Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(diffusion) processes acting competitively. Regional deposition fractions were
calculated for aerosols having lognormal particle size distributions, with geometric
standard deviations taken to be a function of the median particle diameter, increasing
from a value of 1.0 at 0.6 nm to a value of 2.5 above about 1 μm.
(94) No changes are made here to the Publication 66 implementation of the
deposition model for aerosols, except for the distribution of the deposit in the ET
airways between regions ET1 and ET2. In Publication 66 (ICRP, 1994a) it was
assessed, on the basis of the available information, that deposition in ET1 is somewhat
higher than in ET2 during inhalation through the nose, and that most of the particles
deposited in ET1 are cleared by nose-blowing, but some clear to ET2 and hence to the
alimentary tract on a time scale of hours. However, because of the lack of quantitative
information, these judgements were applied in a simplified form in the original
HRTM. It was assumed that particles deposited in the nasal passage during inhalation
are partitioned equally between ET1 and the posterior nasal passage, which is part of
ET2. (However, because of the way the deposition efficiencies were calculated for
polydisperse aerosols during inhalation and exhalation, for most aerosol sizes of
interest in radiation protection the deposition fractions given in Publication 66 are
somewhat higher for ET2 than for ET1.) As described in the section below on particle
transport from the ET airways, recent experimental studies (Smith et al, 2011) enable
a more accurate representation of ET deposition and clearance to be implemented
here. Results for a group of subjects indicated that the distribution of the deposit in
the ET airways can be characterised by mean deposition fractions of 65% to ET1 and
35% to ET2. To calculate the fractions of inhaled material deposited in ET1 and ET2,
the fractions deposited in ET1 and ET2 (calculated using the original HRTM) were
summed to give the total deposit in the ET airways, and then re-partitioned 65% to
ET1 and 35% to ET2. (For mouth breathing there is no deposition in ET1 and the
fraction deposited in ET2 remains as calculated using the original HRTM.)
(95) For inhalation of radionuclides by workers, the reference subjects are taken to
be normal nose-breathing adult males and females at light work. However, for
simplicity, deposition in (and clearance from) the respiratory tract are calculated for
the reference adult male only. For occupational exposure, the default value
recommended for the Activity Median Aerodynamic Diameter (AMAD) is 5 μm
(ICRP, 1994b), consistent with the review of data by Dorrian and Bailey (1995) and
Ansoborlo et al (1997). Fractional deposition in each region of the respiratory tract of
the reference worker is given in Table 3 for aerosols of 5 μm AMAD.
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