2
Assessment of Exposure to the Decay Products of 222Rn in Mines and Homes

INTRODUCTION

The purpose of this chapter is to review the available data on the exposures of underground miners and members of the general public to radon progeny. These data are limited by changes in measurement methodologies and the types of measurements that were made. There are relatively few data available to fully characterize the airborne radioactivity in either mines or homes, and them are no data at all that characterize radon progeny in school or workplace atmospheres. Thus, Chapter 6 summarizes in substantial detail those data that are available and highlights the limitations of the existing knowledge base.

BACKGROUND

In order to estimate the dose of alpha energy from inhaled radon progeny received by miners and by the general public in their homes, it is necessary to know both the concentrations of the airborne progeny and their size distributions. The size of the radioactive particles determines their penetration through the upper respiratory system and the pattern of deposition within the tracheobronchial region. As discussed in Chapter 1, the series of short-lived radon decay products can be deposited directly in the respiratory tract or onto the surface of particles that can be deposited in the respiratory tract. The behavior of airborne progeny is described schematically in Figure 2-1.



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Comparative Dosimetry of Radon in Mines and Homes 2 Assessment of Exposure to the Decay Products of 222Rn in Mines and Homes INTRODUCTION The purpose of this chapter is to review the available data on the exposures of underground miners and members of the general public to radon progeny. These data are limited by changes in measurement methodologies and the types of measurements that were made. There are relatively few data available to fully characterize the airborne radioactivity in either mines or homes, and them are no data at all that characterize radon progeny in school or workplace atmospheres. Thus, Chapter 6 summarizes in substantial detail those data that are available and highlights the limitations of the existing knowledge base. BACKGROUND In order to estimate the dose of alpha energy from inhaled radon progeny received by miners and by the general public in their homes, it is necessary to know both the concentrations of the airborne progeny and their size distributions. The size of the radioactive particles determines their penetration through the upper respiratory system and the pattern of deposition within the tracheobronchial region. As discussed in Chapter 1, the series of short-lived radon decay products can be deposited directly in the respiratory tract or onto the surface of particles that can be deposited in the respiratory tract. The behavior of airborne progeny is described schematically in Figure 2-1.

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Comparative Dosimetry of Radon in Mines and Homes Figure 2-1 Schematic representation of the behavior of radon progeny in an enclosed space such as a mine or a room. Since the discussion of exposure centers on the presence of airborne radioactivity and the aerodynamic behavior of the particles that transport the activity, it is necessary to review the concept of particle size. Although particles can be described in terms of their physical behavior, respiratory deposition and other related problems depend on how the radioactive particles move in the atmosphere. For particles above a few tenths of a micron, the particles are typically described in terms of their aerodynamic diameter. The aerodynamic diameter is the diameter of a unit-density sphere that has the same gravitational settling velocity as the particle. However, for particles below 0.1 µm in size, diffusion is the dominant mechanism for particle deposition. Thus, the diffusion equivalent diameter as defined in Equation 6-1 becomes the appropriate measure of particle size. A detailed discussion of particle size and diffusion coefficient is provided in Chapter 6. Certain other terms need to be initially defined. The amount of radioactivity is described in units of disintegrations per unit time or activity rather than as the mass of contaminant. The activity (A) is the number of decay events per unit time and is calculated: where λ is the probability of decay of the nucleus of a particular atom in a unit time (λ = 0.693/T1/2). T1/2 is the half-life of the isotope, and N is the total number of the atoms present in the sample. The international (SI) unit of activity is the becquerel (Bq). A becquerel is the quantity of radioactive

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Comparative Dosimetry of Radon in Mines and Homes material in which one atom is transformed per second. However, for radon the unit picocuries (pCi) is commonly used in the United States. A picocurie is 10-12 curies (Ci) or 3.7 × 10-2 disintegrations per second. For airborne concentrations (the amount of activity per unit volume), the proper scientific measurement units are becquerels per cubic meter or picocuries per liter. A concentration of 37 Bq/m3 is equal to 1 pCi/liter. If all progeny formed by the decay of radon were to remain in air, then equal activities of all of the radon decay products would be present. Such a mixture is said to be in secular equilibrium. The quantitative relationship between isotopes in secular equilibrium may be derived in the following manner for the general case for isotopes A, B, and C: where the half-life of isotope A is very much greater than that of isotope B. The decay constant of isotope A, λA, is therefore much smaller than λB, the decay constant of isotope B. The product of the decay probability times the number of atoms (N) would be equal for isotopes A and B (λANA = λBNB) because the larger λB compensates for the smaller NB. For radon decay products, the amount of activity equal to that of Po-218, Pb-214, and Bi-214 in secular equilibrium at a concentration of 100 pCi of each isotope per liter is called 1 working level (WL). This terminology was developed for describing the progeny concentrations in uranium mines. The WL is a measure of the concentration of the total potential alpha energy of the short-lived progeny. The total exposure from radon progeny can then be expressed as the length of time a person is present in a room or mine with a concentration of radon progeny multiplied by that concentration. Again, for the uranium mining environment, the unit of exposure was called the working level month (WLM), defined as the average WL multiplied by time in units of 170 h. Since people spend more than 170 hrs per month in their homes, the cumulative exposure is given by where (WL)i is the average concentration of the radon progeny during the exposure interval i expressed in WL, and ti is the number of hours of exposure at the ith concentration. The cumulative exposure in a home at a given decay product level could thus be more than four times that for an occupational exposure (8,766 total hours in a year compared with 2,000 working hours in a year). A better way to express the total activity of all of the radon decay products is as the potential alpha-energy concentration (PAEC) in the air, which is

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Comparative Dosimetry of Radon in Mines and Homes expressed as MeV/m3 or WL. AWL of 100 pCi/liter in equilibrium deposits 1.3 × 105 MeV/liter. where A1, A2, and A3 (in Bq/m3) and I1, I2, and I3 (in pCi/liter) are the activity concentrations of 218po, 214pb, and 214Bi, respectively. Because of the losses of decay products from the air by deposition on surfaces such as walls, ceilings, and furniture, the decay product activity is less than the radon activity. The term characterizing the airborne concentration of PAEC as a fraction of the radon activity is the equilibrium factor, F. F is defined as the ratio of decay products to radon by where a1, a2, and a3 are the relative activities of the three radon progeny relative to that of radon, a0 (i.e., a1 = a1/a0). Swedjemark (1983) found that for low air exchange rates (<0.3 h-1) in 225 dwellings in Sweden, F was about 0.51 (0.28-0.74). For an average air rate (0.3-0.6 h-1), F is about 0.43 (0.21-0.66), and for high air rate (>0.6 h-1), F was about 0.33 (0.21-0.47). Another study (Porstendörfer, 1987) of the relationship between equilibrium factor and aerosol sources indicated that the mean value of the equilibrium factor F measured in homes without aerosol sources was 0.3 ± 0.1. The equilibrium factor increased up to 0.5 with additional aerosol particles (cigarette and candle smoke) in the room air. Measurements of equilibrium factor determined in Swedish homes (Jonassen and Jensen, 1989) showed that F was 0.51 in a home inhabited by smokers and 0.46 in the homes of eight nonsmokers. A distinction is often made in the state of the airborne progeny based on apparent attachment to aerosol particles. The unattached fraction (ultrafine mode, 0.5-5 nm) refers to those progeny existing as ions, molecules, or small clusters. The attached fraction (accumulation mode, 0.1-0.4/µm) is regarded as those radionuclides attached to ambient particles. The unattached fraction fp of the total potential alpha energy of the radon progeny mixture is described as: where Ceq = 0.105 C1 + 0.516 C2 + 0.379 C3, and Cj (j = 1, 2, 3) is the activity concentration of radon progeny. The superscript u stands for unattached fraction. Typically, most of the unattached activity is that of 218Po. Measurements of the unattached fraction in the domestic environment (Reineking et al., 1985;

