrobat Reader to view some of the files on this page. See EPA's PDF page to learn more.

Introduction

Most oral toxicity values (RfDs, slope factors) are based on the empirical relationship between the occurrence of toxic effects and the amount of chemical ingested.  That is, the amount of chemical that is actually absorbed into the body (bioavailability) is not explicitly considered.  Thus, if it is expected that the absorption of a chemical from an on-site medium is significantly different from the medium used in the study supporting the RfD or slope factor (SF), then it is appropriate to adjust the RfD or SF to account for this difference in absorption.  This adjustment increases the accuracy of the subsequent risk calculations while still being protective of public health.

The ratio of the absorption fraction for a chemical in a site medium compared to the medium used in the key toxicity studies is referred to as the Relative Bioavailability (RBA):

RBA = (Fraction absorbed from site medium) / (Fraction absorbed in toxicity studies)

If reliable estimates of RBA are available for chemicals of potential concern in site media, these can be used to adjust the default RfD and SF values as follows:

The concept of RBA is potentially important for all types of contaminants of concern, but is especially important in the case of metals and other inorganics that commonly occur at mining-related sites.  This is because metals in soil and mine waste may occur in a wide variety of chemical and physical forms, not all of which are readily absorbed when ingested.  A good general discussion of bioavailability of metals in soil is presented in the document "Assessing the Bioavailability of Metals in Soil for Use in Human Health Risk Assessments" (National Environmental Policy Institute, Summer 2000; currently not available online).

[Top of page]

Resources

EPA Region 8 Studies on RBA

EPA Region 8 has been engaged in a multi-year study to investigate the RBA of metals (mainly lead and arsenic), both in experimental animals and using in vitro methods, as described below.

In Vivo RBA Studies on Lead

The basic approach for measuring lead absorption in vivo is to administer an oral dose of lead to test animals and measure the increase in lead level in one or more body compartments (blood, soft tissue, bone).  In order to calculate the RBA value of a test material, the increase in lead in a body compartment is measured both for that test material and a reference material (lead acetate).  Equal absorbed doses of lead are expected to produce approximately equal increases in concentration in tissues regardless of the source or nature of the ingested lead, so the RBA of a test material is calculated as the ratio of doses (test material and reference material) that produce equal increases in lead concentration in the body compartment.  This approach has been applied to a number of different soils and soil-like materials from mining-related sites in Region 8 and elsewhere, and the results have been summarized in the technical document Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods (OSWER 9285.7-77, May 2007).

In Vivo RBA Studies on Arsenic

The approach for measuring RBA of arsenic is similar to that for lead, except that the best marker of arsenic absorption is the amount of arsenic excreted in the urine rather than the concentration of arsenic in blood or tissues.  This is because most arsenic that is absorbed into the body is rapidly (within 1-2 days) excreted in urine.  In this case, RBA is calculated in two steps:

This approach has been applied to a number of different soils and soil-like materials from mining-related sites in Region 8 and elsewhere, and the results have been summarized in the following document:

[Top of page]

In Vitro Studies on Lead and Arsenic

Because measurement of lead and arsenic RBA in animals is relatively slow and costly, a number of scientists have been working to develop alternative in vitro procedures that may provide a faster and less costly alternative for estimating the RBA of metals and metalloids in soil or soil-like samples.  These methods are based on the concept that the rate and/or extent of metal ion solubilization in the gastrointestinal fluid are likely to be important determinants of the in vivo bioavailability of the metal in soil, and most in vitro tests are aimed at measuring the rate or extent of ion solubilization from soil into an extraction solvent that resembles gastric fluid.  To help avoid confusion in nomenclature, the fraction of lead or arsenic which solubilizes in an in vitro system is referred to as in vitro bioaccessibility (IVBA), while the fraction that is absorbed in vivo is referred to as bioavailability.

Dr. John Drexler at the University of Colorado in Boulder has been working cooperatively with EPA Region 8 for a number of years to develop an in vitro method that can be used to obtain RBA data for lead, arsenic, and potentially other metals in soils.  As a result of this continuing collaboration, Dr. Drexler and his colleagues have developed a procedure that is relatively fast, simple, and reproducible.  The details of this test system are available at the following site:

In brief, samples of test material (1 gram) are placed into a plastic bottle, and to this is added 100 mL of an extraction fluid (0.4 M glycine, pH 1.5).  Each bottle is placed into a water bath maintained at 37°C, and samples are extracted by rotating the bottles end-over-end for 1 hour.  After 1 hour, the bottles are removed, dried, and placed upright on the bench top to allow the soil to settle to the bottom.  After a few minutes, a 15-mL sample of supernatant fluid is removed directly from the extraction bottle into a disposable 20-cc syringe, and then filtered through an 0.45-um cellulose acetate disk filter (25 mm diameter) to remove any suspended particulate matter.  This filtered sample of extraction fluid is then analyzed to quantify the fraction of lead or arsenic in the sample which had dissolved.  This method has undergone external peer review, as described at the following site:

Tests performed to date have yielded results for lead that correlate well with in vivo RBA measurements for lead.  Data for soils from several different Superfund sites are presented in the document Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods (OSWER 9285.7-77, May 2007).  Use of the IVBA procedure for lead is considered to be a useful strategy for obtaining information on lead RBA at a site (see Assessing Relative Bioavailability in Soil at Superfund Sites).

