6
Aerosols in Homes and Mines

The measurement methods used to determine the effective size of the radon progeny in air are generally based on the use of diffusional deposition in the sampler to remove some of the airborne activity in a well-defined manner. By making a series of such measurements and knowing the theoretical rates at which the deposition should occur, the size distribution of the radioactive aerosol can be inferred. Thus, to understand the experimental methods used for activity-weighted size distribution and/or unattached fraction measurements, it is necessary to present the theoretical considerations upon which the measurement instruments are based. In the next sections, the relationship between size and diffusivity will be presented along with the theory of diffusive deposition of particles on the inside surfaces of cylinders and on strands of wire mesh screen.

RELATIONSHIP BETWEEN PARTICLE SIZE AND DIFFUSION COEFFICIENT

The particle diffusion coefficient, D, for aerosols is commonly estimated by the equation,

where k is the Boltzmann constant (1.38 × 10-16 erg°K-1), T is the temperature in degrees Kelvin (293 K at 20°C and 1 atm), dp is the particle diameter in centimeters, μ is the gas viscosity (1.83 × 10-4 g cm/s for air), and C is the Cunningham correction factor given by Friedlander (1977),



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes 6 Aerosols in Homes and Mines The measurement methods used to determine the effective size of the radon progeny in air are generally based on the use of diffusional deposition in the sampler to remove some of the airborne activity in a well-defined manner. By making a series of such measurements and knowing the theoretical rates at which the deposition should occur, the size distribution of the radioactive aerosol can be inferred. Thus, to understand the experimental methods used for activity-weighted size distribution and/or unattached fraction measurements, it is necessary to present the theoretical considerations upon which the measurement instruments are based. In the next sections, the relationship between size and diffusivity will be presented along with the theory of diffusive deposition of particles on the inside surfaces of cylinders and on strands of wire mesh screen. RELATIONSHIP BETWEEN PARTICLE SIZE AND DIFFUSION COEFFICIENT The particle diffusion coefficient, D, for aerosols is commonly estimated by the equation, where k is the Boltzmann constant (1.38 × 10-16 erg°K-1), T is the temperature in degrees Kelvin (293 K at 20°C and 1 atm), dp is the particle diameter in centimeters, μ is the gas viscosity (1.83 × 10-4 g cm/s for air), and C is the Cunningham correction factor given by Friedlander (1977),

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes where λ is the mean free path of the gas (0.65 × 10-5 cm for air at 20°C and 1 arm of pressure). Equation 6-2 was empirically derived to fit the entire range of values of dp/λ from the continuum to the free molecular regimes (Davies, 1945). However, Equations 6-1 and 6-2 overestimate the particle diffusion coefficient in the 0.5-1.75-nm particle diameter range. To illustrate this overestimation, consider the diffusion coefficient for a radon-222 (222Rn) atom whose atomic diameter is estimated to be 0.46 nm. This diffusion coefficient was measured by Hirst and Harrison (1939) to be 0.12 cm2/s. Equations 6-1 and 6-2 predict a diffusion coefficient of 0.20 cm2/s for a 0.5-nm-diameter cluster. This result is further evident from a comparison (Figure 6-1) with the diffusion coefficient calculated by using the kinetic theory of gases (Loeb, 1961; Porstendörfer, 1968; Raabe, 1969). The kinetic theory equation for the diffusion coefficient of molecular clusters in a gas is where Vr is the root mean square velocity of the gas (5.02 × 104 cm/s for air at 20°C), N is the number concentration of gas molecules (2.51 × 1019 cm3 for air at 760 mm Hg and 20°C), s is the sum of the radii of the gas molecules (0.155 × 10-7 cm for air) and of the cluster, M is the molecular weight of the cluster, and m is that of the gas (28.9 for air). In order to use a single equation for the particle diffusion coefficient over the entire size range for dp > 0.5 nm, the Einstein-Cunningham Equations 6-1 and 6-2 may be fitted to the kinetic theory Equation 6-3 in the 0.5-1.75-nm size range in a manner that yields the original Einstein-Cunningham Equations 6-1 and 6-2 for dp > 1.75 nm. This result may be obtained by the substitution of for dp (in centimeters) in Equation 6-2 for the Cunningham constant C. The molecular weight-cluster size relationship for polonium-218 (218Po)-H2O clusters was used for the molecular weight factor [(M + m)/M]1/2 in the kinetic theory equation (Equation 6-3) and to obtain the fitting parameter d* in Equation 6-4. Figure 6-1 is a plot of the corrected and uncorrected Einstein-Cunningham equations and the kinetic theory diffusion coefficient equation versus particle diameter (the kinetic theory equation is plotted without the [(M + m)/M]1/2 factor, which rapidly approaches unity).