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Comparative Dosimetry of Radon in Mines and Homes Vanmarcke et al., 1987) showed that fp is between 0.05 and 0.15 without any aerosol sources in the room and can decrease to below 0.05 in the presence of aerosol sources (cigarette smoke, cooking, stove heating). However, Jonassen and Jensen (1989) found that fp was 0.01 in a house inhabited by smokers and 0.04 in the homes of eight nonsmokers. As shown schematically in Figure 2-1, the highly diffusive, ultrafine activity (unattached fraction) becomes attached to the particles that are present in the existing aerosol whether it is in the mine or the home. The attachment rate is dependent on both the number of particles suspended in the air as well as their size in a complicated manner (Porstendörfer et al., 1979). The distribution of sizes as measured based on the determination of the amounts of associated radioactivity is referred to as the activity-weighted particle size distribution. MEASUREMENT METHODS Because of limitations in the measurement methods, only two physical parameters were used in the earlier lung dosimetry models for estimating alpha doses from inhaled radon decay products: the activity median diameter of the attached radioactive aerosol and the unattached fraction of PAEC (Jacobi and Eisfeld, 1980; James et al., 1980; Harley and Pasternack, 1982). Traditionally defined, the unattached fraction constitutes free molecular daughter atoms or ions possibly clustered with other molecules such as H2O, as distinct from decay product atoms attached to particles in the preexisting ambient aerosol. The measurement of the unattached fraction of radon progeny has been the subject of extensive research. Diffusion, impaction, and electrostatic deposition methods have been used for the separation of the attached and unattached fractions of radioactive aerosols in ambient and mine atmospheres (Van der Vooren et al., 1982). Interpretation of unattached fraction measurements has been clouded by limited understanding of the physicochemical behavior of the progeny, particularly 218Po. The studies based on diffusional collection of the unattached activity all used a single, constant diffusion coefficient for unattached 218Po based on the first estimate of 0.054 cm2/s for the diffusion coefficient of 218Po (Chamberlain and Dyson, 1956). Subsequent investigations have indicated that the unattached 218Po fraction is actually an ultrafine particle or cluster mode, 0.5-3 nm in diameter, rather than free molecular 218Po (Busigin et al., 1981; Reineking and Porstendörfer, 1986). Busigin et al. (1981) were the first to conclude that the diffusion coefficient of unattached 218Po in air could not be adequately described by a single number, given the experimentally observed range of 0.005 to 0.1 cm2/s. Goldstein and Hopke (1985) have also shown experimentally that the diffusion coefficient for 218Po can be adjusted in the range of 0.03 to 0.08 cm2/s by controlling the admixed trace gases. Raes (1985) has applied classical nucleation theory to describe the growth of clusters

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Comparative Dosimetry of Radon in Mines and Homes in atmospheres containing 218Po, H2O, and SO2. The results of these studies strongly suggest that the so-called unattached fraction is actually an ultrafine particle mode in the size range of 0.5 to 3.0 nm whose nature is dependent upon the gaseous environment surrounding the radon decay products. Results of recent studies by Chu and Hopke (1988) of the neutralization rate for 218Po+ ions initially present at the end of the polonium nucleus recoil path strongly suggest that the ions are rapidly neutralized in the atmosphere and that electrostatic collection underestimates the unattached fraction. Jonassen (1984) and Jonassen and McLaughlin (1985) found that only about 10% of the unattached fraction, as measured by using a wire screen system, was charged. Thus, the unattached fraction measurements of Blanc et al. (1968) and Chapuis et al. (1970), who used electrostatic collection, found that the unattached fractions were extremely small when they actually measured the fraction of the highly diffusive daughter activity that was still charged. These results are in good agreement with later measurements of Jonassen and McLaughlin (1985). Thus, electrostatic measurements of unattached fractions are not considered further in this assessment. The detection of these ultrafine clusters by conventional particle detection devices such as condensation nuclei counters is precluded by the lack of sensitivity of these devices below particle diameters of 4 nm (Agarwal and Sem, 1980; Bartz et al., 1985). However, the existence of the ultrafine cluster mode (0.5-3 nm in diameter) in the activity size distribution has been observed in investigations by Reineking and Porstendörfer (1986) and by Tu and Knutson (1988), who used specially developed wire screen diffusion batteries. Wire screens, which are calibrated by using a single, constant value for the 218Po diffusion coefficient and sampling an unattached fraction consisting of an ultra fine cluster mode in the activity size distribution, would thus be unable to separate the unattached and the attached fractions. In order to measure the amounts of activity in the fractions of various sizes, it is necessary to understand the dynamics of particle deposition in tubes or screens. (The details of the theoretical background to the measurement methods are provided in Chapter 6.) This theoretical framework can then be applied to develop measurement methods. The measurement systems have traditionally attempted to separate the activity into two fractions: a highly diffusive fraction referred to as the unattached fraction and the activity attached to indoor aerosol particles. A review of these measurement methods and their limitations is also presented in Chapter 6.

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Comparative Dosimetry of Radon in Mines and Homes EXPOSURE TO RADON PROGENY Mine Atmospheres The limited data on particle size distribution and unattached fraction measured in uranium mines are presented in Chapter 6. From the study of Bigu and Kirk (1980), who made measurements in active working areas of a mine, under typical working conditions in mines, it appears that the unattached fraction of PAEC is quite small, with values estimated to be below 1%. It is likely that conditions in mines of previous decades were even more dusty as standards were not in place to limit exposures via ventilation. Thus, very low values of the unattached fraction are anticipated to have been typical in the past for active working areas of the mines. For the committee's modeling, an fp value of 0.005 is assumed for the active working areas of mines. While higher values have been reported, values of about 0.03 are more typical for the unattached fraction of 218Po and not PAEC. In nonworking areas such as haulage drifts (sometimes called haulageways), etc., the values are more on the order of 0.02 to 0.03, and a value of 0.03 is assumed. In lunchrooms and other ''clean'' areas (workshops, storage areas), the fp value could be as high as 0.08. It is likely that the miners most highly exposed in the earliest days of underground uranium mining were probably exposed to very low unattached fractions, as the low ventilation that led to high radon concentrations would also produce high aerosol particle concentrations. The size distribution data of Cooper et al. (1973) show that the activity median diameter (AMD) of the attached mode is on the order of 0.25 µm, with a geometric standard deviation (σg) of 2.5. Although transient aerosols with both larger- and smaller-sized modes would be expected immediately following a dynamite blast, the total exposure to aerosols generated by such events should be relatively small compared with the total time underground. In the haulage drifts and other areas away from the areas where active mining is occurring, the distributions are shifted to somewhat smaller sizes, as indicated by Knutson and George (1990). They report an average activity median diameter for unimodal distributions of 151 nm with an average geometric standard deviation of 2.71. Miners are also exposed to airborne radioactivity from long-lived alpha emitters, including uranium-234, thorium-230, radium-226, and polonium-210 present in the suspended ore dusts in mine atmospheres. Harley and coworkers (1981, 1985) have estimated the contribution of these species to the total lung dose of alpha energy. At the higher concentrations of 222Rn present in mines until 1967, the additional dose from these long-lived radionuclides is estimated to be <10% of that from the short-lived 222Rn decay products. Singh et al. (1985) measured the accumulated long-lived activity in a limited number of autopsied miner's lungs and also estimated that the dose from the long-lived activities is a small fraction of that obtained from the short-lived activities.

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Comparative Dosimetry of Radon in Mines and Homes Indoor Atmospheres In general, the data indicate that the unattached fraction of the potential alpha-energy concentration measured in active working areas of uranium mines is much lower than that in typical indoor air. However, the limited number of unattached fraction measurements in unoccupied houses (also summarized in Chapter 6) makes extrapolation to the much larger population of occupied houses subject to substantial uncertainty. The results that have been measured suggest that, in the absence of smoking, unattached fractions are in the range of 0.07 to 0.10. The typical value is taken as 0.08 for this report. In houses with low aerosol particle concentrations, fp values in excess of 0.16 may be observed. Smoking produces relatively high concentrations of 0.1- to 0.5-µm-diameter particles and can reduce unattached fractions to the range of 0.01 to 0.03. Because smoking is an intermittent source of particles, the higher end of this range is taken and the typical value is assumed to be 0.03. Recent measurements also suggest that the equilibrium factor in houses without smokers is lower than the commonly used value of 0.5, probably in the range of 0.3 to 0.4. Thus, although there is less airborne activity in indoor air per unit radon activity concentration, more of the activity is in a more diffuse form. However, there are relatively few measurements of the behavior of radon progeny in houses under normal conditions of occupation over a prolonged period of time. The only reported measurements are those from Japan of Kojima and Abe (1988), and the house and life-style examined did not correspond to those in the United States. To describe better the actual behavior of the airborne activity, multiple wire screens can be used in diffusion battery-type systems to comprehensively determine the activity size distribution in the 0.5- to 500-nm-diameter size range, as described by Reineking and Porstendörfer (1986), Kulju et al. (1986), Tu and Knutson (1988), Strong (1988), and others. These measurements are discussed in Chapter 6. The limited size distributions that have now been measured indicate that there may be unattached activity with an average diffusion coefficient on the order of 0.035 ± 0.010 cm2/s. However, one or two modes may be present below a diameter of 10 nm, depending on the presence of aerosol sources. Thus, earlier definitions of the unattached fraction as that activity on particles of less than 5 nm in diameter do not adequately describe the actual nature of the aerosol behavior of the progeny. Complete characterization of the activity size distribution can now be accomplished with automated or manual systems. Uncertainties remain in obtaining size distributions from the activity measurements (Ramamurthi and Hopke, 1990), and standardization of measurement systems is currently feasible. Further development and refinement of these systems will lead to incremental improvements in their performances and the reliability of the information extracted from them. Only a limited number of such systems are available, and there are few data on both unattached

