Comparative Dosimetry of Radon in Mines and Homes (1991) / Chapter Skim
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Dosimetry and Dosimetric Models for Inhaled Radon and Progeny
Dosimetry and Dosimetric Models for Inhaled Radon and Progeny
Pages 60-89
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From page 60... ...
Such is the case with the dosimetry of inhaled radon and radon progeny in the respiratory tract. The objective of this chapter is to provide both background and a historical perspective of the development of dosimetric models for radon and radon 60
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MORPHOMETRIC MODELS OF THE RESPIRATORY TRACT A morphometric model of the respiratory tract is a fundamental component
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From page 62... ...
Without an accurate anatomic description of the respiratory tract, one is limited in the ability to apply theoretical or empirical principles of airflow patterns and aerosol deposition at the level of resolution that is presumed to be necessary for properly modeling the radiation dose distribution from inhalation of radon and radon progeny, i.e., at the millimeter and submillimeter levels (see Chapter 9~. Although morphometric models of the respiratory tract below the level of the larynx have existed for some time, there is currently no
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From page 63... ...
21 2po 0.30Ss 1~(36%) 4: .~/ 208T' / 3.07min single morphometric model considered to be fully adequate for describing the morphometry of the entire respiratory tract, which includes the nasal airways, oral cavity, nasopharynx and oropharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
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From page 64... ...
223Ra 1 11.4d 1 0t 21 9Rn 3.96s _ 21' iPo 1.78ms 211BI ~,;`L 2.1 4min 211pb 1/ 1 (X 36.1 mIn r ~ ,BL 1 207TI / 4.77mln 2o7pb Third, although the nasal airways have long been recognized as an important deposition site for large-sized aerosol particles (>10 film) , their importance in filtering out smaller particles (<0.1 ,um)
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From page 66... ...
(1978) also formulated a geometrical model for calculating aerosol particle deposition within the nasal airways.
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From page 67... ...
However, the model, together with the accompanying theory of particle deposition in the nasal airways, did show reasonable agreement with the data available from studies of aerosol deposition in human nasal airways (Scott et al., 1978~. Except for the dosimetry model of Bailey (1984)
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These measurements were used to construct two morphometric models of the human lung: the first (Weibel lung model A) , which emphasized the regular features of the airways and their patterns, and the second (Weibel lung model B)
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From page 69... ...
It is not known whether the present models accurately reflect the morphology of the conducting airways of the human lung, nor is it known to what degree the variabilities in the morphometric models can affect dosimetry calculations. Since most of the models currently being used for deposition and dosime
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(1979) also used Weibel lung model A as a basis for constructing a tracheobronchial airway morphometry model that included variability in airway size.
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From page 71... ...
It is not clear, however, what level of accuracy in the morphometric model is needed for the specific case of constructing an adequate dosimetric model for exposure of the conducting airways to radon and radon progeny. In this context, other important facets of the dosimetric model, e.g., thickness of epithelium and mucus, deposition and clearance parameters, and identification of cells at risk, must also be considered.
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From page 72... ...
DEPOSITION MODELS OF THE RESPIRATORY TRACT One of the important factors controlling the distribution of alpha-radiation dose to the different portions of the human respiratory tract is the deposition pattern of radon progeny-containing aerosols. Much research has been done to determine experimentally the deposition fractions for different sizes and types of aerosols in different experimental animal models, including humans.
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From page 73... ...
They also used the variabilities in the data sets that were probably due to intersubject and intrasubject variabilities to calculate variances for the calculated mean deposition fractions, assuming normal statistics. Although these data and models are relevant to many types of inhalation exposures, the particle size range for the studies described above does not include the sizes of most radon progeny-containing aerosols, which are significantly smaller.
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From page 74... ...
In general, for nasal breathing and for a particle size range of greater than 0.5 Am, less than 10% deposition of aerosol is found in the conducting airways, within wide limits of assumed airway sizes and breathing patterns. Oral breathing tends to result in increased deposition of large aerosol particles in the conducting airways by impaction; this is a result of increased penetration into the lung because of the absence of nasal filtration.
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From page 75... ...
