Comparative Dosimetry of Radon in Mines and Homes (1991) / Chapter Skim
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The Committee's Dosimetric Model for Radon and Thoron Progeny
The Committee's Dosimetric Model for Radon and Thoron Progeny
Pages 194-238
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From page 194... ...
DOSIMETRIC ASSUMPTIONS AND MODEL Both secretory and basal cells in the bronchial epithelium and, to a lesser extent, secretory cells in the bronchioles were identified in Chapter 8 as the principal targets for induction of bronchogenic cancer. The location of these cells within the ciliated epithelium is illustrated schematically in Figure 9-1, which shows a diagrammatic section through the wall of a bronchus.
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From page 195... ...
Figure 9-2 illustrates two possible locations of the source, within the sheath of mucous "gel" overlying the cilia and within the epithelium itself, if the progeny move into the epithelium. In a dosimetric model, the distribution of target cell nuclei in the bronchial epithelium can be approximated by the idealized structure shown in Figure 9-3.
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From page 196... ...
An airway caliber of 5 mm in diameter is assumed to typify a bronchus. In adults, the actual airway caliber varies from about 1 cm in TO ~ Source in Mucous Gel Source in \ ~ ~ ~ / Cilia in,, l FIGURE 9-2 Cylindrical model of a bronchial or bronchiolar airway used to calculate doses received by target cell nuclei from alpha-particle decays of radon progeny located in mucus or in the epithelium.
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From page 197... ...
Figure 9-4 also indicates the range of depths at which secretory or basal cell nuclei are assumed to occur. When the depth-dose curves are averaged over these ranges of target depths, it is found that the average dose received by secretory cell nuclei is relatively independent of the location of radon progeny alpha-particle decays, but the dose received by basal cell nuclei is significantly higher if radon progeny decay in the epithelium rather than in mucus.
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From page 198... ...
Figure 9-5 illustrates the model of target cell nuclei and mucus assumed by the committee to represent the epithelial lining of the bronchioles. These airways are devoid of basal cells.
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From page 199... ...
The doses received by these various target cell populations for a given subject, under given conditions of exposure, are evaluated by first modeling the number of Gyro and Typo alpha decays that occur in each airway generation, using the procedures described below. In order to apply the dose conversion coefficients given in Table 9-1, it is necessary to specify the surface areas of the respective airways in each subject.
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From page 200... ...
of 3,000 ml (James, 19881. The model of airway dimensions is needed for two purposes: first, to calculate the fractions of inhaled radon progeny activity that are deposited in each airway generation throughout the bronchial tree (and also in the alveolated respiratory airways)
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From page 201... ...
(1989~. The committee assumes that all airway dimensions in the fully grown lungs of adults (male and female)
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From page 202... ...
These values are used with the coefficients given in Table 9-1, to convert the calculated number of radon progeny alpha-particle decays in each airway generation into radiation doses absorbed by target cell nuclei. CLEARANCE MODEL The process of breathing continuously deposits radon progeny at all levels in the bronchial tree.
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From page 203... ...
, since the number of airways is halved each time subsidiary branches converge into a common parent. The combination of these processes with that of radioactive decay determines the number of alpha decays that occur in each airway generation, the surface density of the decays, and thus the dose received by target cells.
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From page 204... ...
-it ,,! ,,,,,.,,,1,,,,,,,,,,,,.,.,.,.,.,~.1 2 4 6 8 10 12 14 16 18 20 22 24 26 Airway Generation Number FIGURE 9-8 Scaling of airway length as described for Figure 9-7.
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From page 205... ...
The model also has a pathway along which radon progeny may be transferred through the mucous sol layer to be retained temporarily in epithelial tissue, before radioactive decay or absorption into the blood. through 214Bi)
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From page 206... ...
In the absence of contrary data, the committee assumed that these values also typify mucous clearance times in children and infants. The clearance model shown in Figure 9-9 also enables the effect on doses to target cells of any transfer of deposited radon progeny from the surface fluid to the underlying epithelium to be evaluated (NEA, 1983~.
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From page 207... ...
However, the impact of this uncertain aspect of radon progeny behavior on doses to target cells is examined in Chapter 3. DEPOSITION MODEL The committee used the theoretical model of aerosol transport and deposition in the lung that was developed by Egan and Nixon (1985)
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From page 208... ...
made by inertial impaction is modeled directly from the experimental data obtained with hollow bronchial casts of human lung (Gurman et al., 1984~. Figure 9-10 shows that the efficiencies, ~2, with which particles are deposited by impaction in each bronchus of a particular airway generation, i, can be approximated by an expression of the form 9~ = aStk~ (9-6)
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From page 209... ...
The stokes number was varied experimentally by changing both the particle size arid the airflow rate through the cast. where Stki is the stokes number of the flow in that airway generation, given by Stki = pd2 ui/18 Audi, where p is the particle density, dp is the particle diameter, ~ is the fluid viscosity, and ui and di are the mean flow velocity and airway diameter, respectively, in generation i.
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From page 210... ...
The proper treatment of thermodynamic and aerodynamic processes as competing processes (represented by Equation 9-9 gives significantly lower estimates of their combined efficiency. There is some uncertainty in the use of a theoretical expression to evaluate the thermodynamic deposition efficiency in the complex flow fields that occur in the upper bronchial airways.
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From page 211... ...
averaged over airway generations 1 through 6, for each combination of mean tracheal flow rate and particle size. The error bars show +1 standard deviation of individual values of F observed in different airway generations (there was no apparent correlation of F with airway generation number)
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From page 212... ...
The committee used the net correction factors shown in Figure 9-12 to evaluate bronchial deposition as a function of radon progeny aerosol size. The effect of this correction for enhanced thermodynamic deposition on calculated doses from inhaled radon progeny is examined in Chapter 4.
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From page 213... ...
The attached or so-called accumulation mode of the radon progeny aerosol has a median size in ambient air that ranges from about 0.15 to 0.25 ,um diameter (Chapter 6~. However, the carrier aerosol particles are considered to be partly hydroscopic and to grow in the respiratory tract to about double their ambient size (Chapter 6; Sinclair et al., 19744.
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From page 214... ...
The net effects of variations in breathing rate and radon progeny aerosol size distributions on doses received by different subjects are examined below. Another significant factor that must be accounted for in calculating the deposition profile of radon progeny within the respiratory tract is the typical variability or dispersion in size of the aerosol particles.
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From page 215... ...
Each component or mode of the radon progeny aerosol is assumed to be represented by a log normal distribution of activity with particle size, in which the geometric standard deviation (denoted by ag) is related to the activity median diameter (denoted by AMD)
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From page 216... ...
More recently, hollow casts of the human nasal and oral passages have been used to study the mechanisms of particle deposition, in particular, the influence of flow rate and particle diffusion coefficient on the deposition of particles in the size range of 0.2 to 0.005 ,um in diameter (Cheng et al., 1988, 19901. For these particles, the deposition efficiency, E, of both the nasal and oral passages was found to be represented by an empirical expression of the form E = 1—exp(—kQ-l/8D2/3 (9-13)
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From page 217... ...
(1989) found that it was necessary to modify the exponent of the particle diffusion coefficient in Equation 9-13 from 2/3 to 1/2 to fit the measured deposition efficiencies.
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From page 218... ...
The impact of this uncertainty on estimates of lung dose in mine and home environments was examined elsewhere in this report.* To evaluate lung dose in subject's who breathe habitually through their mouth, it has been customary to assume that the filtration efficiency of the oral passageway for unattached radon progeny is negligibly low (NCRP, 1984~.
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From page 219... ...
The curves show the respective efficiency functions fitted to these data. in view of the preliminary nature of these data and the lack of confirmatory evidence in viva, the committee took a more conservative approach by assuming that the filtration efficiency of the oral passageway for unattached radon progeny is only 50% of the value for nasal filtration efficiency.
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From page 220... ...
Therefore, to scale nasal and oral deposition efficiency for body size, the factor to be applied to Q in Equation 9-14 is Lref/Ls, where Lref is a characteristic airway dimension for the reference adult male and Ls is the
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From page 221... ...
Finally, it is also necessary to consider the efficiencies of the nose and mouth for removing that part of the radon progeny aerosol spectrum that overlaps the aerodynamic size range. The experimental data on nasal deposition of particles in the size range from about 1 to 10 Am in aerodynamic diameter in human subjects are shown in Figure 9-21 (Stahlhofen et al., 19891.
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From page 222... ...
The factor 1/L3 can therefore be applied to Q to scale inertial effects in the nasal or oral passages for airway dimensions. As assumed above to scale thermodynamic deposition efficiencies, the committee adopted the diameter of the trachea as the reference airway dimension (Yu and Xu, 1987; Swift, 19894.
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From page 223... ...
This is done to evaluate the conversion coefficient between "exposure to potential oe-energy" and the "dose" received by the tissues of the respiratory tract that are deemed sensitive to bronchogenic cancer. The exposure-dose conversion coefficient is examined as a function of the particular conditions of exposure and determined by the radon progeny aerosol size and characteristics of the exposed subject.
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From page 224... ...
Unattached = 0 This mixture of typo and 2~4Bi activities is assumed below to calculate dose conversion coefficients for unit exposure to unattached radon progeny (1 WLM) in the size range 0.0006 sum (0.6 nm)
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From page 225... ...
= 0.2. 225 Exposure-dose conversion coefficients are derived below as functions of the attached radon progeny aerosol size over the continuous range of size that may extend from 0.01 ,um (10 nm)
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From page 226... ...
Exposure-Dose Conversion Coefficients In this final section, the dependence of the radon progeny exposure-dose conversion coefficient on influencing factors such as aerosol size, breathing rate, the assumed clearance behavior of the progeny, the choice of target cell population for which dose is evaluated, the presence of airway disease, and the gender or age of the exposed subject are examined. In each case, the dose received by the nuclei of target cells is evaluated as a continuous function of the assumed size of radon progeny on entry into the respiratory tract.