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Comparative Dosimetry of Radon in Mines and Homes fractions and complete activity-weighted size distributions in indoor air. The available measurements have mostly been made under artificial conditions, not during normal occupancy. Therefore, current estimates of unattached fractions cannot be assumed, with a high degree of confidence, to represent typical values in indoor air. From the available size distribution data, the AMD of the attached mode in houses without cigarette smokers is estimated to be 0.15 µm, with a σg of 2.0. With cigarette smokers present, the average aerodynamic diameter increases to 0.25 µm, with a σg of 2.5. The presence of active aerosol sources such as a vacuum cleaner motor or burning gas burners yields an activity mode at 20 nm, with a σg of 1.5, containing approximately 10 to 15% of the PAEC. A summary of the characteristics of the radon progeny aerosols assumed by the committee is given in Chapter 3. Listed there are the different scenarios that are assumed, the fp values for these scenarios, and the AMD's of the aerosols in the rooms and in the human respiratory tract. REFERENCES Agarwal, J. K., and G. J. Sem. 1980. Continuous flow, single-particle counting condensation nuclei counter. J. Aerosol Sci. 11:343-358. Bartz, H., H. Fissan, C. Helsper, Y. Kousaka, K. Okuyama, N. Fukushima, P. B. Keady, S. Kerrigan, S. A. Fruin, P. H. McMurry, D. Y. H. Pui, and M. R. Stolzenburg. 1985. Response characteristics for four different condensation nucleus counters to particles in the 3-50 nm diameter range. J. Aerosol Sci. 5:443-456. Bigu, J., and B. Kirk. 1980. Determination of the unattached radon daughter fractions in some uranium mines. Presented at the Workshop on Attachment of Radon Daughters, Measurement Techniques and Related Topics, October 30, 1980, University of Toronto. (Report available from CANMET, P.O. Box 100, Elliot Lake, Ontario, Canada.) Blanc, D., J. Fontan, A. Chapuis, F. Billard, G. Madelaine, and J. Pradel. 1968. Dosage du radon et de ses descendants dans une mine d'uranium. Repartition granulometrique des aerosols radioactifs. Pp. 229-238 in Symposium on Instruments and Techniques for the Assessment of Airborne Radioactivity in Nuclear Operations. Vienna: International Atomic Energy Agency. Busigin, A., A. W. Van der Vooren, J. C. Babcock, and C. R. Phillips. 1981. The nature of unattached 218Po (RaA) particles. Health Phys. 40:333-343. Chamberlain, A. C., and E. D. Dyson. 1956. The dose to the trachea and bronchi from the decay products of radon and thoron. Br. J. Radiol. 29:317-325. Chapuis, A., A. Lopez, J. Fontan, F. Billard, and G. J. Madelaine. 1970. Spectre granulometrique des aerosols radioactifs dans mine d'uranium. J. Aerosol Sci. 1:243-253. Chu, K. D., and P. K. Hopke. 1988. Neutralization kinetics for polonium-218. Environ. Sci. Technol. 22:711-717. Cooper, J. A., P. O. Jackson, J. C. Langford, M. R. Petersen, and B. O. Stuart. 1973. Characteristics of attached radon-222 daughters under both laboratory and field conditions with particular emphasis upon underground mine environments. Report to the U.S. Bureau of Mines under contract H0220029. Richland, Wash.: Battelle Pacific Northwest Laboratories.