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-1 Diffusion coefficient for particles 0.5-10 nm in diameter. Penetration of Aerosols Through a Tube Theoretical equations for diffusional deposition in circular tubes are well documented in the literature (e.g., Gormley and Kennedy, 1949; Fuchs, 1964). Of particular interest when sampling highly diffusive ultrafine cluster aerosols are tube penetration equations for the prediction of diffusional deposition or wall losses in the entrances of sampling tubes. Typically, the ''wall loss'' lengths L in these situations are smaller than the tube length required for the development of a laminar flow profile ("entrance length"). This entrance length z beyond which a parabolic flow field is established is (Fuchs, 1964), where R is the radius of the tube, and Ret = 2UρR/μ is the Reynolds number of the tube. Within this entrance distance, z, a developing flow exists and could enhance diffusional deposition. Theoretical studies by Tan (1969) and Chen and Comparin (1976) suggest that for highly diffusive aerosols with small Schmidt numbers (Sc= v/D) and penetration parameter, μ* = DL/R2U < 0.05 (small entrance length L and high flow velocity U), assuming a uniform flow profile may be a suitable first-order approximation for the prediction of diffusional deposition losses. Ingham (1975) developed an analytical, matched-asymptote solution for uniform flow penetration through a circular tube, Pt, given by,

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-2 Assessment of penetration of radioactive particles into a cylindrical tube compared to the theoretical predictions of Ingham (1975). The parameters (=5.783186), (=30.471262), (=74.887007) are the zeros of the zero-order Bessel function of the first kind, J0(an) = 0 (Tan, 1969). Equation 6-6 is valid for tube Reynolds number Ret < 1,200 and tube Peclet number (Pet = RU/D) > 100. Thomas (1955) attempted to verify the laminar flow tube penetration theory equations using gas molecules and reported agreement to within 20-30%. The discrepancies were attributed to experimental difficulties and possible entrance effects. Scheibel and Porstendörfer (1984) attempted a verification of tube penetration theory using monodisperse particles (dp > 4 nm). They reported good agreement with the laminar flow theory (Gormley and Kennedy, 1949) for particle sizes greater than 15 nm and large deviations for particle sizes below 5 nm. The discrepancies were resolved by including terms for uniform flow deposition in the entrance of the tube and for deposition on the front face of the tubes. Recent studies by Ramamurthi et al. (in press) have found that tube penetration theory accurately predicts the deposition behavior of highly diffusive radioactive particles if the value . For μ* > 0.05, Equation 6-6 systematically underpredicts the penetration of particles through the tube. Figure 6-2 shows the results of this test of penetration of a single tube as a function of the diffusion parameter, μ*/D.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Deposition of Ultrafine Particles onto Wire Screens An equation of penetration through wire screens based on the theory of fibrous filtration was derived by Cheng and Yeh (1980) and Cheng et al. (1980). The penetration equation for a wire screen, with wire diameter df, thickness w, volume fraction α, and sampling face velocity U, for ultrafine particles, dp < 0.1 μm, is given by, where and D is the particle diffusion coefficient. Equation 6-7 is valid for wire Reynolds numbers (Ref = Uρdf/μ, where ρ is the density of air) less than 1 (Emi et al., 1982). The fan model filtration theory has been applied to wire screens and experimentally verified for particle sizes dp > 15 nm (0.015 μm) (Cheng and Yeh, 1980). Scheibel and Porstendörfer (1984) have further verified the fan model for particle sizes nm. Recent work with charged and uncharged 218Po clusters in the 0.5-1.5-nm size range has also indicated general agreement with the wire screen fan model penetration Equation 6-7 (Holub and Knutson, 1987; Ramamurthi et al., in press), although calibration studies remain necessary for dp < 4 nm. The penetration characteristics for a wire screen operating in the 0.5-100 nm size range can thus be determined from Equations 6-7, 6-8, and 6-9. Typical penetration curves are shown in Figure 6-3. Further, each screen-velocity combination can be conveniently parameterized by its dp(50%) (Ramamurthi and Hopke, 1989), the particle diameter for 50% collection efficiency, because of the form of the penetration in Equation 6-7. This parameter can be determined (using a log-linear diffusion coefficient approximation) as, for 0.001 < KVF < 0.325. These dp(50%) values are a convenient way of characterizing the particular screen parameters and face velocity of the sampling flow. However, as can be seen by the curves in Figure 6-3, there is a distinct difference between the relatively slowly changing penetration-versus-diameter behavior of screens and the sharp cutoffs that are observed with inertial collections systems such as impactors or cyclones.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-3 Fractional penetration through various mesh screens at a face velocity of 10 cm s-1. MEASUREMENT METHODS FOR UNATTACHED FRACTION Diffusion Sampler Most of the early work on "unattached" fraction measurements was carried out in uranium mines. In the earliest studies, including the work of Chamberlain and Dyson (1956), the unattached fraction was determined by measurement of the penetration of the activity through a right circular cylinder. Using the tube penetration theory of Gormley and Kennedy (1949), the fractional penetration can be related to πDL/2Q, where D is the diffusion coefficient of the radioisotope, L is the length of the tube, and Q is the volumetric flow rate through the tube. Details of the theory of penetration of particles through a tube are given in the previous section. The diffusion coefficients of the radon decay products are assumed to have a single, very much larger value than the diffusion coefficients of the condensation nuclei to which the radon progeny become attached. Thus, by comparing the amount of activity penetrating through a tube of given dimensions at a given flow rate with the total airborne decay product activity, the fraction of unattached activities could be estimated. The amount of unattached radioactivity has been measured in this manner by a number of investigators (Craft et al., 1966; Fusamura et al., 1967; Duggan and Howell, 1969). A problem that tends to confuse the literature of unattached fraction measurements is the number of ways in which the unattached radioactivity is reported. The International Commission on Radiological Protection

