Occupational Intakes of Radionuclides Part 1 - ICRP

2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
DRAFT REPORT FOR CONSULTATION
observed. The optimised parameter values derived by Kuempel et al (2001) were a
rate mT = 0.001 d -1 for clearance from the alveolar compartment to the bronchiolar
region, a rate mI = 0.00047 d -1 for clearance from the alveolar compartment to the
interstitium, and a rate mLN = 10 -5 d -1 for clearance from the interstitium to lymph
nodes. The main difference from the original HRTM AI model is that a significant
fraction of the AI deposit is sequestered in the interstitium [mI/(mI+mT) = 0.32].
Kuempel et al (2001) noted that the HRTM underestimated lung retention in the
miners by about a factor of four.
(125) Gregoratto et al (2010) showed that the Kuempel et al (2001) model provides
an adequate representation of AI retention for the data in the other three studies
outlined above. They developed a new model using the Kuempel et al model structure
but fitted to both the experimental datasets on which the HRTM parameter values
were based, and the more recent long-term studies (Figure 8). They obtained particle
transport rates from the alveolar compartment of mT = 0.0017 d -1 and mI = 0.0010 d -1 .
These values are adopted here, but the value of mT is rounded to 0.002 d -1 , reflecting
the underlying uncertainty in the model (Figure 6). These rates give a clearance halftime
from the alveolar compartment of about 250 days (mI+mT = 0.003 d -1 ), and about
33% of the alveolar deposit of insoluble particles is sequestered in the interstitium.
The greater AI retention than in the original HRTM is likely to result in lung doses
per unit intake that are 50-100% higher for Type S long-lived alpha-emitters, but will
have little, if any effect on more soluble forms.
(126) No clear difference was observed by Gregoratto et al (2010) between smokers
and non-smokers in the long-term studies they analyzed. This contrasts with the
greater retention in smokers than in non-smokers observed in those studies reviewed
in Publication 66 in which the comparison could be made, although it is noted that the
earlier studies were of relatively short duration. It also contrasts with the much greater
retention in smokers than in non-smokers observed in studies of alveolar retention of
iron oxide followed using magnetopneumography (see section on iron in Part 2), but
for which absorption to blood rather than particle transport is considered to be the
dominant clearance mechanism.The modifying functions proposed in Table 19 of
Publication 66 relating to the effect of cigarette smoking on particle transport from the
AI region are therefore not considered applicable to the revised model. Furthermore, it
is not recommended that the other modifying factors in that table are applied in
individual dose assessments.
(127) In the original HRTM, the transport rate from the AI region to the thoracic
lymph nodes, LNTH, was set at 2x10 -5 d -1 to give the ratio of material concentration in
lymph nodes and lungs equal to that estimated from autopsy data: for non–smokers
[LN]/[L]≈20 after 10,000 days after inhalation of Pu (Kathren et al, 1993). Because of
the smaller fraction of the deposit in the BB and bb regions cleared to LNTH via the
airway walls (BBseq and bbseq), and the longer AI retention in the model adopted here
than in the Publication 66 model, the amount cleared to LNTH from the BB and bb is
now negligible compared to that from the AI region. The ratio [LN]/[L]≈20 is
obtained with a transport rate from the interstitium to LNTH of 3x10 -5 d -1 (Gregoratto
et al, 2010).
61