Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
Results from different studies are consistent;
It was considered that occupational exposure to the material is possible;
The specific parameter values are sufficiently different from default Type F, M
or S parameter values to justify providing specific dose coefficients and
bioassay functions.
(141) Other materials were assigned to default Types using current information.
Publication 66 did not give criteria for assigning materials to absorption Types on the
basis of experimental results. Criteria were developed in Publication 71 (ICRP,
1995c) and their application was discussed further in Guidance Document 3 (ICRP,
2002b). Type M is assumed for all particulate forms of most elements in the absence
of information. A material is assigned to Type F if the amount absorbed into body
fluids by 30 d after an acute intake is greater than the amount that would be absorbed
over the same period from a hypothetical material with a constant rate of absorption
of 0.069 d –1 (corresponding to a half time of 10 d) under identical conditions.
Similarly, a material is assigned to Type S if the amount absorbed into body fluids by
180 d after an acute intake is less than the amount that would be absorbed over the
same period from a hypothetical material with a constant rate of absorption to body
fluids of 0.001 d –1 (corresponding to a half-time of about 700 d) under identical
conditions.
(142) Particulate forms of each element were assigned to the HRTM default
absorption Types using these criteria. However, strict application of the criterion for
assigning materials to Type S requires experiments of at least 180 days duration, and
since this would exclude much useful information, extrapolation has been used in
some cases, as indicated in the text. For studies where it was possible to apply the
criteria, a statement is made to the effect that results “are consistent with” (or “give”)
assignment to Type F (M or S). For studies where the results point towards a
particular Type, but there was insufficient information to apply the criteria, a
statement is made to the effect that the results "indicate" or "suggest" Type F (M or S)
behaviour. For some elements, for which there is little or no experimental data on
absorption from the respiratory tract, some materials have been assigned to default
Types based on chemical analogy.
(143) For soluble (Type F) forms of each element, estimates are made of the overall
rate of absorption from the respiratory tract to blood (where information is available).
In general this might result from a combination of processes including: (i) dissolution
of the deposited material (if not inhaled as droplets and so already in solution); (ii)
transfer through the lining fluid to the epithelium, especially in the conducting
airways; (iii) transfer across the epithelium. Strictly, in terms of the model structure,
the first two of these would be described as ‘dissolution’ and be represented by the
rapid dissolution rate, sr, because the material is subject to particle transport, whereas
transfer across the epithelium, unless extremely rapid, should be represented by a
bound fraction. In practice it would often be difficult to assess how much of the
overall rate should be assigned to each process, and for simplicity sr is used to
represent the overall absorption. However, it is assumed that sr is a characteristic of
the element, and this would be expected for transfers through the lining fluid and
epithelium. Wide variation in values of sr was found between elements, ranging from
about 1 d –1 (e.g. yttrium) to 100 d –1 (e.g. caesium). Some justification for this
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