Once an intake is confirmed, sufficient samples must be obtained to establish a
reasonably anticipated baseline against which future measurements can be compared.
This is important both to provide future verification of the accuracy of the assessment
and to identify potential additional intakes.
The statistical fluctuation of low-level measurements can be particularly troublesome
for long-term excretion patterns. Factors of 2 can be easily expected due to day-to-
day variability and imprecise adherence by the worker to urine collection protocols.
Fecal Sample Results
Fecal samples are much more sensitive to detection of intakes than are urine samples
and, consequently, are an important part of follow-up bioassay monitoring for
potential intakes initially identified by workplace indications. Pitfalls to the data
interpretation include highly variable individual fecal voiding patterns, ranging from
more than one per day to one every few days. This makes it extremely important to
know what time interval is represented by a collected fecal sample. While
normalizing a single set of fecal data to reference man daily excretion rate can be
done, it is not likely to improve the quality of assessment.
The preferred fecal sampling protocol following an intake is to collect all the early
fecal clearance (meaning total feces for the first five-to-seven days). This method will
allow a good estimation of inhalation or ingestion intake, but does not readily permit
discrimination of inhalation from ingestion, or identify whether inhaled material
exhibits class D, W, or Y clearance patterns. For optimum interpretation, total fecal
collection should be interpreted in light of early urine and in vivo data for
preliminary estimates. The urine data is likely to be particularly valuable in
conjunction with fecal data to classify an intake as class W or Y. Longer-term follow-
up fecal samples at nominally 30, 60, and 90 days post intake should substantially
improve the classification of material as class W or Y.
Fecal sampling can also be applied to monitor excretion at long times post-intake.
One caveat in such sampling is that a worker still active in a plutonium facility may
be incurring very minor chronic exposure, which can significantly interfere with
long-term interpretation of acute exposure data. Bihl et al. (1993) have discussed
experience with a routine fecal sampling program.
Use of Air Sample Data in Internal Dosimetry
Results of air sampling and continuous air monitoring implying more than 40 DAC-
hours exposure should be used to initiate special bioassay to assess intakes of
plutonium. Although bioassay data are the preferred method for assessing intakes and
internal doses, air sample data can be used for assessing internal doses if bioassay
data are unavailable or determined to be inadequate or nonrepresentative. Air sample
data can be used to calculate an exposure to airborne material either in terms of
DAC-hours or potential radioactivity intake as follows: