5
Dosimetry and Dosimetric Models for Inhaled Radon and Progeny

Dosimetry is defined as the measurement of the amount, type, rate, and distribution of radiation emitted from a source of ionizing radiation and the calculation of both spatial and temporal patterns of energy deposition in any material of interest as a result of ionizing radiation. Instruments and techniques exist to measure fields of penetrating radiation such as X rays or gamma rays that are external to the body, and provide means for directly quantifying the amount of energy deposited per unit mass of material (air, tissue, water). These dose measurements can then be related to a person present in the radiation field and the radiation dose that he or she would receive. In the case of internally deposited radionuclides, however, direct measurement of the energy absorbed from the ionizing radiation emitted by the decaying radionuclide is rarely, if ever, possible. Therefore, one must rely on dosimetric models to obtain estimates of the spatial and temporal patterns of energy deposition in tissues and organs of the body. In the simplest case, when the radionuclide is uniformly distributed throughout the volume of a tissue of homogeneous composition and when the size of the tissue is large compared with the range of the particulate emissions of the radionuclide, then the dose rate within the tissue is also uniform and calculation of absorbed dose can proceed without complication. However, if nonuniformities in the spatial and temporal distributions of radionuclide are coupled with heterogeneous tissue composition, then calculation of absorbed radiation dose becomes complex and uncertain. 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



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Comparative Dosimetry of Radon in Mines and Homes 5 Dosimetry and Dosimetric Models for Inhaled Radon and Progeny Dosimetry is defined as the measurement of the amount, type, rate, and distribution of radiation emitted from a source of ionizing radiation and the calculation of both spatial and temporal patterns of energy deposition in any material of interest as a result of ionizing radiation. Instruments and techniques exist to measure fields of penetrating radiation such as X rays or gamma rays that are external to the body, and provide means for directly quantifying the amount of energy deposited per unit mass of material (air, tissue, water). These dose measurements can then be related to a person present in the radiation field and the radiation dose that he or she would receive. In the case of internally deposited radionuclides, however, direct measurement of the energy absorbed from the ionizing radiation emitted by the decaying radionuclide is rarely, if ever, possible. Therefore, one must rely on dosimetric models to obtain estimates of the spatial and temporal patterns of energy deposition in tissues and organs of the body. In the simplest case, when the radionuclide is uniformly distributed throughout the volume of a tissue of homogeneous composition and when the size of the tissue is large compared with the range of the particulate emissions of the radionuclide, then the dose rate within the tissue is also uniform and calculation of absorbed dose can proceed without complication. However, if nonuniformities in the spatial and temporal distributions of radionuclide are coupled with heterogeneous tissue composition, then calculation of absorbed radiation dose becomes complex and uncertain. 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

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Comparative Dosimetry of Radon in Mines and Homes progeny and to provide a basis for more detailed descriptions of the scientific issues that relate to the different component parts of the present radon progeny dosimetric model. The physical dosimetry and microdosimetry of alpha particles in tissue are discussed in Appendix I of the BEIR IV report (NRC, 1988). SOME PHYSICAL CHARACTERISTICS OF RADON AND RADON PROGENY Three isotopes of radon occur naturally in the environment as a result of being part of the so-called uranium (238U), thorium (232Th), and actinium (235U) decay series. These are 222Rn, 220Rn, and 219Rn, respectively. Their importance as environmental sources of radioactivity stems from the fact that radon is a noble gas that can be carried in air or water streams far from its source of creation in soil and rock and can reach the indoor environment where people work and live. Radon's importance as an environmental source of radiation depends principally on the local concentrations of the parent radionuclide, the physical characteristics of the rocks and soil, and on its half-life. The last two factors determine the time available to migrate into air and water. The decay characteristics of the isotopes of the three decay series are summarized in Figures 5-1A through C. Of least importance is 219Rn, which, because of its 3.96 s half-life, has limited capacity for migration in the environment. Moreover, its progenitor, 235U, has a relatively low concentration in the environment. 220Rn, with a 55.6 s half-life, is more readily available to the environment, and in geographic regions that have shallow deposits of thorium-rich soils, it can be a major contributor to the potential radiation dose to people. However, the majority of the concern for risks from radon exposure is due to the environmental presence of the isotope 222Rn, which has the longest half-life of the radon isotopes, 3.824 days, and is ubiquitous because of the pervasive presence of its precursor radionuclides, 226Ra and 238U, in the earth's crust. The apparent complexity of the 222Rn decay scheme in Figure 1-1 can be simplified by neglecting the decays and , both of which occur with very low probabilities , and by truncating the decay scheme at 210Pb, which has a 22-yr half-life and is of little consequence as a respiratory hazard. Only three alpha-emitting radionuclides of consequence. then, remain, 222Rn, 218Po, and 214Po. Table 5-1 provides additional physical data on 222Rn and its progeny, including the historical designations for the decay products. Note that because the very short half-life for 214Po (164 s), it is in virtual equilibrium with its parent 214Bi under any conditions. Thus, decay chain calculations need only be done for the chain . MORPHOMETRIC MODELS OF THE RESPIRATORY TRACT A morphometric model of the respiratory tract is a fundamental component

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Comparative Dosimetry of Radon in Mines and Homes Figure 5-1A Uranium decay series. of any dosimetric model intended for use in estimating alpha-radiation doses from radon and radon progeny, because of the heterogeneous distribution of deposited activity that occurs upon inhalation and the diverse structures, airway sizes, and geometries that are found at the different levels of the respiratory tract. 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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Comparative Dosimetry of Radon in Mines and Homes Figure 5-1B Thorium decay series. 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. Additionally, the roles of subject size and age on the dimensions of the various regions of the respiratory tract have only recently received attention by researchers. HEAD AIRWAY MORPHOMETRY For several reasons, morphometric models have not yet been developed for either the nasal or the oral airways. First nasal airways in humans, although much simpler geometrically than those in most other animal species, are complex anatomic structures that do not lend themselves to description in simple geometric terms, e.g., as tubes with circular or elliptical cross sections. Second, significant interindividual variabilities in the size and shape of nasal airways, particularly as a function of the size of the individual, have been recognized.

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Comparative Dosimetry of Radon in Mines and Homes Figure 5-1C Actinium decay series. Third, although the nasal airways have long been recognized as an important deposition site for large-sized aerosol particles , their importance in filtering out smaller particles (<0.1 µm) has only recently been appreciated (Cheng et al., 1988). The nasal airways consist of several distinct anatomic regions that differ in terms of their size, shape, physiological function, and types of epithelial cells lining the airways (Swift and Proctor, 1976). Air enters through elliptically shaped openings, the anterior nares, and passes into a conically shaped vestibular region that extends approximately 10 to 15 mm in the posterior direction. The cross-sectional area decreases posteriorly to a minimum value at the location of the nasal valve or ostium internum, which has been determined by Haight and Cole (1983) to be the location of maximum airway resistance. Proctor

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Comparative Dosimetry of Radon in Mines and Homes TABLE 5-1 Some Physical Properties of 222Rn and Its Short-Lived Decay Products   No. of Atoms Element Historical Symbol Principal Radiation(s) Decay Energies (MeV) Half-Life Per µCi Per Bq 226Ra Ra α 4.8 1,620 yr 2.7 × 1015 7.4 × 1010 222Rn Rn α 5.5 3.82 day 1.8 × 1010 4.8 × 105 218Po RaA α 6.0 3.05 min 9.77 × 106 2.6 × 102 214Pb RaB β,γ 1.0 max 26.8 min 8.58 × 107 2.3 × 103 214Bi RaC β,γ 3.3 max 19.7 min 6.31 × 107 1.7 × 103 214Po RaC α 7.7 164 µs 8.8 2.4 × 10-4

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Comparative Dosimetry of Radon in Mines and Homes and Swift (1971) estimated that the average cross-sectional area at this point is probably between 20 and 40 mm2 per side. Posterior to the nasal valve are the nasal fossae, which contain the inferior, medial, and superior turbinates attached to the lateral walls. The left and right nasal airway cavities are separated by a thin cartilaginous nasal septum, whose thickness and shape can vary significantly among individuals, but it is typically 3- to 7-mm thick in the anterior cartilaginous part and 2- to 3-mm thick in the posterior bony part (International Commission on Radiological Protection [ICRP], 1974). Because of the intrusion of the turbinates, the air passages of the nasal fossae have ribbon-like cross sections that are complexly folded within the turbinate structures. Because of this complexity, there is a high surface area-to-volume ratio in this region of the airways (ICRP, 1974). At the posterior end of the turbinate region, about 50 mm in length, the nasal septum terminates, and the two nasal passages combine (nasal choanae) and join the pharynx, which then bends 70° downward to the remainder of the respiratory tract. The length of the nasal passage from nostril to oropharynx is about 130 mm (Swift, 1981). Limited morphometric information is available regarding the sizes of the different parts of the nasal airways. Additionally, most of the published measurements on nasal airway dimensions were obtained from cadaver studies. Postmortem measurements result in overestimation of the cross-sectional areas and airway volumes of the region containing the turbinates, because of the marked shrinkage of the nasal mucosa after death (Guilmette et al., 1989). For calculational purposes, Landahl (1950) assumed the following schematic representation of the nasal airways: Each of the external nares had a cross-sectional area of 75 mm2, with hairs of 100 µm in diameter occupying one-half of the projected area. The second region was the region containing the nasal valve, 2 cm behind the opening of the nares, and was assumed to have a rectangular cross section 1.2-cm high and 0.25-cm wide and to bend at an angle of 30°. The third region was also assumed to have a cross-sectional profile of a rectangular tube 3-cm tall, 0.2-cm wide, and 1-cm long, with a bend of 20°. The fourth region was subdivided into two elements. The first was the narrow and tortuous upper passage of 1 mm in width; the second was the more direct lower passage of 2 mm in width. The total height for both elements was 40 mm, with a length of 50 mm, so that the total effective surface area for both nasal passages was 80 cm2. The appropriateness of the various simplifying assumptions used in this nasal airway model has not been evaluated to date. Scott et al. (1978) also formulated a geometrical model for calculating aerosol particle deposition within the nasal airways. In this model, a symmetrical, bilateral model was divided into five contiguous parts that represented the anterior nares, the nasal valve, the expansion region just posterior to the nasal valve, the turbinate area, and the posterior bend. There are many similarities between this model and that of Landahl (1950). The major differences between them are the increased geometrical complexity of the turbinate areas, which

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Comparative Dosimetry of Radon in Mines and Homes is accomplished by using coupled rectangular shapes that simulate the meatal folds, the graded cross-sectional area in the region of the nasal valve, and the presence of the posterior nasopharyngeal bend. Again, as with the Landahl model, the adequacy of this particular model in representing nasal deposition has not been evaluated experimentally. 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), in which the nasal airways were assumed to be two sets of parallel disks, representing the anterior and posterior portions of the airway, no other morphometric models of the nasal airways have been presented. To date, no morphometric models of the oral airways have been used either experimentally or for theoretical calculations. The reasons for this lack are not clean It is clear, however, that oral breathing can be important, and even predominant, in cases of acute or chronic nasal airway obstruction and in cases of significant physical exertion, in which increased ventilation can only be accomplished by decreasing respiratory resistance, i.e., by compensatory oral breathing (see Chapter 7). Intuitively, it would appear to be difficult to construct a single morphometric model of the oral cavity that would be adequate for deposition and dosimetry purposes, as the cavity dimensions are likely to depend significantly on the size of the individual, as well as the position of the tongue and jaw. These latter positions depend on the ventilation rate requirements and the fractionation of breathing between the nasal and oral routes. TRACHEOBRONCHIAL AIRWAY MORPHOMETRY Anatomically, the tracheobronchial airways consist of a series of bifurcating tubes with circular or near-circular cross sections and ever-decreasing diameters that extend from the trachea at the proximal end to the terminal bronchioles in the distal region of the lung. The trachea divides into the left and right major bronchi, which in turn divide by dichotomous branching into smaller bronchi and bronchioles. In humans, there are about 18 to 20 branchings before the level of the respiratory bronchioles is reached. The bronchi have a ciliated epithelium covered by a layer of mucus, which is produced by the goblet cells and mucous glands. The bronchi also have irregularly shaped cartilage plates situated on the outside of the bronchial walls that, along with the smooth muscle layers, provide structural support to maintain airway patency during ventilation. The smallest conducting airways, the bronchioles, occur at the ends of the bronchi and have neither cartilage nor mucous glands. They do, however, have a ciliated epithelium and mucus-producing cells, although the ciliated regions and the mucus blanket are no longer continuous, as they tend to be in the larger bronchi.

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Comparative Dosimetry of Radon in Mines and Homes Morphometric models of the conducting airways have been developed based on measurements made by using airway corrosion casts or in vivo bronchoscopic or radiographic measurements or at autopsy. These measurements have included airway diameters, segment lengths, and branching angles. From these, luminal surface areas and volumes can be calculated. The earliest models of the respiratory airways were made assuming symmetry of size and length for airways of a given generation (Findeisen, 1935; Landahl, 1950; Davies, 1961). These models have been summarized by Raabe (1982). More recent morphometric models have been based on sets of data obtained primarily from measurements of conducting airway dimensions by using corrosion cast techniques (Weibel, 1963; Horsfield et al., 1971; Yeh and Schum, 1980; Phalen et al., 1985). Weibel (1963) used a plastic airway cast prepared by Liebow and measured the lengths, diameters, and branching angles completely for the first 5 airway generations and incompletely through 10 airway generations. Sampling frequencies decreased gradually from 91 to 95% for generations 6 to 18 to 21% for generation 10. The results confirmed earlier views that the conducting airway system has a characteristic irregular dichotomous branching. For a given generation, there was a dispersion of both airway diameters and airway lengths, with a greater variability being seen in the lengths of the airway segments. The above measurements were then combined with data collected from the more peripheral regions of the lung by quantitative morphometry of histologically prepared sections. 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), which attempted to take into account the irregularities encountered in the measurements (Weibel, 1963). The Weibel morphometry models, particularly Weibel lung model A, have been incorporated into deposition and dosimetric models by many investigators. Horsfield et al. (1971) described a morphometric model based on data obtained from measurement of a resin cast of a normal human bronchial tree (Horsfield and Cumming, 1968; Parker et al., 1971). Their measurements, which included airway diameters, segment lengths, and branching patterns, indicated that asymmetric branching was needed to produce a more realistic model of the airway structure. Thus, they developed two models that included asymmetric branching, unequal numbers of airways in different lung lobes, and variations in the airway segment lengths (Horsfield et al., 1971). Of note was their determination that the mean diameter of the smaller of a pair of daughter bronchi was similar to that of a larger bronchial branch occurring four generations distal to the original. Their model has been implemented mathematically by Yeates and Aspin (1978). Yeh and Schum (1980) prepared a flexible silicone rubber airway cast of the lungs of a 60-yr-old male. Their measurements included diameters and lengths of each airway segment together with the respective branching angles

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Comparative Dosimetry of Radon in Mines and Homes and angle of each airway with respect to gravity. Measurements were made of all airways to 2 or 3 mm in diameter, with randomized sampling of 20% of the smaller-diameter airways being made to the level of the terminal bronchioles. They then developed a typical path lung model in which n number of identical pathways (down to the level of the terminal bronchioles) was used to represent n number of actual or estimated pathways of a complicated tree structure such as those present in each lobe of a lung or the lung as a whole. The concept is similar to that of Weibel lung model A, except that it does not impose a symmetry requirement on the bronchial tree structure. Phalen et al. (1985) developed a morphometric model of the tracheobronchial airways based on measurements obtained from 20 humans ranging in age from 11 days to 21 yr. Complete measurements of diameters, lengths, branching angles, and angles with respect to gravity were made for the first three generations; successive generations were sampled, and pathways to the level of the terminal bronchioles were marked and measured at random. Data were then analyzed with respect to the height of the subject, and linear regressions were performed on the data sets grouped by airway generation. Although several studies have measured morphometric parameters of human conducting airways, no single study has made a complete set of measurements for the length of the tracheobronchial tree. This is understandable, considering that analysis of all airways for 15 generations of a symmetrical dichotomous tree would require more than 65,000 measurements. Thus, each investigator must make assumptions as to the structure of the tracheobronchial airways and how representative their sampling is with respect to the real morphology of the conducting airways. Additionally, very few human lung specimens have been analyzed, and their ages and sizes have differed significantly. Thus, a standard human conducting airway model is not available, nor has variability been adequately described by age, size, gender, or other anthropomorphic characteristic. However, James (1988) compared the generation-specific surface areas and volumes of airways from Weibel lung model A, the Yeh and Schum model, and the Phalen (University of California, Irvine [UCI]) model. The data for the Weibel and Yeh and Schum models were derived from Yu and Diu (1982), in which the respective models were scaled to an initial lung volume of 3,000 cm3. Figure 5-2 illustrates the relationships between the generation-specific volumes for the three models. Although there is relatively good agreement between the model values for the first four generations, there is a significant divergence of the volumes of the Yeh and Schum model for generations 5 to 12 from those of the other two models, with a maximum variance of a factor of 3 in generation 8. 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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Comparative Dosimetry of Radon in Mines and Homes Figure 5-2 Calculated airway volumes of tracheobronchial airways using different models. try calculations rely on simplifying assumptions about the generation-specific structure and size of the airways, it would be useful to evaluate the effect of introducing asymmetry assumptions into the morphometric models. These assumptions, which can now be characterized by using statistical distributions for the generation-specific variables of airway size, length, and branching angle, would undoubtedly add complexity to the models. However, with current computer technologies, it would not be difficult to perform the necessary calculations. Weibel, in lung model B, attempted to account for the irregular dichotomy of the lung by applying a binomial distribution to the occurrence of airways with 2-mm diameters with respect to generation number. The distance from the root of the trachea at which these 2-mm bronchi occurred was then described by a normal distribution with a mean of 24.5 cm. Asymmetries for the smaller airways (1.0-and 0.5-mm diameters) were then estimated based on the assumption of normal distributions analogous to that used for the 2-mm-diameter airways. Thus, a model accounting for airway diameter and length asymmetries was developed. To date, however, this model has not been applied to deposition or dosimetry calculations, although there is no reason why this could not be done. Soong et al. (1979) also used Weibel lung model A as a basis for constructing a tracheobronchial airway morphometry model that included variability in airway size. Initially, they estimated coefficients of variation for 24 generations of airways based on data obtained from available morphometric studies, and then applied the coefficients to the mean diameters and lengths for the different generations of airways taken from Weibel (1963). They later determined that the distribution of airway diameters and lengths could be characterized by either log-normal or gamma probability distributions. Yu et al. (1979) then applied

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Comparative Dosimetry of Radon in Mines and Homes Measurement of mucous clearance rates in the conducting airways below the larynx has been limited to measurements made in the trachea and major bronchi, but primarily the former. The techniques used to make these measurements have been similar to those used to measure nasal mucous clearance, i.e., roentgenographic localization of insufflated powder (Gamsu et al., 1973) or radiopaque Teflon disks emplaced bronchoscopically (Friedman et al., 1977; Goodman et al., 1978); direct observation of the movement of Teflon disks through a bronchoscope (Santa Cruz et al., 1974; Wood et al., 1975); or measurement of the clearance of radioactively labeled microscopic particles by using gamma imaging devices, the particles being emplaced either by bronchoscopy (Chopra et al., 1979) or by inhalation (Thomson and Short, 1969; Yeates et al., 1975; Foster et al., 1976; van Hengstum et al., 1989). The measured average values of tracheal mucous clearance velocities varied between studies that used normal nonsmoking adults: 10.1 ± 3.5 (young adult) and 5.8 ± 2.6 mm/min (Goodman et al., 1978), 15.5 ± 0.69 mm/min (Chopra et al., 1979), 21.5 ± 5.5 mm/min (Santa Cruz et al., 1974), 20.1 ± 1.4 mm/min (Wood et al., 1975), and 5.5 ± 0.4 mm/min (Foster et al., 1980). Yeates et al. (1982) described their measured tracheal mucous velocities in terms of a log-normal distribution with a geometric mean of 4.0 mm/min with a coefficient of variation of 48%. It is not clear whether the differences in the measured velocities among the various studies were due to differences in the subjects or in the different measurement methods used. In addition to the values for normal nonsmoking adults given above, other measurements have been made on smokers and on subjects with lung disease. Foster et al. (1980) measured clearance velocities for ferric oxide particles deposited in the large bronchi by gamma camera imaging and found average velocities to be 2.4 ± 0.5 mm/min, compared with tracheal velocities of 5.5 mm/min. However, no studies have measured the mucous clearance rates for the different generations of the bronchial and bronchiolar airways. 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), Lee et al. (1979), Haque and Geary (1981), Yu et al. (1983), and Cuddihy and Yeh (1988) are summarized in Table 5-2. In general, the values differ within a range of a factor of three. The basis for the differences seen in these models is the selection of the appropriate tracheal mucous velocity, since with all of the model structures there is a mathematical scaling of generation-specific velocities to that of the trachea, which is the only experimentally measured value. The second basis for the differences lies in the assumptions that were made regarding the dynamics of mucous and lung water production and absorption. For example, Lee et al. (1979) assumed a

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Comparative Dosimetry of Radon in Mines and Homes TABLE 5-2 Mucous Clearance Velocities Derived from Theoretical Models   Mucous Clearance Velocity (mm/min) from the Following Models Weibel Airway Generation Harley and Pasternack Lee et al. Haque and Geary Yu et al. Cuddihy and Yeh 0 15.0 5.5 10 13 5.5 1 8.0 4.1 4.9 8.98 3.5 2 2.5 3.0 4.9 3.45 2.0 3 2.5 2.2 4.9 1.52 1.1 4 0.9 1.4 0.4 0.94 0.9 5 0.9 0.88 0.4 0.53 0.9 6 0.9 0.55 0.4 0.28 0.7 7 0.9 0.34 0.09 0.15 0.6 8 0.25 0.21 0.09 0.14 0.4 9 0.25 0.13 0.09 0.14 0.3 10 0.01 0.074   0.15 0.2 11 0.01 0.044   0.14 0.1 12 0.01 0.025   0.13 0.05 13 0.01 0.015   0.12 0.02 14 0.01 0.0082   0.11 0.007 15 0.01 0.0046   0.04 0.001   SOURCES: Harley and Pasternack (1972), Lee et al. (1979), Haque and Geary (1981), Yu et al. (1983), and Cuddihy and Yeh (1988). 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., vα = v0d0/2αdα, where vα is the velocity in airway generation a, da is the diameter of an airway of generation α, and v0 and d0 are the velocity of clearance and diameter of the trachea, respectively. On the other hand, Altshuler et al. (1964) assumed that mucus is produced throughout the length of the conducting airways and at different generation-specific rates. Their methodology was used by Harley and Pasternack (1972), albeit with a different morphometric model. Currently, there is no firm basis on which to select one theoretical clearance model because of the lack of biological understanding of the mechanisms of secretion and absorption of either lung water or mucus. Nor are the sites of production and absorption well known. It is interesting to note, however, that the elimination of clearance in the Harley and Pasternack (1972) model resulted in a 50% increase in alpha-radiation dose to the larger bronchi compared with that obtained by assuming normal clearance. Clearance of particles from the nonciliated or parenchymal regions of the lung have generally been done by quantitating the long-term retention of inhaled gamma-emitting radioactive particles within the thorax region by using appropriate gamma-ray detectors. As described above, the clearance of particles during the first 24 h after a brief exposure have been attributed to clearance from the ciliated tracheobronchial airways. Likewise, decreases in radioactivity

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Comparative Dosimetry of Radon in Mines and Homes beyond the first 24 h has been assigned to clearance of particles from the nonciliated regions of the lung. The use of insoluble particles in which the radioactive label has been firmly bound has facilitated the measurement of long-term clearance, as the confounding influence of clearance via translocation of the radiolabel to blood becomes minimized. Studies have been reported in which humans have inhaled 51Cr-labeled Teflon particles (Philipson et al., 1985), 198Au-Fe2O3 (Stahlhofen et al., 1986), or 85Sr-or 88Y-labeled fused aluminosilicate particles (Bailey et al., 1985). The half-lives for retention have varied somewhat, but all have been long. Philipson et al. (1985) measured a mean half-life of 1,200 days for 68% of the deposited particles; Bailey et al. (1985) estimated a mechanical clearance half-life of about 700 days beyond 200 days after inhalation. This latter value was corroborated by the modeling of Cuddihy and Yeh (1988). The Task Group on Lung Dynamics (1966) has assigned a clearance half-life of 500 days to "class Y" insoluble particles. Thus, it is clear that retention of particles that deposit in the respiratory region of the lung is very long, i.e., hundreds of days. When compared with the effective radiological half-life of radon progeny, , then retention of the progeny in the pulmonary region can be considered to be infinite. DOSIMETRY MODELS OF THE RESPIRATORY TRACT Radon and radon progeny dosimetric models have evolved since the first efforts were made to develop themin the 1940s. During this radon period, a large number of models have been published. 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. NCRP (1984) lists 48 references through 1981 and remarks on the evolution in thinking and increasing sophistication of the models as well as the dose conversion factors in rad per working level month (WLM), and derived limits for exposure to both radon gas and radon progeny. The investigators noted a trend of decreasing values of the dose conversion factor toward 1 rad/WLM as the unattached fraction decreased and the activity becomes attached to intermediate-sized aerosols (tenths of a micrometer). James (1988) has updated the listing of radon dosimetry models to include

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Comparative Dosimetry of Radon in Mines and Homes the newer works of Hofmann (1982), Harley and Pasternack (1982), and the NEA Experts Report (1983), as well as the model of James (1988). In examining the dose conversion factors, James (1988) pointed out the importance of determining separately the dose conversion factors for attached and unattached radon progeny. This facilitates comparison of the different dosimetric models on a more equivalent basis. Thus, for the attached fraction, the dose conversion factors have converged somewhat since 1980, to a range of 0.2 to 1.3 rad/WLM; the variations were attributed mainly to the assumed aerosol particle size and the depth of the target cells. 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, 1988). Nor is there agreement as to which choices in dosimetric model structure and parameter values are most appropriate. This difficulty arises because of a general lack of either relevant data (e.g. in upper airway morphometry, regional deposition, and mucous clearance) or understanding of the nature of the different processes that must be considered in calculating alpha-radiation doses to airway epithelial cells. As the knowledge base matures, construction of better models will be possible. Later sections of this report deal with updating knowledge of radon and radon progeny dosimetry as it applies to the different scenarios specific to exposure within a mine environment, for which epidemiological data are available, and in homes, for which human exposure-response data are being developed. REFERENCES Albert, R. E., M. Lippmann, J. Spiegelman, A. Liuzzi, and N. Nelson. 1967. The deposition and clearance of radioactive particles in the human lung. Arch. Environ. Health 14:10-15. Altshuler, B. 1959. Calculation of regional deposition of aerosol in the respiratory tract. Bull. Math. Biophys. 21:257-270. Altschuler, B., N. Nelson, and M. Kuschner. 1964. Estimation of lung tissue dose from the inhalation of radon and daughters. Health Phys. 10:1137-1161. Altshuler, B., E. D. Palmes, and N. Nelson. 1967. Regional aerosol deposition in the human respiratory tract. Pp. 323-337 in Inhaled Particles and Vapors II, C. N. Davies, ed. Oxford: Pergamon Press. Andersen, I., G. R. Lundquist, and D. F. Proctor. 1971. Human nasal mucosa . Arch. Environ. Health 23:408-420. Bailey, M. R. 1984. Assessment of the dose to the nasopharyngeal region from inhaled radionuclides. Pp. 273-280 in Lung Modelling for Inhalation of Radioactive Materials, H. Smith, and G. Gerber, eds. Brussels: Commission of the European Communities. Bailey, M. R., F. A. Fry, and A. C. James. 1985. Long-term retention of particles in the human respiratory tract. J. Aerosol Sci. 16(4):295-305.

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Comparative Dosimetry of Radon in Mines and Homes Haque, A. K. M. M., and M. J. Geary. 1981. The radiation dose to the respiratory system due to the daughter products of radon. Pp. 113-127 in Seventh Symposium on Microdosimetry, J. Booz, G. Ebert, and H. D. Hartfiel, eds. Oxford: Harwood Academic Publishers Ltd. Harley, N. H., and B. S. Pasternack. 1972. Alpha absorption measurements applied to lung dose from radon daughters. Health Phys. 23:771-781. Harley, N. H., and B. S. Pasternack. 1982. Environmental radon daughter alpha dose factors in a five-lobed human lung. Health Phys. 42:789-799. Heyder, J., and G. Rudolf. 1977. Deposition of aerosol particles in the human nose. Pp. 107-126 in Inhaled Particles IV, Part 1, W. H. Walton and B. McGovern, eds. Oxford: Pergamon Press. Heyder, J., and G. Rudolf. 1984. Mathematical models of particle deposition in the human respiratory tract. J. Aerosol Sci. 15:697-707. Heyder, J., J. Gebhart, G. Rudolf, C. E Schiller, and W. Stahlhofen. 1986. Deposition of particles in the human respiratory tract in the size range 0.005-15 µm. Aerosol Sci. 17:811-825. Hofmann, W. 1982. Cellular lung dosimetry for inhaled radon decay products as a base for radiation-induced lung cancer risk assessment. 1. Calculation of mean cellular doses. Radiat. Environ. Biophys. 20:95-112. Horsfield, K., and G. Cumming. 1968. Morphology of the bronchial tree in man. J. Appl. Physiol. 24:373-383. Horsfield, K., G. Dart, D. E. Olson, G. F. Filley, and G. Cumming. 1971. Models of the human bronchial tree. J. Appl. Physiol. 31:207-217. Hounam, R. F., A. Black, and M. Walsh. 1971. The deposition of aerosol particles in the nasopharyngeal region of the human respiratory tract. Pp. 71-80 in Inhaled Particles III, Volume 1, W. H. Walton, ed. Old Woking, Surrey, England: The Gresham Press. Ingham, D. B. 1976. Diffusion of aerosols from a stream flowing through a cylindrical tube. J. Aerosol Sci. 6:125-132. International Commission on Radiological Protection (ICRP). 1974. Report of the Task Group on Reference Man, W. S. Snyder, M. J. Cook, E. S. Nasset, L. R. Karhausen, G. P. Howells, and I. H. Tipton, eds. Oxford: Pergamon Press. International Commission on Radiological Protection (ICRP). 1987. Lung Cancer Risk from Indoor Exposure to Radon Daughters. Publ. No. 50. Oxford: Pergamon Press. James, A. C. 1988. Lung dosimetry. Pp. 259-310 in Radon and Its Decay Products in Indoor Air , W. W. Nazaroff and A. V. Nero, eds. New York: John Wiley & Sons. Johnson, J. R., K. D. Isles, and D. C. F. Muir. 1977. Inertial deposition of particles in human branching airways. Pp. 61-73 in Inhaled Particles IV, Part 1, W. H. Walton and B. McGovern, eds. Oxford: Pergamon Press. Koblinger, L., and W. Hofmann. 1985. Analysis of human lung morphometric data for stochastic aerosol deposition calculations. Phys. Med. Biol. 30:541-556. Lambert, B. E., M. L. Phipps, P. J. Lindop, A. Black, and S. R. Moores. 1982. Induction of lung tumours in mice following the inhalation of 239PuO2. Pp. 370-375 in Proceedings of the 3rd International Symposium on Radiation Protection—Adv. in Theory and Practice, Vol. 1. Inverness, Scotland: Society for Radiation Protection. Landahl, H. D. 1950. On the removal of air-borne droplets by the human respiratory tract. II. The nasal passages. Bull. Math. Biophys. 12:161-169. Landahl, H. D., and S. Black. 1947. Penetration of air-borne particulates through the human nose. J. Ind. Hyg. Toxicol. 29:269-277.

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