Occupational Intakes of Radionuclides Part 1 - ICRP

1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
DRAFT REPORT FOR CONSULTATION
(113) Most of the relevant recent human studies thus suggest that slow clearance in
the conducting airways is associated with particles deposited in the bronchioles: a
simpler explanation than the particle-size dependent clearance mechanism assumed in
Publication 66. In the revised HRTM, it is assumed that slow clearance in the
conducting airways occurs only in the bb region, and following Falk et al (1997,
1999) as described above, particles are taken to be cleared from the bb region to the
BB region at a rate of 0.2 d -1 (t½ ~3.5 d) (except for the small sequestered fraction, see
below). The rate of rapid clearance from the BB region to the ET region is unchanged
at 10 d -1 .
(114) The results of Falk et al (1997, 1999) suggest that only a fraction of particles
deposited in bb is cleared slowly, perhaps 25% for the conditions of their
experiments. If so, it is reasonable to suppose that it occurs mainly in the smaller
bronchioles, as proposed by Camner et al (1997). However, given the remaining
uncertainties, (and the lack of deposition fractions available for subdivisions of the bb
region) it is assumed here for simplicity that it applies to all particles deposited in the
bb region. It is also assumed that it applies to all particles cleared from the AI region
to the bb region, unlike the Publication 66 implementation of the HRTM which
assumed that slow clearance applied only to particles deposited directly in the BB and
bb regions. These changes result in a simplification of the model: a single
compartment BB′ replaces BB1 and BB2 and a single compartment bb′ replaces bb1
and bb2 (Figure 6). Associated changes to the dosimetric model are described in
Section 3.2.4.
Sequestration in the airway walls
(115) The original HRTM assumes that the fraction of particles deposited in the BB
and bb regions retained in the airway wall (BBseq and bbseq) is 0.7% at all sizes, and
that this material clears to lymph nodes at a rate of 0.01 d –1 . When the original HRTM
was finalised the phenomenon had only been well quantified by Patrick and
colleagues (e.g. Takahashi and Patrick, 1987), who followed retention of activity after
deposition of radio-labelled particles onto the distal trachea of rats. Subsequently,
Takahashi et al (1993) conducted similar experiments, instilling 133 Ba-labelled BaSO4
onto the distal trachea of rabbits, dogs and monkeys. The amounts retained 1 week
after injection were 0.145%, 0.044% and 0.043% of the injected amount, respectively.
These values are far lower than found in rats, suggesting inter-species differences.
The value chosen above for retention of particles in the wall of the nasal epithelium,
ETseq, 0.2%, which was based on results for several different materials in several
species, is within the range observed for the trachea. On that basis it is assumed here
that values for both the retained fractions and clearance rates from BB and bb to LNTH
are the same as those for ETseq, i.e., 0.2% and 0.001 d –1 . The revised model thus
assumes less transfer to LNTH from BB and bb, but more transfer from the AI region
(see below), maintaining consistency with the ratio of lung to LNTH contents observed
in autopsy studies.
(116) The changes from the treatment of slow clearance from the bronchial tree in
the original HRTM will in many cases decrease dose coefficients. The decreases will
be considerable for Type M alpha-emitting radionuclides with half-lives of weeks or
more, for which slow clearance gave the largest component of the effective dose
57