It is important to stress that the IVBA method described above was specifically developed for lead, and that the method has not been specifically designed to optimize the correlation of in vivo and in vitro results for arsenic.  Thus, the current in vitro procedure is not recommended as the sole basis for derivation of IVBA/RBA data for arsenic.  Further efforts are currently underway to develop an IVBA procedure that will be specifically intended for arsenic.

[Top of page]

Geochemical Speciation

The RBA value for lead or arsenic in any test soil is believed to depend on (at least) three important attributes of the metal-bearing grains in the soil:

Mineral Phase.  Mineral phase refers to the chemical form in which the metal occurs in the soil.  For example, lead might occur in grains of galena (PbS) or cerrusite (PbCO3).  In general, the less soluble a mineral phase is, the more likely it is to be associated with a low RBA value.  A category list of the grouped lead phases based on the soils tested in the in vivo swine study is provided in Table 1 (PDF) (May 2007) (1 p, 11K).  Similarly, a category list of consolidated arsenic phases based on the soils tested in the in vivo swine study are presented in Table 2 (PDF) (March 2005) (1 p, 16K).

Matrix Association.  Some metal-bearing grains in soil are entirely free (“liberated”) in the soil, while others may exist in association with other mineral phases or matrices that may limit the contact of the grain with gastrointestinal fluid.  For example, most slags contain the majority of their metal content in small grains or inclusions located within the glassy (vitreous) phase of the slag.  When fully encased in the glassy phase, these grains are essentially isolated from gastrointestinal fluid, and are not believed to be available for dissolution or absorption.  However, if the slag is crushed and the grains are released, then the metal in the grains become available for contact with gastrointestinal fluid and the RBA value would be expected to increase.  Based on the soils tested in the in vivo swine studies, matrix associations of lead particles are presented in Table 3 (PDF) (May 2007)(1 p, 11K) and matrix associations of arsenic particles are presented in Table 4 (PDF) (March 2005)(1 p, 83K).

Particle Size.  Metals occur in soils as particles that may have a wide range of sizes.  Because the process of dissolving a metal from a particle into gastrointestinal fluid occurs only at the surface of the particle, it is thought that large particles are likely to be associated with a lower RBA value than smaller particles of the same mineral, since the ratio of surface area to volume (mass) tends to decrease as particle size increases.  Based on the soils tested in the in vivo swine studies, Table 5 (PDF) (May 2007) (1 p, 50K) summarizes data on the particle size distributions (frequency) for liberated lead particles and Table 6 (PDF) (March 2005) (1 p, 13K) summarizes particle size distributions for liberated arsenic particles.

Based on these general concepts of the factors that influence RBA of metals in soil, valuable information on potential sources of contamination at a site and the potential for differences in RBA at different locations within a site can often be gained by observing the mineral forms, particle size distribution, and matrix associations for metal-bearing grains using electron microprobe analysis (EMPA).  A detailed description of the EMPA method is provided at the following site:

Although the relationships between RBA and the geochemical parameters measured by EMPA are not yet well enough understood to derive reliable quantitative estimates based only on EMPA data, it is nevertheless possible to use EMPA data to support semi-quantitative inferences on the likely range of RBA values in site samples.  This is done by comparing the EMPA data (especially phase composition data) for samples from the site to EMPA data for samples that have been characterized by in vivo RBA studies.  If a good match can be identified between the phase composition of the site sample and one or more samples from the library of tested materials, then it may be reasonably expected that the RBA of the samples will be similar.

Data on the mineral phase composition of 19 soil samples that have been tested in vivo for lead RBA are presented in Table 2-4 of the document titled "Estimation of Relative Bioavailability of Lead in Soil and Soil-Like Materials Using In Vivo and In Vitro Methods" (OSWER 9285.7-77, May 2007).  Likewise, data on the mineral phase composition of 20 soil samples that have been tested in vivo for arsenic RBA are presented in Table 2-9 of the Estimation of Relative Bioavailability of Arsenic in Soil and Soil-Like Materials by In Vivo and In Vitro Methods (Region 8 Review Draft, March 2005).  For convenience, derivations from both of these table are produced here as Table 7 (PDF) (May 2007) (1 p, 12K) for lead and Table 8 (PDF) (March 2005) (1 p, 61K) for arsenic.

