More technical considerations and procedures for biological monitoring are summarized in a recent issue of an international journal (IAOEH, 1999), and in the U.S. EPA's guidance document on exposure monitoring (1998). The numerical example presented here serves to reflect some of these more technical considerations as well as those referenced in the last slide.

Suppose from a previous pharmacokinetics study, it was learned that 70% of Chemical A (Ch-A) would be excreted to the urine in the form of its Metabolite A (Met-A) at 24 hours following a single oral dose, for which the default absorption is 100% and considered to be almost instantaneous. Met-A is further assumed to have one-half the molecular mass of Ch-A.

In the present biomonitoring study, a male test subject's total level of Met-A in his urine collected over a 24-hour period was measured to be 0.1 mg. Thus, the total absorbed dose of Ch-A in his body on the monitoring day would be 0.29 mg [= (0.1 mg of Met-A as measured) x (2/1 for difference in molecular weight between Ch-A and Met-A)/(70% for a 24-hour urinary recovery)].

This type of back-calculation becomes more complicated, however, when the individual in the study is biomonitored for nonbolus dose such as from dermal exposure acquired throughout a workday. Efforts must be made to account for the fact that dermal absorption and dermal acquisition generally proceed more slowly than via the oral or injection route. That is, nonbolus dose or urinary excretion from dermal exposure is often more protracted.