Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(98) In this series of reports, parameter values are adopted for gaseous and vapour
forms of compounds of a number of elements, including hydrogen, carbon, sulphur
and iodine. In each case, values are given for total deposition, regional deposition and
absorption.
3.2.2 Clearance: particle transport
(99) The model describes several routes of clearance from the respiratory tract
(Figure 4). Some material deposited in ET1 is removed by extrinsic means such as
nose-blowing. In other regions, clearance is competitive between the movement of
particles towards the alimentary tract and lymph nodes (particle transport), and the
absorption into blood of material from the particles in the respiratory tract. Removal
rates due to particle transport and absorption to blood are taken to be independent. It
is assumed that all clearance rates are independent of age and sex.
Lymph nodes
Body fluids
ET1
Respiratory tract
excluding ET1
52
Environment
Alimentary tract
Figure 4. Routes of clearance from the respiratory tract
(100) As in the original HRTM, it is assumed that particle transport rates are the
same for all materials. A generic compartment model is therefore provided to describe
particle transport of all materials. The original model is shown in Figure 5. Reference
values of rate constants were derived, as far as possible, from human studies, since
particle transport rates are known to vary greatly among mammalian species. Figure 5
as it stands would describe the retention and clearance of an insoluble material.
However, as noted above, there is in general simultaneous absorption to blood. New
studies enable more reliable particle transport parameter values to be chosen for the
extrathoracic regions (ET); bronchial (BB); bronchiolar (bb) and alveolar-interstitial
(AI) regions, than was possible when Publication 66 was issued in 1994.
(101) The revised particle transport model adopted here is shown in Figure 6.
Region ET2 is described in the model by two compartments, ETseq and ET 2 . Because
the oral passage is no longer included in Region ET2 (see above), compartment ET 2
is redefined as consisting of the posterior nasal passage, pharynx and larynx. The
compartments used to represent the retention of particles deposited in the BB and bb
regions that are cleared slowly (compartments BB2 and bb2 in Figure 5) are no longer