Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
approach comes from the fact that the value of sr tends to have more effect on the
overall biokinetics of an inhaled material deposited in the conducting airways (where
the lining fluid is relatively thick) than on material deposited in the alveolar region,
because it competes with particle transport rates of similar magnitude (10 d –1 from
BB’ to 2 T E and 100 d–1 from 2 T E to oesophagus). Because of the wide variation
between elements in the estimated value of sr, element-specific values are adopted in
this series of documents for those elements for which an estimate of the value could
be made.
(144) For soluble (Type F) forms of some elements, however, part of the dissolved
material is absorbed rapidly into body fluids, but a significant fraction is absorbed
more slowly. To represent this time-dependent uptake, it is assumed that a fraction
(fb) of the dissolved material is retained in the ‘bound’ state, from which it goes into
body fluids at a rate sb, while the remaining fraction (1 – fb) goes to body fluids
instantaneously (Figure 9). Evidence for retention in the bound state, rather than by
transformation into particulate material may be in one or more forms: e.g. systemic
uptake rather than faecal clearance of the retained material, or autoradiography
showing diffuse rather than focal retention of activity. In Part 2, bound state
parameter values are used for cobalt, ruthenium and lead.
Revision to default absorption parameter values
(145) As noted above, the specific parameter values derived from experimental data
(from both in vivo and in vitro studies) provided a database to give guidance on
selecting values that are representative of materials that are generally considered to
clear at ‘fast’, ‘moderate’ or ‘slow’ rates.
(146) When about 100 sets of parameter values were available (i.e. when most of
the reviews for Part 2 elements were completed) the results were collated and
analyzed. It is emphasised that this was not a representative survey from which central
values could be derived by some objective statistical means. Rather it provided a basis
for informing judgements as described below.
(147) Parameter values given in the text of the current draft element sections were
sorted into Types F, M and S according to the Publication 71 criteria given above, and
tabulated. Some selection was made. A few values noted to be particularly uncertain
were excluded. Where there was more than one set of results for a material (or very
similar materials) they were merged, and central values taken, to avoid giving too
much weight to a few compounds. Note that for some sets of parameter values,
because of limitations in data fitting, the value of sr was fixed and only the values of fr
and ss were assessed. In such cases the assumed value of sr was not included in the
derivation of central values.
(148) Medians, geometric means, and geometric standard deviations (GSD) of the
assessed values of fr, sr and ss are given in Table 6. Except for the value of fr for Type
F materials, GSDs are very large (4 – 14) reflecting the wide ranges of estimated
values, and hence indicating large uncertainties in the central values.
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