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Comparative Dosimetry of Radon in Mines and Homes Goldstein, S. D., and P. K. Hopke. 1985. Environmental neutralization of polonium-218. Environ. Sci. Technol. 19:146-150. Harley, N. H., and I. M. Fissenne. 1985. Alpha dose from long-lived emitters in underground uranium mines. Pp. 518-522 in Occupational Radiation Safety in Mining, Vol. 2, H. Stocker, ed. Toronto: Canadian Nuclear Association. 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. Harley, N. H., D. E. Bohning, and I. M. Fissenne. 1981. The dose to basal cells in bronchial epithelium from long-lived alpha emitters in uranium mines. In Radiation Hazards in Mining: Control, Measurement, and Medical Aspects, M. Gomez, ed. New York: American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Jacobi, W., and K. Eisfeld. 1980. Internal dosimetry of radon-222, radon-220 and their short-lived daughters. GSF Report S-626. Munich-Neurherberg, Germany: Gesellschaft für Strahlen- und Umweltforschung. James, A. C., J. R. Greenhalgh, and A. Birchall. 1980. A dosimetric model for tissues of the human respiratory tract at risk from inhaled radon and thoron daughters. Pp. 1045-1048 in Radiological Protection—Advances in Theory and Practice , Vol. 2, Proceedings of the 5th Congress IRPA, Jerusalem, March 1980. Oxford: Pergamon Press. Jonassen, N. 1984. Electrical properties of radon daughters. Presented at the International Conference on Occupational Radiation Safety in Mining, Toronto, Canada. Jonassen, N., and B. Jensen. 1989. Radon daughters in indoor air. Final Report to Vattenfall Technical University of Denmark. Lyng, Denmark. Jonassen, N., and J. P. McLaughlin. 1985. The reduction of indoor air concentrations of radon daughters without the use of ventilation. Sci. Total Environ. 45:485-492. Knutson, E. O., and A. C. George. 1990. Reanalysis of data on the particle size distribution of radon progeny in uranium mines. Proceedings of the 29th Hanford Life Sciences Symposium, Indoor Radon and Lung Cancer: Reality or Myth?, October 16-19, 1990, Richland, Washington. Kojima, H., and S. Abe. 1988. Measurement of the total and unattached radon daughters in a house. Radiat. Prot. Dosim. 24:241-244. Kulju, L. M., et al. 1986. The detection and measurement of the activity size distribution of ultrafine particles. Paper No. 86-40.6. Pittsburgh, Pa.: Air Pollution Control Association. Porstendörfer, J., G. Röbig, and A. Ahmed. 1979. Experimental determination of the attachment coefficients of atoms and ions on monodisperse particles. J. Aerosol Sci. 10:21-28. Porstendörfer, J. 1987. Free-fractions, attachment rates, and plate-out rates of radon daughters in houses. Pp. 285-300 in Radon and Its Decay Products: Occurrence, Properties and Health Effects, P. K. Hopke, ed. Symposium Series 331. Washington, D.C.: American Chemical Society. Raes, F. 1985. Description of properties of unattached 218Po and 212Pb particles by means of the classical theory of cluster formation. Health Phys. 49:1171-1187. Ramamurthi, M., and P. K. Hopke. 1990. Simulation studies of reconstruction algorithms for the determination of optimum operating parameters and resolution of graded screen array systems (non-conventional diffusion batteries). Aerosol Sci. Technol. 12:700-710. Reineking, A., and J. Porstendörfer. 1986. High-volume screen diffusion batteries and a-spectroscopy for measurement of the radon daughter activity size distributions in the environment. J. Aerosol Sci. 17:873-879.

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Comparative Dosimetry of Radon in Mines and Homes Reineking, A., et al. 1985. Measurements of the unattached fractions of radon daughters in houses. Sci. Total Environ. 45:261-270. Singh, N. P., et al. 1985. Concentrations of 210Pb in uranium miners lungs and its states of equilibria with 238U, 234U, and 230Th. In Occupational Radiation Safety in Mining, Vol. 2, H. Stocker, ed. Toronto: Canadian Nuclear Association. Strong, J. C. 1988. The size of attached and unattached radon daughters in room air. J. Aerosol Sci. 19:1327-1330. Swedjemark, G. A. 1983. The equilibrium factor. F. Health Phys. 45:453-462. Tu, K. W., and E. O. Knutson. 1988. Indoor radon progeny particle size distribution measurements made with two different methods Radiat. Prot. Dosim. 24:251-255. Van der Vooren, A. W., A. Busigin, and C. R. Phillips. 1982. An evaluation of unattached radon (and thoron) daughter measurement techniques. Health Phys. 42:801-808. Vanmarcke, H., A. Janssens, F. Raes, A. Poffijn, P. Perkvens, and R. Van Dingenen. 1987. The behavior of radon daughters in the domestic environment. Pp. 301-323 in Radon and Its Decay Products: Occurrence, Properties and Health Effects, P. K. Hopke, ed. Symposium Series 331 Washington, D.C.: American Chemical Society.