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes (ICRP) has defined the unattached fraction as the fraction of the equilibrium number of 218Po ions that are unattached to particles (ICRP, 1959). However, the method used to measure the attached fraction readily yields the fraction of unattached 218Po atoms to the total number of 218Po atoms actually present. Chamberlain and Dyson (1956) made the first report of unattached fraction to be 0.1 using what became the ICRP definition (ICRP, 1959). It is important to carefully determine in each case what the investigator means by "unattached" fraction in order to compare results. Wire Screen Samplers The diffusion samplers were fairly cumbersome devices to use, and therefore a simpler and more portable system was developed based on the collection of the activity on wire mesh screens. Wire mesh screens have become the most commonly used method for estimating unattached radon daughter fractions. The early development of these systems was described by James et al. (1972), Thomas and Hinchliffe (1972), and George (1972). These systems were much easier to use, although initially they suffered from the lack of a well-developed theory to relate the screen properties to their collection efficiency. The collection efficiency of wire screens for unattached 218Po was therefore determined empirically from calibration experiments with fresh 218Po in the absence of ambient aerosols as a function of screen parameters and face velocity. Compared with the currently accepted theory described below, the equation of Thomas and Hinchliffe (1972) overestimates wire screen collection efficiencies for particle diameters dp < 1.0 to 2.0 nm and dp > 5.0 to 7.0 nm for most screen-velocity combinations. George (1972) developed a standard method for measuring unattached 218Po fractions: a 60-mesh stainless steel screen is sampled simultaneously with a parallel filter, followed by counting of the alpha particles on both the screen and filter; 218Po data are extracted from both the screen and the filter by the modified Tsivoglou technique, and the unattached fraction is calculated as the ratio of twice the activity on one face of the screen to the activity on the filter. This method has been widely used to obtain estimates of the unattached fraction in a variety of different environments. The wire screen penetration theories developed by Cheng and Yeh (1980), Cheng et al. (1980), and Yeh et al. (1982) are presented earlier, along with a semi-empirically corrected diffusion coefficient equation in the molecular cluster size range to characterize unattached fraction measurements reported in earlier studies, Equations 6-1 and 6-2. A number of wire screen measurement studies of the unattached 218Po fraction are reported in the literature (George, 1972; James et al., 1972; Raghavayya and Jones, 1974; Bigu and Kirk, 1980; Stranden and Berteig, 1982; Bigu, 1985; Reineking et al., 1985). Table 6-1 provides a compilation

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes TABLE 6-1 Wire Screen Parameters and Face Velocities Used in Previously Published Wire Screen "Unattached" Fraction Measurements   "Unattached" Mode Underestimation (%)   1-C 1-C/E Study Mesh No. df U dp(50%) "E" C C/E James et al. (1972) 200 50.8 12.0 2.7 — 15 — George (1972) 60 178.0 11.5 1.7 0.76 45 10 George (1972) 80 127.0 11.5 1.9 — 36 — Raghavayya and Jones (1974) 120 94.0 12.2 2.0 — 29   Bigu and Kirk (1980) 150 76.0 9.6 2.8 — 14 — Stranden and Berteig (1982) 174 56.0 21.0 2.3 0.86 22 5 Bigu (1985) 150 76.0 27.3 1.6 0.77 48 14 Reineking et al. (1985) 188 50.0 8.5 2.0 — 31 — NOTE: Underestimation of "unattached" cluster fraction is based on a log-normally distributed cluster mode, 0.5 < dp-< 3.0 nm, with dm = 1.0 nm and σg = 1.5. "C" is the fraction of activity collected by the screen, and ''E" is a "collection efficiency" correction factor used in some of the studies. of the wire screen parameters and face velocities used in these studies. Each screen-velocity combination is characterized by its dp(50%) parameter obtained from Equation 6-1. Nominal values for commonly undocumented parameters such as w and α were taken from wire screen manufacturers' catalogs for screens with the appropriate combination of mesh number and wire diameter reported in the studies. Figure 6-4 shows the calculated particle collection efficiency characteristics for each of the studies listed in Table 6-1, as determined by the theory of Cheng and Yeh (1980) discussed above. The collection efficiencies for the 60-mesh screens used by George (1972) and the 150-mesh screens used by Bigu (1985) axe also plotted, although the wire Reynolds numbers are greater than 1. Wire screens with collection efficiencies that differ only slightly are plotted together for comparison purposes. The collection efficiencies of the screens are between 70 and 90% and 6 and 12%, respectively, for 1-and 10-nm-diameter particles and decrease rapidly in the 1-to 3-nm size range. The concept of an unattached fraction measurement is to have a system that will collect all of the highly diffusive activity without collecting any activity attached to particles. The separation of aerosol size distributions into well-defined modes has been used to great advantage in studying the much larger sized modes in the ambient aerosol. However, such measurements

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes FIGURE 6-4 Collection characteristics for the wire mesh screen systems used in a number of the reported "unattached" fraction measurements. are possible because the separation of large particles is based on their inertial properties. In this case, devices with sharp cutoff points such as cyclones or impactors can be designed (Lodge and Chan, 1986). However, because of the stochastic nature of diffusional deposition, the collection curves for wire screens or tubes are much more gradual functions of particle size. To illustrate the problem with using a single screen for making such a dichotomous measurement, Figure 6-5 shows the collection efficiency curves with dp(50%) along with the characteristics of a log-normally distributed aerosol size distribution with a median diameter of 1.0 nm and a geometric standard deviation of 1.5 in the range suggested by Reineking and Porstendörfer (1986). Nominal values for w and α were obtained from screen manufacturers' catalogs (see Hopke et al. [in press] for details and references).

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-5 Collection efficiency curves for "typical" wire screens characterized by dp(50%) values of 1, 2, 3, and 4 nm and presentation of a log-normal size distribution characteristic of the "unattached" fraction. It can be seen that the use of wire screens underestimates the unattached fraction if it indeed consists of an ultrafine cluster mode in the 0.5-to 3-nm size range (Reineking and Porstendörfer, 1986). The cumulative collection of activity as a function of size is presented in Figure 6-6. The underestimation of the unattached cluster mode in each of the studies is presented in Table 6-1. The wire screens used in these studies would underestimate such an unattached cluster mode by 14 to 48%, depending on the dp(50%) parameter for the particular screen-velocity combination. The choice of a single, optimized screen dp(50%) parameter that maximizes the collection of the unattached mode while minimizing the collection of attached activity may hence be beneficial for single-screen unattached fraction measurements. To facilitate the appropriate choice of this parameter, the collection efficiency of a log-normally distributed aerosol particle mode was determined as a function of the wire screen dp(50%), as shown in Figure 6-6. From the data in Figure 6-6 it appears that a wire screen dp(50%) is an optimal choice yielding about 90% collection of the unattached mode (dm = 1 nm, σg = 1.5, 0.5 < dp < 3 nm), while minimizing the collection of activity in the second aerosol mode (dm = 25 nm, σg = 1.75, 10 < dp < 60 nm) to less than 8%. Activity in the latter size range may be significant in indoor air following cooking, as reported by Tu and Knutson (1988a). The collection of activity attached to the larger aerosol particle mode (dm = 125 nm, σg = 2.0, 40 < dp < 400 nm) is minimal for a dp(50%) of <10 nm, and is about 1% for a dp(50%) of 4 nm. This

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-6 Cumulative collection fraction of an ultrafine activity mode. result is consistent with the calculations by Van der Vooren et al. (1982) for the collection of attached activity in this size range by wire screens sampling the unattached mode. In the sampling of dusty atmospheres with particle sizes of a dp of >0.5 μm (500 nm), collection by impaction and interception may become significant; but for wire screens with a df of >100 μm, an α of <0.3 and a U of <10.0 cm/s the wire screen collection efficiency for a 5-μm-diameter particle is less than 5%. More recent work by Reineking and Porstendörfer (1990) suggests that there can be errors in unattached fraction measurements caused by inertial collection of radioactivity-carrying particles of >100 nm in diameter. Another consideration of wire screen systems is the activity measurements. Measurement of the activity collected by a wire screen is complicated by the deposition of activity on the front and back faces of the screen as well as alpha absorption losses in the screen weaves. The ratio of activity collected on the front and back faces of a wire screen has been found to be clearly dependent on both the screen parameters and the activity distribution sampled by the screen (Holub and Knutson, 1987). For single wire screen samplers, analysis of total or reference filter (At) and wire screen backup filter (Abf) activities would yield more reliable estimates of the unattached fraction and would help to circumvent the problems associated with analyzing the wire screens themselves for collected activity. However, depending on the amount of activity that attaches to the screen, such a procedure may lead to low statistical precision in the activity determination on the screen backup filter.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes the activity attached to the larger ambient aerosol particles (diameter, >50 nm). The corresponding 214Pb and 214Bi distributions showed much smaller activity fractions in the 0.9-nm size range. The longer lifetime of these decay products permits a greater fraction of activity to become attached to the ambient aerosol. For all three distributions, the attached mode peaked in the 160-500-nm size range. However, this measurement system cannot be used to determine particle sizes greater than 500 nm in diameter. The activity distributions obtained are in general agreement, with respect to both the 218Po cluster fraction and the size range of attached activity, with the distributions measured by Tu et al. (1989) in the basement of this house under similar conditions at an earlier date. Several measurements of activity-size distributions were also made in the kitchen on the first level of the house. The initial measurements were performed under typical conditions of 20,000 particles/cm3 and a radon concentration of . The results of the measurement are shown in Figure 6-28. The 218Po size distribution showed a large fraction of the 218Po activity with a diffusivity similar to that of the classical unattached fraction. However, a significant fraction was in the 1.6-5.0-nm size interval. The corresponding 214Pb and 214Bi distributions indicate insignificant activity fractions in the 0.5-1.6-nm size interval, but a significant mode between 1.6 and 5.0 nm. The differences in the size distributions obtained in the basement and in the kitchen area related primarily to the 1.6-to 5.0-nm size interval, with the attached activity modes remaining in the 160-to 500-nm size range. This result suggests the presence of condensable constituents leading to the formation of particles in the 1.6-to 5.0-nm size interval or a source of very fine primary particles. This process may then allow the radon decay products to become associated with the 1.6-to 5.0-nm size mode in varying fractions depending upon the relative lifetimes. The presence of six large gas range pilot lights may be related to the formation of this mode, and similar effects are believed to have been observed by Tu et al. (1989) in other houses. In a final experiment, activity-size distributions were measured following the continuous addition of aerosols generated in the kitchen from lighting the gas stove burners of the kitchen range. Figure 6-29 shows the distributions measured with particle number concentrations of and a radon concentration of . The large concentrations of particles generated could be presumed to be rapidly coagulating soot cluster aggregates. The 218Po and 214Pb activity distributions measured under these conditions (Figure 6-29) were dramatically different from those measured in the basement and background kitchen conditions. The 218Po distribution revealed very little activity in the 0.5-to 1.6-nm size interval (unattached fraction), with most of the activity spread out over the size interval range from 1.6 to 50 nm. The fraction of 218Po attached to particles of > 100 nm in diameter was reduced to a negligible level, probably because of the very large number of smaller particles produced by the gas burners. The distribution of 214Pb revealed that the activity