[Top of page]

Region 8 Site-specific Relative Bioavailability Studies

As noted above, Region 8 has measured the in vivo RBA of lead and/or arsenic at a number of sites and has used the results to help improve the accuracy of risk assessments for these two contaminants in soil.  Links to the site-specific reports are provided below.

Site-specific Arsenic Studies
Site-specific Lead Studies
Note: The PDF files listed below are very large (4 to 8 MB in file size)

[Top of page]

Region 8 Recommendations for Quantifying the Bioavailability of Lead and Arsenic in Soil for Use in Human Health Risk Assessments

Based on the information presented above, this section summarizes the general strategies that Region 8 recommends for obtaining information to make RBA adjustments for lead and arsenic in soil at Region 8 Superfund sites.

Strategy for Lead

The default value used in the IEUBK model for the relative bioavailability of lead in soil is 60% (USEPA, 1994).  That is, it is assumed that lead in soil is absorbed about 60% as well as soluble lead that is ingested in water.  When risk calculations based on the default RBA for lead are close to (either above or below) a level of health concern, then acquisition of site-specific data may be needed to help increase the accuracy of the assessment.  As always, measurements using a reliable in vivo assay is considered to be the most reliable, but this option is usually only feasible for large sites. If in vivo studies will not be performed, the best option for obtaining this information is through the EPA-approved IVBA assay for lead (OSWER 9285.7-77, May 2007).  This technique is relatively fast and inexpensive, and data may be obtained for a number of site samples.  If it is not possible to obtain reliable IVBA data, then comparison of the mineral phase composition of site samples with the data listed in Table 7 (PDF) (May 2007) (1 p, 12K) may be used to support semi-quantitative inferences regarding the likely RBA values of the site samples.

Strategy for Arsenic

In the past, Region 8 utilized a default relative bioavailability factor of 80% for arsenic in soil from mining and smelting sources.  However, in vivo testing of arsenic in soil and mine waste has been conducted at a variety of sites in the Rocky Mountain West (Region 8 Review Draft, March 2005).  In 26 test materials, the relative bioavailability of arsenic ranged from 8 - 61% with a mean of 34%.  Of the 26 test materials investigated, only 5 exceeded 50%, and 1 exceeded 60%.  Similarly, bioavailability studies conducted by Roberts et al. (2006) in Cynomolgus monkeys measured the bioavailability of arsenic in 14 soil samples from 12 different sites, including mining and smelting sites, pesticide facilities, cattle dip vat soil, and chemical plant soil.  The relative bioavailabilties ranged from 5% to 31%.  Based on this, Region 8 has concluded that a relative bioavailability of 50% can be considered a generally conservative default value for arsenic in soil.

In cases where risk predications based on this default RBA value are near a level of concern (either higher or lower), collection of site-specific RBA information may be appropriate to increase the accuracy of the assessment.  As above, results obtained using a reliable in vivo RBA assay are preferred, but may not be feasible in all cases.  Given the relatively weak correlation between in vivo RBA and in vitro IVBA estimates for arsenic, Region 8 does not recommend using IVBA studies as the sole basis for determining a relative bioavailability factor.  However, Region 8 does support the use of a weight-of-evidence approach which combines the in vitro studies and geochemical speciation results at the site in question, with the in vivo and geochemical results from the studies in the arsenic library conducted to date (see Table 8 (PDF) (March 2005) (1 p, 61K)).  If the geochemical speciation results of the site in question are similar to the speciation results of a site in the arsenic library, the in vivo bioavailability results from the library site may be used as a reasonable surrogate, although the uncertainty associated with this approach must be identified and discussed in the uncertainty section.

As always, Region 8 highly recommends that anyone interested in adjusting the bioavailability factor of lead and arsenic in soil work closely with the Region 8 toxicologists during the development of the risk assessment.

[Top of page]

References

Roberts, S.M., Munson, J.W., Lowney, Y.W. and Ruby, M.V.  2006.  Relative Oral Bioavailability of Arsenic from Contaminated Soils Measured in the Cynomolgus Monkey.  Toxicological Sciences 95(1):281-288.

USEPA. 1994.  Guidance Manual for the Integrated Exposure Uptake Biokinetic Model for Lead in Children.  U.S. Environmental Protection Agency, Office of Emergency and Remedial Response.  Publication Number 9285.7-15-1.  EPA/540/R-93/081.

[Top of page]

Local Navigation

Jump to main content.