Occupational Intakes of Radionuclides Part 1 - ICRP

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1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 Anterior nasal Posterior nasal, pharynx, larynx Bronchial Bronchiolar Alveolarinterstitial Extrathoracic LNTH Thoracic 0.001 0.001 0.001 0.00003 DRAFT REPORT FOR CONSULTATION Figure 6. Revised compartment model representing time-dependent particle transport from each respiratory tract region. Rates shown alongside arrows are reference values in units of d –1 . It is assumed that 0.2% of material deposited in regions ET2, BB and bb is retained in the airway wall (ETseq, BBseq and bbseq respectively). Particle transport: extrathoracic airways BBseq bbseq (102) The Publication 66 model assumed that of material deposited in the ET airways, about 50% deposits in ET1 (Figure 5), which is cleared by nose blowing at a rate of 1 d –1 , and the rest deposits in ET2, which clears to the GI tract at a rate of 100 d –1 . However, there was little information available to quantify clearance from ET1. It was recognised that the fraction deposited in ET1 was generally greater than that in ET2 and that there was slow transfer from ET1 to ET2, but insufficient information was available to quantify these factors and transfer rates. In experiments intended to address this deficiency, subjects inhaled 1.5-, 3- or 6-µm aerodynamic diameter (dae) radiolabelled insoluble particles through the nose while sitting at rest or performing light exercise (Smith et al, 2002, 2011). Retention in the nasal airways and clearance by voluntary nose blowing were followed until at least 95% of the initial ET deposit (IETD) had cleared (typically about 2 days). On average, 19% IETD was cleared by nose blowing (geometric mean time for 50% clearance was 8 hours), and the rest was cleared to the alimentary tract: 15% within a few minutes, 21% between a few minutes and an hour, and 45% on a similar time-scale to the fraction cleared by noseblowing. Measurements in this study, and the previous studies on which the original model was based, indicate that most particles that have not cleared within an hour are retained in the anterior nasal passage. 54 INT 0.001 1.5 10 0.2 0.002 ET1 LNET ETseq ET’2 BB’ bb’ ALV 0.6 100 Environment Oesophagus
1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 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 55
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Occupational Intakes of Radionuclides Part 1 - ICRP

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