Occupational Intakes of Radionuclides Part 1 - ICRP

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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
2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088 2089 2090 2091 2092 DRAFT REPORT FOR CONSULTATION (121) A recent study by Davis et al (2007) provides better in vivo information on long-term lung retention than any available when Publication 66 was adopted. A group of workers had a simultaneous brief inhalation exposure to particles containing cobalt-60, and most (seven) had been followed for about 15 years. It is reasonable to assume that they are representative of nuclear industry workers. They all showed much slower clearance than the HRTM predicts, consistent with the few data on retention beyond 2000 days available at the time that the HRTM was published (Figure 7). Lung Retention R(tf) Lung retention R(300) 1.00 0.10 0.01 Ta1 U1 Co3 Co3Ce1 Pu5 Pu2T1 Co1 U10 Co3 Pu1Am3 Co4 U4 Co3 U2 U6 Pu2 U5 U3 Am1 Co2 Pu2 U7 Pu4 Co5 Co5 Co5 Pu4 Co1 Am2 U8 Co5 U8 Am4 Co5 U11 Co1 U8 Co5 Co5 Pu3 (122) Figure 7 Long term thoracic retention following accidental inhalation. References for data included in the figure are listed in Table 4. Separate symbols are used for each element. The solid and dashed curves show retention of insoluble particles as predicted by the original HRTM and the revised HRTM, respectively. Thoracic retention R(tf) at tf, the time of the final measurement, is expressed as a fraction of R(300), retention at 300 d. 59 U9 Pu7 0 3000 6000 9000 12000 15000 Pu6 Pu6 Time of Final Measurement t f (d) Pu6 Pu6 Pu6 Pu6 Pu6 Pu6 Pu6
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Occupational Intakes of Radionuclides Part 1 - ICRP

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