incorporate feedback from organs to the systemic circulation may be more appropriate for interpreting
Biokinetic models used for intake, uptake or retention assessment should be appropriate for the
intake mode (e.g., by ingestion, inhalation, or injection)
duration of the intake (e.g., acute, continuous, or intermittent)
time period of interest
sites of uptake and retention
intake radionuclide and its progeny.
Biokinetic models should relate well to the available bioassay data, should account specifically for
the chemical and physical characteristics of the contaminant, and should account for the influence of
decorporation therapy if used. Models, default assumptions, methodologies, and computer codes used for
assessments of doses from intakes should be recorded and maintained.
18.104.22.168 Selection of Intake Default Assumptions
Many different factors influence the resulting distribution, retention, and excretion of radionuclides
following an intake. The following default assumptions should be applied to assessments of intakes and
their resulting doses unless more appropriate values are available. As discussed in the introduction to
Section 7, there are levels of intake and dose that make it more appropriate to determine values or
parameters more accurately and realistically.
Entry Pathway and Duration of Intake. If the intake mode is not known, acute inhalation should be
assumed. Acute inhalation represents the most common type of occupational intake. This assumption
will tend to maximize the committed effective dose equivalent evaluated from bioassay data.
Time of Intake. If it is not reasonably possible to establish the time of an intake identified by a
routine bioassay measurement, it may be assumed that the intake occurred at the midpoint of the period
during which it could have occurred (ICRP 1982), or the time at which the expectation value of intake
would have occurred (see Section 22.214.171.124). The midpoint is usually the date halfway between the sample
from which an intake was detected and the previous routine bioassay measurement. If no prior sample
exists (baseline result) or if a baseline bioassay measurement exceeds the decision level, effort should be
expanded to examine the person's previous work history, in an attempt to assign an intake date.
Particle-Size Distribution. The particle-size distribution influences the probability of aerosols
depositing in the nasopharyngeal, tracheobronchial, and pulmonary (parenchymal) regions of the lung.
Particle size also influences the relationship between lung deposition, retention times, and excretion rate.
Therefore, assessments of quantities retained in lung and assumptions regarding lung clearance should be
determined using direct lung counting, wherever possible.
When lung counts cannot be used to determine the activity retained in lung, assessments may be
made from urinalysis or fecal analysis data. The ICRP Publication 30 (1979) model for the respiratory
tract shows that deposition in the pulmonary region will vary by a factor of about 3 over a range of
activity median aerodynamic diameters (AMADs) between 0.3 - 3.0 :m. In the absence of specific
information on particle size, a particle size distribution with an AMAD of 1 :m and the associated
deposition fractions should be assumed. If the newer respiratory tract model is used (ICRP 1994a),