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From page 227... ...
(19891. Influence of Aerosol Size, Clearance Behavior, and Target Cells The calculated dependence on radon progeny aerosol size of doses received by the nuclei of various target cells in an adult male is examined in Figures 9-22 through 9-24.
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From page 228... ...
With Assumed Nasal Deposition Exposure-Dose Conversion Coefficient (mGy per WLM) for the Following Level of Physical Exertion Sleep Rest Light Exercise Heavy Exercise Adult Male Adult Female Child age 5 yr 0.001 1b 0.0011C 0.02 0.15 0.25 0.3 0.5 0.001 1b 0.0011C 0.02 0.15 0.25 0.3 0.5 Child age 10 yr O.OOl lb 0.0011C 0.02 0.15 0.25 0.3 0.5 0.001 1b 0.0011C 0.02 0.15 0.25 0.3 0.5 Infant age 1 yr O.OOllb 0.0011C 0.02 0.15 0.25 0.3 0.5 Infant age 1 mo O.OOllb 0.0011C 0.02 0.15 0.25 0.3 0.5 48.9 59.6 23.4 28.9 18.3 20.0 4.66 5.04 3.35 3.64 3.03 3.31 2.63 2.93 39.9 48.8 18.6 23.1 19.1 21.4 4.66 5.17 3.29 3.64 2.95 3.27 2.48 2.77 48.8 60.2 23.0 28.8 22.3 24.9 5.38 5.98 3.81 4.25 3.43 3.84 2.89 3.30 55.7 74.9 26.1 36.0 25.8 29.7 6.04 6.98 4.34 5.05 3~93 4.61 3.33 4.08 63.8 94.3 29.6 45.2 33.2 39.6 7.77 9.06 5.56 6.65 5.02 6.11 4.17 5.47 50.1 22.4 36.8 9.02 6.25 5.54 4.25 153.0 80.9 31.5 7.86 6.31 6.22 7.51 152.5 80.0 36.2 8.62 6.75 6.59 7.77 166.5 87.5 40.1 9.58 7.61 7.47 8.80 129.4 65.6 38.2 8.94 6.66 6.26 6.42 148.6 74.3 48.2 10.9 8.19 7.69 7.67 78.8 36.7 45.9 10.9 7.35 6.47 5.01 321.2 210.7 41.6 11.8 14.9 18.4 38.6 340.9 223.2 49.2 13.4 17.1 21.1 44.4 aAverage value of the exposure-dose conversion coefficient calculated on the alternative assumptions that radon progeny are i)
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From page 229... ...
..... .0001 1Lobar/Segmental Bronchi All Bronchi Bronchioles .001 .01 .1 1 10 Activity Median Thermodynamic Diameter, Em FIGURE 9-22 Effects of radon progeny aerosol size on calculated dose to secretory cell nuclei adult male (insoluble/light exercise/nose breather)
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From page 230... ...
However, if basal cells are instead assumed to be the principal targets, significantly different values of the exposure-dose conversion coefficient are calculated (Figure 9-24~. In this case, dose conversion coefficients are approximately twofold lower for ultrafine radon progeny aerosols than they are for secretory cells, and uncertainty in the clearance behavior of radon progeny has a greater impact.
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From page 231... ...
. 1 1 10 Activity Median Thermodynamic Diameter, Am FIGURE 9-25 Effects of radon progeny aerosol size and exercise on calculated dose to secretory cell nuclei in the bronchi adult male (nose breather)
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From page 232... ...
_ it' 100o a) o COMPARATIVE DOSIMETRY OF RADON IN MINES AND HOMES .0001 .001 .01 Newborn Infant 1-y Child 5-y Child 1 0-y Adult Female Adult Male .1 1 10 Activity Median Thermodynamic Diameter, Am FIGURE 9-26 Effects of radon progeny aerosol size on calculated dose to secretory cell nuclei in bronchi for different subjects (resting/nose breather)
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From page 233... ...
.... .1 1 10 Activity Median Thermodynamic Diameter, Am FIGURE 9-29 Effect of epithelial regeneration on calculated dose to bronchial secretory cell nuclei adult male (light exercise/nose breather)
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From page 234... ...
Finally, Figure 9-29 compares doses calculated for secretory cells in thinned epithelium that is undergoing regeneration with those in epithelium of normal thickness. In this case, target cells in the damaged epithelium are calculated to receive about twofold higher doses than those in normal epithelium.
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From page 235... ...
1984. Particle deposition in replicate casts of the human upper tracheobronchial tree under constant and cyclic inspiratory flow.
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From page 236... ...
1970. Experimental study of the distribution of short-lived radon daughters in the respiratory tract.
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From page 237... ...
1990. Deposition of unattached radon daughters in models of human nasal airways.
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