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-28 Po-218, Pb-214, and Bi-214 activity-size distributions measured under typical conditions in the kitchen of the test house. was spread out over the size spectrum dp > 1.6 nm, while the 214Bi distribution remained similar to those measured prior to the addition of external aerosols. However, these latter results may be due to the timing of the sampling interval, which was between 20 and 35 min after the start of continuous addition of the external aerosols. Consequently, steady-state 214Pb and 214Bi distributions may not have been attained. A stability analysis was performed for each of the size distributions shown in Figures 6-27, 6-28, and 6-29. This analysis provides an estimate of the stability of the inferred solutions with respect to errors in the input penetration data, and the results are represented by the error bars indicated in the figures. The size distributions obtained in the experiments were found to be stable and relatively insensitive to perturbations in the input data of the order of the associated measurement errors. The errors in the size interval fractions estimated from this procedure are too small to be seen in these figures and thus were not included. Additional measurements were made in another one-story residence with living room, dining room, kitchen, two bedrooms, a study room, two bathrooms, and basement in the Princeton, N.J., area (Li, 1990; Hopke et al., 1990a,b). Activity-size distributions were measured in the living room and one of the bedrooms over a 2-week period (January 16 to 31, 1990). A total of about 10 measurements were made in the living room, and more than 100 measurements were made in the bedroom with different types of particle generation. Aerosols were generated from candle burning, cigarette smoking, vacuuming (electric

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Figure 6-29 Po-218, Pb-214, and Bi-214 activity-size distributions measured during the continuous generation of aerosols from the kitchen gas stove burners. motor), cooking, and opening doors from normal activities in the domestic environments. The particle concentrations were measured by using a Gardner manual condensation nucleus counter. The concentration and size distribution of radon progeny were determined by a semicontinuous graded screen array system. A sequence (0-15, 15-35, 35-75 min) with 75-min sampling was chosen because the radon concentration was in the range of 5 to 50 pCi/liter. The influence of cigarette smoking (20 min) on the radon progeny size distributions in a closed bedroom are shown in Figure 6-30. The measurements were made 5 min after lighting the cigarette (5-20 min), 80 min later (80-95 min), and 155 min later (155-170 min). The fraction of 218Po in the 0.9-nm size range changed from 60 to 8%. The fraction of 214Pb and 214Bi in the 0.9-nm size range was about 10% and essentially became zero. The fraction of three distributions in the 1.5-to 15-nm size range stayed the same. There was a large increase (from 40 to 80%) of 218Po in the attached mode (50-to 500-nm size range), with insignificant changes (from 35 to 40%) in 214Pb and 214Bi fractions in this mode. The influence of cooking on the radon progeny size distributions with an open bedroom door is shown in Figure 6-31. A steak was pan fried for 20 min (0-20 min) by using a gas stove burner in the kitchen. The measurements were made 5 min later (5-20 min), 80 min later (80-95 min), and 155 min later (155-170 min). The fraction of 218Po in the 0.9-nm size range changed from 60 to 15%. The fractions of 214Pb and 214Bi in the 0.9-nm size range changed from 15 to 10%. There was a very low fraction of activity in the 1.5-to 15-nm

