Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(103) On the basis of these data, it is assumed here that material deposited in ET1
(now taken to be 65% of the deposit in ET, as described in Section 3.2.1) is cleared at
a rate of 2.1 d –1 (half-time about 8 hours): about one-third, by nose blowing and twothirds
by transfer to ET2. This is implemented with particle transport rates of 0.6 d –1
from ET1 to the Environment and 1.5 d –1 from ET1 to ET 2 . Clearance from ET 2 is
unchanged, with a rate to the alimentary tract of 100 d –1 (half-time about 10 minutes).
As in the original HRTM, a small fraction of particles deposited in ET2 (but not
cleared to it from ET1) is sequestered in the airway wall (ETseq) and transferred to
lymph nodes. However, the fraction sequestered is increased from 0.05% of the
deposit in ET2 in the original HRTM, to 0.2% here, partly because of the smaller
fractional deposition in ET2, but also from reconsideration of the experimental data
relating to long-term retention of inhaled particles in the nasal passages, which were
reviewed in Publication 66.
(104) The changes from the original HRTM treatment of ET will in many cases
increase dose coefficients because of the transfer from ET1 to ET2 and hence greater
systemic uptake in ET2 and the alimentary tract. The changes will also affect
interpretation of measurements of radionuclides in faecal samples: a larger fraction of
the material deposited in the nose (which is typically about 50% of the material
inhaled) is cleared through the alimentary tract.
Particle transport: bronchial and bronchiolar airways
Slow clearance
(105) The original HRTM includes a slow phase of clearance of particles deposited
in the BB and bb regions (compartments BB2 and bb2 in Figure 5), with a half-time of
23 days. It was based mainly on the results of experiments in which volunteers
inhaled a ‘shallow bolus’ of radio-labelled particles i.e., a small volume of aerosol at
the end of each breath, designed to deposit particles in the major airways. A ‘slowcleared’
fraction was observed, which was considered to show a better correlation
with particle geometric diameter, dp than with dae (ICRP, 1994a). The original HRTM
assumes that the slow-cleared fraction of particles deposited in BB and in bb (fs) is 0.5
for dp ≤2.5 μm, and decreases exponentially for larger particles.
(106) In the revised HRTM, a different approach has been taken to slow clearance
from the bronchial tree based on more recent human volunteer experiments. In
particular, in a series of studies, large particles (6-μm dae) were inhaled extremely
slowly, which theoretically should result in most deposition occurring in the
bronchioles (e.g. Anderson et al, 1995; Camner et al, 1997; Falk et al, 1997, 1999;
Philipson et al, 2000; Svartengren et al, 2001). Retention at 24 hours was much
greater than the predicted AI deposition, supporting the concept of slow clearance in
the bronchial tree.
(107) Falk et al (1997, 1999) compared lung retention of 6 µm dae Teflon particles
inhaled slowly (~45 cm 3 s –1 ) with retention of similar particles inhaled at a normal
flow-rate (~450 cm 3 s -1 ) for up to 6 months. About 50% of the initial lung deposit
(ILD) cleared in the first 24 hours following both modes of inhalation. Retention after
24 hours was well described by a two-component exponential function, the clearance
rates having half-times of about 3.7 days (‘intermediate’ phase) and 200 days
(attributed to clearance from the AI region). The fractions associated with the
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