Methods of Estimating Intake - doe-std-1136-20040137

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

deposited in the respiratory tract; the other 37% is immediately exhaled (ICRP 1979). For a wound

intake, material may be initially deposited at the wound site. Once the material has been deposited, it can

be taken up into systemic circulation either as an instantaneous process (e.g., direct intravenous injection

of a dissolved compound) or gradually (e.g., slow absorption from a wound site or the pulmonary region

of the lung). Both the instantaneous and slow absorption processes are often referred to as uptake to the

systemic transfer compartment (i.e., blood). Once material has been absorbed by the blood, it can be

translocated to the various systemic organs and tissues.

An understanding of this terminology is important to review of historical cases. Before DOE Order

5480.11, many sites reported internal doses not as dose equivalent estimates but as an uptake (or

projected uptake) expressed as a percentage of a maximum permissible body burden (MPBB). The

standard tabulated values for MPBBs were those in ICRP Publication 2 (ICRP 1959). Many archived

historical records may have used this approach. DOE Order 5480.11 (now superseded) required

calculation of dose equivalent. Now, 10 CFR 835 has codified the calculation of intakes and committed

doses.

5.8.1 Methods of Estimating Intake

There are several published methods for estimating intake from bioassay data (Skrable et al. 1994;

Strenge et al. 1992; ICRP 1988b; King 1987). These methods each employ an idealized mathematical

model of the human body showing how materials are retained in and excreted from the body over time

following the intake. An intake retention function (IRF) is a simplified mathematical description of the

complex biokinetics of a radioactive material in the human body. These functions are used to predict the

fraction of an intake that will be present in any compartment of the body, including excreta, at any time

post-intake. Intake retention functions incorporate an uptake retention model that relates uptake to

bioassay data and a feed model that relates intake to uptake and bioassay data. ICRP Publication 54 (ICRP

1988b) and others (Lessard et al. 1987) contain compilations of IRFs.

In its simplest form, a compartment content at any time post-intake (Q t) can be expressed as the

product of intake multiplied by the intake retention function value for compartment Q at time t post-

intake, or:

Qt = Intake IRF(Qt)

(5.9)

Results predicted by the model can then be compared with the observed bioassay data. Such results are

often referred to as expectation values.

Simple algebraic manipulation of the model allows calculation of intake from the compartment

content at time t, as shown below:

(5.10)

When multiple data points are available for a compartment, the intake can be estimated using an

unweighted or weighted least-squares fitting procedure, as described by Skrable (Skrable et al. 1994) and

Strenge (Strenge et al. 1992) or as can be found in most statistics textbooks. As an alternative, data can be

fit by eye to a graphical plot; however, the apparent fit can be misleading if data have been logarithmically

transformed.

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