bone (measured at study termination on day 15). The measured lead concentrations in blood and tissue samples from animals exposed to test soils were compared to those for animals exposed to lead acetate, and the relative bioavailability was calculated for each endpoint medium (blood, liver, kidney, bone). The relative bioavailability results for the two samples from the Palmerton site are given in Table 2-6.

In interpreting the results, Casteel et al. (1996) recommended emphasis on the blood lead data because they are less susceptible to random errors than the tissue lead data. They defined the “plausible range” to extend from the relative bioavailability based on blood AUC to the mean of the three tissues (liver, kidney, and bone). They defined the “preferred range” to be the interval from the relative bioavailability based on blood to the mean of all four relative bioavailability values. Their “suggested point estimate” is the mid-point of the preferred range. These relative bioavailability values are presented in Table 2-7.

Because soluble forms of lead are about 50 percent absorbed (absolute bioavailability) by a child, estimates of the absolute bioavailability of lead in soil can be determined by multiplying the relative bioavailability value by 0.5. This would result in absolute bioavailability values for the two Palmerton soils (EPA’s suggested point estimate) of 0.33 (Location 2) and 0.27 (Location 4). This conversion is important because the IEUBK model (EPA, 1994a), which is used by EPA to estimate the effect of lead in soil on children’s blood lead, contains a default value of 30 percent absolute bioavailability of lead in soil to children. The results of the Palmerton bioavailability study bracketed the default values used in the IEUBK model for predicting blood lead levels (Ioven and Hubbard, 2000). Thus, the juvenile swine testing served to confirm the oral bioavailability value used in risk assessment modeling for lead in soil at the Palmerton site, and no special adjustments for bioavailability were needed or used for this site.