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes A: Background Size Distributions. B: During the active smoking period. C: 60 minutes after the cigarette burn. D: 135 minutes after the cigarette burn. Figure 6-30 Activity-size distributions in a bedroom before, during, and after smoking a cigarette.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes A: Background Size Distributions. B: 5 minutes into the cooking period. C: 60 minutes after the cooking period. D: 135 minutes after the cooking period. Figure 6-31 Activity-size distributions measured in a bedroom during and after cooking in the kitchen.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes size range for background, and it increased to 10% because of cooking. A large increase (from 35 to 70%) of 218Po was observed in the attached mode and peaked at the 50-to 500-nm size range, with only small changes (from 40 to 50%) in 214Pb and 214Bi distributions. Because of the large number of particles generated by normal activities in the domestic environment, the working level increases for a period of time, while the unattached fraction decreases. The particles generated from cigarette smoke and cooking dramatically shifted almost all of the radon progeny to the attached fraction and remained for a long period of time. The particles produced from candle burning and vacuuming were much smaller, with an average attachment diameter around 15 nm. The candle and vacuuming particles did decrease the unattached fraction, but returned to the original background distributions about 150 min later. Summary of Indoor Exposure In evaluating the information on the aerodynamic size of the particles carrying the radioactivity in the indoor environment, the results of Reineking and Porstendörfer (1986, 1990), Tu and Knutson (1988a,b), Ramamurthi and Hopke (1990), Li (1990), and Hopke et al. (1990a,b) were reviewed. From these results, the unattached activity appears to have a diffusion equivalent diameter of 0.0011 µm and typically represents about 8% of the airborne alpha activity energy. The typical indoor radioactive aerosol has a mode with an AMD of 0.15 µm with a geometric standard deviation of 2.0. The presence of sidestream cigarette smoke provides a substantial number of larger sized particles so that the AMD increases to 0.25 µm with a geometric standard deviation of 2.5. During periods of active smoking, the unattached fraction diminishes to 0.1, and on average, the unattached activity represents about 3% of the total activity in houses with smokers. Other activities can produce particles with small diameters so that during vacuuming or cooking, an additional mode in the activity-size distribution with an average diameter of 0.02 µm and containing 15% of the airborne alpha energy can be observed. Because of the high mobility of these particles, this mode is quite transient and will disappear in a few hours time. Finally, there are times in closed rooms such as bedrooms with relatively low air exchange rates that the particle concentration can be sufficiently low that a much higher fraction of the activity is in the unattached mode. It is estimated that a typical value for the unattached fraction under these circumstances is 16%.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes REFERENCES Becker, K. H., A. Reineking, H. G. Scheibel, and J. Porstendörfer. 1984. Radon daughter activity size distributions. Radiat. Prot. Dosim. 7:147-150. Bigu, J. 1985. Radon daughter and thoron daughter deposition velocity and unattached fraction under laboratory conditions in underground uranium mines. J. Aerosol Sci. 16:157-165. 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.) Busigin, A., A. W. Van der Vooren, and C. R. Phillips. 1981. Measurement of the total and radioactive aerosol size distributions in a Canadian uranium mine. Am. Ind. Hyg. Assoc. J. 42:310-314. 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. Chen, R. Y., and R. A. Comparin. 1976. Deposition of aerosols in the entrance of a tube. J. Aerosol Sci. 7:335-341. Cheng, Y. S. 1989. Deposition of thoron daughters in human head airways. Technical Exchange Meeting, Grand Junction, Colo., September 18-19, CONF 8909190. NTIS. Cheng, Y. S., and H. C. Yeh. 1980. Theory of screen type diffusion battery. J. Aerosol Sci. 11:313-319. Cheng, Y. S., J. A. Keating, and G. M. Kanapilly. 1980. Theory and calibration of a screen-type diffusion battery. J. Aerosol Sci. 11:549-556. 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, contract H0220029. Richland, Wash.: Battelle Pacific Northwest Laboratories. Craft, B. F., J. L. Oser, and E W. Norris. 1966. A method for determining relative amounts of combined and uncombined radon daughter activity in underground uranium mines. Am. Ind. Hyg. Assoc. J. 27:154-159. Davies, C. N. 1945. Definitive equations for the fluid resistance of spheres. Proc. Phys. Soc. 57:259-270. Duggan, M. J., and D. M. Howell. 1969. The measurement of the unattached fraction of airborne RaA. Health Phys. 17:423-427. Emi, H., C. Kanaoka, and Y. Kuhabara. 1982. The diffusion collection efficiency of fibers for aerosol over a wide range of Reynolds numbers. J. Aerosol Sci. 13:403-413. Friedlander, S. K. 1977. Smoke, Dust and Haze. New York: John Wiley & Sons . Fuchs, N. A. 1964. The Mechanics of Aerosols. New York: MacMillan Press. Fusamura, N, R. Kurosawa, and M. Maruyama. 1967. Determination of f-value in uranium mine air. Pp. 213-227 in Symposium on Instruments and Techniques for the Assessment of Airborne Radioactivity in Nuclear Operations. Vienna: International Atomic Energy Agency. George A. C. 1972. Measurement of the uncombined fraction of radon daughters with wire screens. Health Phys. 23:390-392.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes George, A. C., and A. J. Breslin. 1980. The Distribution of Ambient Radon and Radon Daughters in Residential Buildings in the New Jersey-New York Area, National Radiation Environment III, Vol. 2. CONF-780422. Oak Ridge, Tenn.: Technical Information Center, U.S. Department of Energy. George, A. C., and L. Hinchliffe. 1972. Measurements of uncombined radon daughters in uranium mines. Health Phys. 23:791-803. George, A. C., L. Hinchliffe, and R. Sladowski. 1975. Size distribution of radon daughter particles in uranium mine atmospheres. Am. Ind. Hyg. Assoc. J. 36:484-490. George, A. C., L. Hinchliffe, and R. Sladowski. 1977. Size Distribution of Radon Daughter Particles in Uranium Mine Atmospheres. HASL-326. New York: Health and Safety Laboratory. George, A. C., M. H. Wilkening, E. O. Knutson, D. Sinclair, and L. Andrews. 1984. Measurements of radon and radon daughter aerosols in Socorro, New Mexico. Aerosol Sci. Technol. 3:277-281. Gormley, P., and M. Kennedy. 1949. Diffusion for a stream flowing through a cylindrical tube. Proc. R. Irish Acad. 52A:163-167. Hirst, B. W., and G. E. Harrison. 1939. The diffusion of Rn gas mixtures. Proc. R. Soc. Lond. Ser. A 169:573-586. Holub, R. F., and E. O. Knutson. 1987. Measuring polonium-218 diffusion-coefficient spectra using multiple wire screens. Pp. 340-356 in Radon and Its Decay Products: Occurrence, Properties and Health Effects, P. K. Hopke, ed. Symposium Series 331. Washington D.C.: American Chemical Society. Holub, R. F., E. O. Knutson, and S. Solomon. 1988. Tests of the graded wire screen technique for measuring the amount and size distribution of unattached radon progeny. Radiat. Prot. Dosim. 