PNNL

Abstract

Routine bioassay programs sometimes find evidence of an unsuspected intake. If there were no workplace indicators of exposure or intake, it is necessary to assume a value for the time of intake. Under these circumstances, the International Commission on Radiological Protection (ICRP) continues to recommend using the midpoint of the interval between routine bioassay measurements (ICRP Pub. 78, para. 106). The assumption of T/2 as the time of intake, where T is the interval between bioassay measurements, represents the expectation value of the time of intake, , assuming uniform probability of an intake at any given time. In virtually all cases presented here, this assumption results in a significant underestimation, in many cases by a factor of 2 or 3, of the expectation value of the intake, , that would have been received by a population of workers who had uniform probability over time of intake. This underestimation leads to a negative bias in dose estimates derived in this fashion. The bias is characterized for realistic, routine urinalysis programs for Pu, U, and 3H, as well as for in vivo measurements of 125I and 137Cs. Simple methods are presented for finding t, the time of using software such as IMBA, LUDEP, or CINDY. Since the primary concern is estimating intake rather than time, the assumed time of intake should be chosen as t rather than . The ICRP should consider revising some of the tables in its Publication 78 to reflect this.