Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
information to warrant a revision in the value. Specific data of absorption from other
regions are considered in the small number of cases for which they were available,
although in some cases (e.g. isotopes of iodine) doses to alimentary tract regions and
other tissues are insensitive to assumptions regarding the site of absorption (ICRP,
2006).
(184) The extent of absorption of radionuclides will depend on the element and its
chemical forms. Changes in chemical forms are likely to occur during digestive
processes, beginning in the mouth, but principally occurring in the stomach and the
small intestine. These changes in chemical form or speciation will determine the
availability of the radionuclide for absorption and hence the extent of uptake through
the intestinal epithelium to bloodstream (ICRP, 2006).
(185) Radionuclides entering the alimentary tract in the form of an insoluble matrix
represent a specific case since absorption is likely to be controlled by the biokinetics
of the matrix rather than that of the radionuclide. In the absence of material-specific
data, it is proposed that the fractional absorption of the radionuclide should be taken
to be that of the particle matrix.
(186) A further specific case arises for ingestion of insoluble particulate material
containing radionuclides with decay chains, where the decay products formed in the
stomach or the intestine may be more soluble than their parents. In the absence of
material-specific data, it is proposed (as for the respiratory tract) that the fractional
absorption should be taken to be that of the particle matrix, which could be
predominantly made up of a compound containing the parent radionuclide, or another
material in which the parent radionuclide is a minor constituent.
(187) For inhaled particles reaching the alimentary tract after clearance from the
respiratory tract, it is appropriate to take account of solubility in the lungs in
specifying fA values. For some elements exhibiting a range in solubility according to
their physicochemical form, there is evidence that the reduced solubility of Type M or
S materials is also associated with reduced intestinal absorption. For inhaled and then
ingested Types M and S materials, the default fA value is determined here as the
product of fr for the absorption Type and the fA value for soluble forms of the element.
However, because of the need for realism in estimates of absorption for application to
bioassay interpretation, attempts have been made wherever possible to use available
data to specify fA values for different forms rather than rely on defaults.
3.3.4 Retention in the alimentary tract regions
(188) The model structure allows, where information is available, for the use of data
on retention of radionuclides in different compartments. Human and animal data
suggesting or showing retention of ingested radionuclides on teeth or in mucosal
tissues of the walls of alimentary tract regions, principally the small intestine, can be
used to refine calculation of doses to the alimentary tract. An example given in
Publication 100 (ICRP, 2006) for cadmium shows that retention of 115 Cd on teeth
increases the estimated dose to the oral mucosa by almost two orders of magnitude
compared to that calculated using the Publication 30 model. Similarly, retention of
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Fe in the wall of the small intestine may increase the dose by about a factor two,
compared to that calculated with the ICRP 30 model. However, in both examples,
these increases in organ doses do not lead to significant changes in the committed
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