24:265-268. Hopke, P. K. 1989a. Use of electrostatic collection of 218Po for measuring Rn. Health Phys. 57:39-42. Hopke, P. K. 1989b. The initial behavior of 218Po in indoor air. Environ. Int. 15:299-308. Hopke, P. K., M. Ramamurthi, and C. S. Li. 1990a. Measurements of the size distributions of radon progeny in indoor air. In Aerosols: Science, Industry, Health, and Environment, Vol. 2, S. Masuda and K. Takahashi, eds. Oxford: Pergamon Press. Hopke, P. K., M. Ramamurthi, and C. S. Li. 1990b. Measurements of size distributions of indoor radioactive aerosol. Presented at the 29th Hanford Symposium on Health and the Environment. Richland, Washington, October 1990. Hopke, P. K., M. Ramamurthi, and E. O. Knutson. In press. A measurement system for Rn decay product lung deposition based on respiratory models. Health Phys. Ingham, D. B. 1975. Diffusion of aerosols for a stream flowing through a cylindrical tube. J. Aerosol Sci. 6:125-132. International Commission on Radiological Protection (ICRP). 1959. Report of Committee II on Permissible Dose for Internal Radiation, ICRP Publication 2. Oxford: Pergamon Press. James A. C., G. F. Bradford and D. M. Howell. 1972. Collection of unattached RaA atoms using wire gauze. J. Aerosol Sci. 3:243-254. Khan, A., C. R. Phillips, and P. Duport. 1987. Analysis of errors in the measurement of unattached fractions of radon and thoron progeny in a Canadian uranium mine using wire screen methods. Radiat. Prot. Dosim. 18:197-208. Knutson, E. O., A. C. George, R. H. Knuth, and B. R. Koh. 1984. Measurements of radon daughter particle size. Radiat. Prot. Dosim. 7:121-125.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Knutson, E. O., K. W. Tu, S. B. Solomon, and J. Strong. 1988. Intercomparison of three diffusion batteries for the measurement of radon decay product particle size distributions. Radiat. Prot. Dosim. 24:261-264. Kojima, H., and S. Abe. 1988. Measurement of the total and unattached radon daughters in a house. Radiat. Prot. Dosim. 24:241-244. Kotrappa, P., and Y. S. Mayya. 1976. Revision of Raghavayya and Jones' data on the radon decay in mine air. Health Phys. 31:380-382. Li, C. S. 1990. Field Evaluation and Health Assessment of Air Cleaners in Removing Radon Decay Products in Domestic Environments. Ph.D. thesis. University of Illinois, Urbana, Ill. Department of Energy Report DOE ER61029-2. Liu, B. Y. H., and D. Y. H. Pui. 1975. On the performance of the electrical aerosol analyzer. J. Aerosol Sci. 6:249-264. Lodge, J. P., Jr., and T. Chan. 1986. Cascade Impactor: Sampling and Data Analysis. Akron, Ohio: American Industrial Hygiene Association, Loeb, L. B. 1961. The Kinetic Theory of Gases, 3rd ed. New York: Dover Publications. Maher, E. F., and N. M. Laird. 1985. Algorithm reconstruction of particle size distribution from diffusion battery data. J. Aerosol Sci. 16:557-570. Mercer, T. T. 1975. Unattached radon decay products in mine air. Health Phys. 28:158-161. Mercer, T. T., and W. A. Stowe. 1969. Deposition of unattached radon decay products in an impactor stage. Health Phys. 17:259-264. Porstendörfer, J. 1968. Die Diffusionkoeffizienten und mittleren freien Weglangen der Geladenen und neutral radon-folge Produkte in Luft. Z. Physik. 213:384-396. 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. Porstendörfer, J., and T. T. Mercer. 1979. Influence of electric charge and humidity upon the diffusion coefficient of radon decay products. Health Phys. 15:191-199. 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. Raabe, O. G. 1969. Concerning the interactions that occur between radon decay products and aerosols. Health Phys. 17:177-185. Raes, F., A. Janssens, A. DeClercq, and H. Vanmarcke. 1984. Investigation of the indoor aerosol and its effect on the attachment of radon daughters. Radiat. Prot. Dosim. 7:127-131. Raghavayya, M., and J. H. Jones. 1974. A wire screen-filter paper combination for the measurement of fractions of unattached daughter atoms in uranium mines. Health Phys. 26:417-430. Ramamurthi, M., and P. K. Hopke. 1989. On improving the validity of wire screen ''unattached'' fraction Rn daughter measurements. Health Phys. 56:189-194. 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. Ramamurthi, M., and P. K. Hopke. 1991. An automated, semi-continuous system for measuring indoor radon progeny activity-weighted size distributions, dp: 0.5-500 nm. Aerosol Sci. Technol. 14:82-92.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Ramamurthi, M., P. K. Hopke, R. Strydom, K. W. Tu, E. O. Knutson, R. F. Holub, W. Winklmayr, W. Marlow, and S. C. Yoon. 1989. Radon decay product activity size distribution measurement methods—A laboratory intercomparison. Presented at the American Association for Aerosol Research Meeting, Reno, Nev., October 1989. Ramamurthi, M., R. Strydom, and P. K. Hopke. In press. Assessment of wire and tube penetration theories using a 218PoOx cluster aerosol. J. Aerosol Sci. 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. Reineking, A., and J. Porstendörfer. 1990. "Unattached" fraction of short-lived decay products in the indoor and outdoor environment: An improved single screen method and results. Health Phys. 58:717-727. Reineking, A., K. H. Becker, and J. Porstendörfer. 1985. Measurements of the unattached fractions of radon daughters in houses. Sci. Total Environ. 45:261-270. Reineking, A., K. H. Becker, and J. Porstendörfer. 1988. Measurement of activity size distributions of the short-lived radon daughters in the indoor and outdoor environment. Radiat. Prot. Dosim. 24:245-250. Scheibel, H. G., and J. Porstendörfer. 1984. Penetration measurements for tube and screen type diffusion batteries in the ultrafine particle size range. J. Aerosol Sci. 15:673-682. Shimo, M., and Y. Ikebe. 1984. Measurements of radon and its short-lived decay products and unattached fraction in air. Radiat. Prot. Dosim. 8:209-214. Shimo, M., Y. Yoshihiro, K. Hayashi, and Y. Ikebe. 1985. On some properties of 222Rn short-lived decay products in air. Health Phys. 48:75-86. Sinclair, D., A. C. George, and E. O. Knutson. 1977. Application of diffusion batteries to measurement of submicron radioactive aerosols. Pp. 103-114 in Airborne Radioactivity. La Grange Park, Ill.: American Nuclear Society. Stranden, E., and L. Berteig. 1982. Radon daughter equilibrium and unattached fraction in mine atmospheres. Health Phys. 42:479-487. Stranden, E., and T. Strand. 1986. A dosimetric discussion based on measurements of radon daughter equilibrium and unattached fraction in different atmospheres. Rad. Prot. Dosim. 16:313-318. Strong, J. C. 1988. The size of attached and unattached radon daughters in room air. J. Aerosol Sci. 19:1327-1330. Strong, J. C. 1989. Design of the NRPB activity size measurement system and results. Presented at the Workshop on "Unattached" Fraction Measurements, University of Illinois, Urbana, Ill., April 1989. Subba Ramu, M. C. 1980. Calibration of a diffusion sampler used for the measurement of unattached radon daughter products. Atmosph. Environ. 14:145-147. Tan, C. W. 1969. Diffusion of disintegration products of inert gases in cylindrical tubes. Int. J. Heat Mass Trans. 12:471-478. Thomas, J. W. 1955. The diffusion battery method for aerosol particle size determination. J. Colloid Sci. 10:246-255. Thomas, J. W. 1970. Modification of the Tsivoglou method for radon daughters in air. Health Phys. 19:691-693. Thomas, J. W., and L. E. Hinchliffe. 1972. Filtration of 0.001 µm particles with wire screens. J. Aerosol Sci. 3:387-393. Tu, K. W., and E. O. Knutson. 1988a. Indoor radon progeny particle size distribution measurements made with two different methods. Radiat. Prot. Dosim. 24:251-255.

