Occupational Intakes of Radionuclides Part 1 - ICRP

2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
DRAFT REPORT FOR CONSULTATION
coefficient. Changes to parameter values relating to sequestration have little effect on
effective dose coefficients because it only ever makes a small contribution to them.
Particle transport: alveolar-interstitial (AI) region
(117) In the original HRTM, the AI region was represented by three compartments:
AI1, AI2 and AI3, which mainly clear to the GI tract via the bronchial tree at rates of
0.02, 0.001 and 0.0001 d –1 , respectively (approximate half-times 35, 700 and 7000 d)
(Figure 5). Human lung clearance had been quantified in experimental studies up to
about a year after inhalation (ICRP, 1994a). It was considered that lung retention of
insoluble particles over this time typically follows a two-component exponential
function: about 30% with a half-time of about 30 d, and the rest with a half-time of
several hundred days, giving about 50% retention of the initial alveolar deposit (IAD)
at 300 d. This information was used to define the parameter values for AI1.
(118) Measurements of activity in the chest after occupational exposure, and of
activity in the lungs at autopsy, indicate that some material can be retained in the
lungs for decades. Information on thoracic retention in humans following accidental
inhalation, based on in vivo measurements of radionuclides, was reviewed in
Publication 66 (ICRP, 1994a). Because retention up to 300 d after intake had been
characterised in controlled experiments, only studies of accidental intakes in which
retention was followed for at least 400 d were included. Since the aim was to obtain
guidance on the likely fate of the approximately 50% IAD that remains at 300 d after
intake, thoracic retention R(tf) at tf, the time of the final measurement, was expressed
as a fraction of R(300), retention at 300 d. This also facilitated the inclusion of
information in cases where the first measurement was made some time after intake,
and avoided the effects of differences in early clearance due to factors such as aerosol
size, breathing patterns, and soluble components. In Figure E.10 of Publication 66,
thoracic retention R(tf), as a fraction of R(300), was plotted against tf: the information
is shown here in Figure 7. Evidence for very long term retention of a significant
fraction (> 10%) of the material remaining in the thorax at 300 d was seen for each of
the elements (cobalt, uranium, plutonium, and americium) for which measurements
extended to 10 y after acute intake of the oxide.
(119) The results were not used to set parameter values for AI2 and AI3
quantitatively because it was considered possible that the published in vivo studies
represented unusually slow lung clearance. It was noted (ICRP, 1994a) that: “The
fraction of the AI deposit that goes to AI3 (a3) is not easily quantified. Since only 50%
IAD is retained at 300 d, a3 is less than 0.5. Since there is measurable thoracic
retention at 5000 d after intake in some subjects (Figure 7), a3 is likely to be at least a
few percent of the IAD. As a rounded value it is assumed that a3 = 0.1, and, hence, by
difference, that a2 = 0.6.” Figure 7 also shows retention of insoluble particles as
predicted by the original HRTM: it fits quite well to results where the final
measurement was made less than 2000 days after intake, but underestimates those
with later measurements.
(120) In the revised model, account has been taken of additional human studies
published since the original HRTM was adopted, which all show greater long term
retention in the AI region than was assumed.
58