This increased deposition is important to consider with respect to the deposition of radon progeny aerosols within the tracheobronchial region of the respiratory tract. Nonuniform enhanced deposition of aerosol particles within the regions of conducting airway bifurcations has been reported in studies in which cast replica models of the upper airways have been used (Martin and Jacobi, 1972; Gurman, 1983; Martonen and Lowe, 1983; Schlesinger et al., 1983; Cohen et al., 19881.
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From page 76... ...
As diffusional mechanisms increasingly predominate with decreasing particle size, the effect of localized flow patterns becomes less important, leading to more and more uniform localized depositions (Gradon and Orlicki, 1990~. Thus, although the data are very limited, it appears that enhanced deposition at airway bifurcations may not be a significant factor to be considered for the expected particle sizes to which radon progeny would be attached.
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From page 77... ...
Such deposition values have been functionally defined based on observed clearance rates of particles from the thorax region. CLEARANCE MODELS OF THE RESPIRATORY TRACT Models of clearance of inhaled particles from the respiratory tract differ
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From page 78... ...
Measurements of mucous clearance velocities and particle clearance for the ciliated region of the nasal airways have shown that clearance rates are large, but with substantial intersubject variability (Ewert, 1965; Proctor and Wagner, 1965; Bang et al., 1967; Quinlan et al., 1969; Andersen et al., 1971; Fry and Black, 1973; Proctor, 1973; Yergin et al., 19781. These results have been obtained by a combination of methods, including radiographic imaging of radiopaque particles, direct optical visualization of the streaming of dye particles, radiometric imaging of instilled radionuclide-containing particles, and subjective perception of a substance by taste (saccharine test)
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From page 79... ...
Since clearance of particles from the different levels of the tracheobronchial tree has been considered to be an important variable in assessing the radiation dose to the bronchial epithelium from radon progeny-containing aerosols, investigators have resorted to theoretical models to calculate the generation-by-generation mucous clearance rates for the conducting airways. Clearance velocities calculated by Harley and Pasternack (1972)
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From page 80... ...
. uniformly thick mucous blanket that undergoes neither secretion nor absorption throughout the length of the conducting airways; thus, the generation-specific mucous velocities scale as the ratio of the total airway perimeters, i.e., v,> = vodo/2~d~, where vet is the velocity in airway generation or, do is the diameter of an airway of generation or, and v0 and do are the velocity of clearance and diameter of the trachea, respectively.
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From page 81... ...
The impetus for the continued development of increasingly sophisticated models stems from the fact that the actual alpha-radiation doses to the presumed target tissue, the epithelial cells of the bronchial airways, cannot actually be measured, thus requiring dosimetric modeling, and because better data relating to the key components of the dosimetric models have become available, i.e., the physical characteristics of mine and home atmospheres, improved morphometric measurements of the respiratory tract, better deposition modeling of aerosols in the different regions of the respiratory tract, more detailed modeling of clearance phenomena, particularly in the tracheobronchial airways, and evolving views concerning the critical cells at risk to the development of alpha-radiation-induced lung cancer. Several documents have summarized the radon dosimetric models that have developed over the years.
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From page 82... ...
For unattached radon progeny, the dose conversion factors for most estimates were mostly in the range of 10 to 20 rad/WLM. To date, there remains little consensus as to the atmospheric characteristics of radon progeny aerosols in mine and home environments (NCRP, 1984; ICRP, 1987; NRC, 1988; United Nations Scientific Committee on the Effects of Atomic Radiation, 19881.
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From page 83... ...
1982. Measurements of the total and regional deposition of inhaled particles in the human respiratory tract.
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1984. Particle deposition in replicate casts of the human upper tracheobronchial tree under constant and cyclic inspiratory flow.
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1971. The deposition of aerosol particles in the nasopha~yngeal region of the human respiratory tract.
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1969. The effect of particle size on the regional deposition of inhaled aerosols in the human respiratory tract.
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From page 87... ...
1972. Particle deposition in casts of the human upper tracheobronchial tree.
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1980. Experimental determination of the regional deposition of aerosol particles in the human respiratory tract.
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From page 89... ...
1983. Mucociliary transport and particle clearance in the human tracheobronchial tree.
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