OCR for page 90
Comparative Dosimetry of Radon in Mines and Homes Tu, K. W, and E. O. Knutson. 1988b. Indoor outdoor aerosol measurements for two residential buildings in New Jersey. Aerosol Sci. Technol. 9:71-82. Tu, K. W., A. C. George, and E. O. Knutson. 1989. Summary of results of radon progeny particle size in indoor air. Presented at the American Association for Aerosol Research Meeting, Reno, Nev., October 1989. Twomey, S. 1975. Comparison of constrained linear inversion and an iterative nonlinear algorithm applied to the indirect estimation of the particle size distribution. J. Comp. Phys. 18:188-200. 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, and F. Raes. 1985. The equilibrium of attached and unattached radon daughters in the domestic environment. Sci. Tot. Environ. 45:251-260. 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. Vanmarcke, H., A. Reineking, J. Porstendörfer, and F. Raes. 1988. Comparison of two methods for investigating indoor radon daughters. Radiat. Prot. Dosim. 24:281-284. Vanmarcke, H., P. Berkvens, and A. Poffijn. 1989. Radon versus Rn daughters. Health Phys. 56:229-231. Yeh, H. C., Y. S. Cheng, and M. M. Orman. 1982. Evaluation of various types of wire screens as diffusion battery cells. J. Colloid Interface Sci. 86:12-16.