Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p20
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 20/24)
Character Range: 995888–998944

applies an a priori uptake factor to an estimated dose to estimate blood lead concentration. The US EPA integrated exposure uptake biokinetic model (IEUBK) for lead in children is widely known and used in this respect. In addition, the US EPA adult lead methodology may be an appropriate tool to assist in the assessment of adult lead exposures. Both of these tools are available at www.epa.gov/superfund/lead/products.htm.

This approach with the appropriate justifications is considered suitable at Tier 2 for assessing risks from lead.

The US EPA continues to develop a research-oriented biokinetic model, the all-ages lead model, which is designed to potentially replace the integrated uptake biokinetic model for lead in children and includes a full population age range (090 years) and updated uptake and biokinetic modules. This model is, however, still under peer review and has not been formally approved for application in contaminated land assessments. Information on the all-ages lead model is available at:
 http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=139314.

    4.8.2          Bioavailability and bioaccessibility
In contaminated land health risk assessment, we are commonly estimating the intake of a chemical in soil, and comparing this to a tolerable daily intake or reference dose. The intake of contaminant is estimated from the intake of soil, using soil concentration data that nominally represents the 'total' concentration of the substance in soil. The toxic effect of a contaminant actually depends upon the uptake or absorbed dose of contaminant, that is, the amount that gets into the bloodstream after being ingested, inhaled or via skin contact. The fraction of a compound that is absorbed into the body (systemic dose) following exposure via all pathways (administered dose) is generically termed the 'bioavailable fraction'.

More specifically:
    * absolute bioavailability is the fraction of a compound which is ingested, inhaled or applied to the skin that actually is absorbed and reaches systemic circulation and
    * relative bioavailability is referred to the comparative bioavailability of different forms of a chemical or for different exposure media containing the chemical. In the context of contaminated land risk assessment, relative bioavailability is the ratio of the absorbed fraction from the exposure medium in the risk assessment (e.g. soil) to the absorbed fraction from the dosing medium used in the critical toxicity study.
The assessment of contaminant bioaccessibility may also be considered for estimating contaminant uptake. Bioaccessibility is related to the solubility of the contaminant in the gastrointestinal tract. More specifically, in the context of soil contamination, it is defined as the fraction of a contaminant in soil that is soluble in the relevant physiological milieu (usually the gastrointestinal tract) which is potentially available for absorption. This can be assessed by validated in vitro test systems.

TRVs used in risk assessment have been derived from animal