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Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
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Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract W. Zhuo, S. Tokonami, H. Yonehara, Y. Yamada Radon Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan
For simultaneous measurements of indoor 222 Rn and 220 Rn and estimation of their deposition fractions in the respiratory tract, a new type of passive integrating 222 Rn and 220 Rn monitor and a portable bronchial dosimeter were developed. The passive 222 Rn and 220 Rn monitor was rebuilt from a commercially available passive 222 Rn monitor. Besides its simple construction, the volume and weight of the new monitor are only 110 cm3 and 20 g. Calibration factors of 222 Rn and 220 Rn for the new monitor were systematically studied through calibration experiments. The results indicated that indoor 220 Rn could be discriminated from 222 Rn by using the new passive monitor. The bronchial dosimeter consists of three sets of progeny integrating sampling units (PISUs) with different configurations of sampling heads. Multiple metal screens are used to mimic the penetrating and deposition behaviour of 222 Rn/220 Rn progeny in the nasal and tracheo-bronchial (T-B) regions of the human respiratory tract. The potential alpha energy concentrations (PAEC) of 222 Rn/220 Rn progeny are directly measured with the allyl diglycol carbonate (CR-39) detectors inside the PISUs. The deposition fractions of 222 Rn and 220 Rn progeny in the T-B region were measured, with averages of 4.5 and 4.0% for ordinary room conditions, in general agreement with other reported values. Both the new monitor and device are simple and compact as well as of low cost, and they are considered to be practical for large-scale and long-term surveys of indoor 222 Rn and 220 Rn.
1. Introduction Public exposure to 222 Rn and its progeny has been of worldwide concern in the past decades. It is well known that the health detriment associated with radon (222 Rn) mainly arises from the inhalation of its progeny and the resultant alpha particle exposure to the epithelium of the respiratory tract [1]. Previous studies have shown that the tracheo-bronchial (T-B) region is the most radiosensitive in the respiratory tract, and the radiation dose in the region due to 222 Rn progeny constitutes a dominant fraction of the total lung dose [2]. Recent surveys revealed that the exposure to thoron (220 Rn) and its progeny could equal or even exceed that RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07040-8
© 2005 Elsevier Ltd. All rights reserved.
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
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of 222 Rn and its progeny in some areas of the world [3–5]. Therefore, measurements of 220 Rn are indispensable for more precise assessment of public exposure to 222 Rn and 220 Rn. For simultaneous measurements of indoor 222 Rn and 220 Rn, several types of 222 Rn and 220 Rn discriminative monitors have been developed. Doi et al. [6] developed a unified 222 Rn and 220 Rn discriminative monitor with polycarbonate film as detectors. Besides its large volume (about 560 cm3 ), the construction of monitor and the etching processes of detectors were complicated. The 222 Rn and 220 Rn monitors developed by Guo et al. [7] and Iida et al. [8] were somewhat inconvenient for field applications because they were two separated hemispherical cups. Moreover, it was also complicated and time-consuming to exchange the detectors. The traditional methods for measuring the deposition of 222 Rn progeny in the T-B region are tedious and difficult, and generally require complex equipment [9,10]. For simulating and measuring the deposition of 222 Rn progeny in the respiratory tract, several type of sampling systems consisting of multiple layers of wire screens with appropriate wire factors and air flow velocity have been developed, and the deposition fractions of 222 Rn progeny in the N and TB regions have been measured under certain conditions [11–16]. However, the inadequacy of the grab sampling method used in the measuring systems may cause large measurement uncertainties [12]. Moreover, the whole measuring systems were generally complicated, largesized and expensive, and so they are unpractical for field surveys. In this paper, a new type of passive 222 Rn and 220 Rn monitor and a portable device for measuring the deposition fractions of 222 Rn and 220 Rn progeny in the respiratory tract are described. Because of their simple construction and low cost, they are considered to be suitable for large-scale and long-term field surveys.
2. Materials and methods 2.1. Passive 222 Rn and 220 Rn monitor Figure 1 shows the construction of the passive 222 Rn and 220 Rn monitor. The 222 Rn monitor made in Hungary is commercially available. The diffusion chamber is a small cylindrical pot (φ = 35 mm, H = 55 mm) made of anti-statically treated plastic. The allyl diglycol carbonate (CR-39) detector is fixed on the middle of the pot cover with its sensitive side towards the pot interior. The cover is screwed to match the pot bottom firmly during measurements. 222 Rn in air can penetrate into the pot through the non-visible air gaps between its cover and bottom through diffusion. The mean 222 Rn concentration during the exposure period can be derived from the track density on the CR-39, the 222 Rn calibration factor and the exposure time. Because of its simple construction and low cost, the 222 Rn monitor has been widely used for 222 Rn surveys worldwide. For this study, in order to measure 220 Rn concentrations, the air exchange rate in the original pot needed to be enhanced. Therefore, four holes (φ = 12 mm) were evenly made on the wall of the pot bottom and covered with the cellulose fiber filter (Whatman® No. 41) to form a 220 Rn monitor for this study. The filter paper used here was chosen for its low cost, high mechanical strength and high permeability. Filtration prevents particulate 222 Rn/220 Rn progeny from entering the pot, thus ensuring the establishment of radioactive equilibrium between 222 Rn/220 Rn and their short-lived progeny. In order to discriminate 222 Rn from the 220 Rn monitor, the 222 Rn monitor is attached to the 220 Rn monitor
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Fig. 1. Construction diagram of the remodelled passive 222 Rn and 220 Rn monitors.
using a piece of double-sided tape. The rebuilt 220 Rn and 222 Rn monitor has a total volume of only 110 cm3 and a weight of as low as 20 g. The etched-track densities on the CR-39 detectors set in the 222 Rn and 220 Rn monitors (NRn and NTn ) can be expressed by the following equations: NRn = QRn CF Rn1 T + QTn CF Tn1 T + B,
(1)
NTn = QRn CF Rn2 T + QTn CF Tn2 T + B,
(2)
where QRn and QTn are the mean concentrations of 222 Rn and 220 Rn in the exposed period in Bq m−3 , CF Rn1 and CF Tn1 are the 222 Rn and 220 Rn calibration factors for the 222 Rn monitor in tracks cm−2 (Bq m−3 h−1 )−1 , CF Rn2 and CF Tn2 are the 222 Rn and 220 Rn calibration factors for the 220 Rn monitor in tracks cm−2 (Bq m−3 h−1 )−1 , T is the exposure time in h, and B is the background track density of the CR-39 detector in cm−2 . Therefore, provided the etched-track densities, calibration factors and the exposure time were known, 222 Rn and 220 Rn concentrations could be derived from the simultaneous equations. 2.2. Bronchial dosimeter A conceptional diagram of the measuring system is shown in Fig. 2. The whole system consists of three sets of progeny integrating sampling units (PISUs) and flow meters as well as a mini air pump. The single mini pump with combined flows was used for air sampling. The sampling flow rate is adjusted to be 13.6 cm3 s−1 for each PSIU by the flow meters, resulting in a face velocity of 12.0 cm s−1 on the wire screens. PISU-A collects the total airborne activity (Aa ) on an AA type MF membrane filter with a pore size of 0.8 µm. PISU-B containing
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
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Fig. 2. Schematic diagram for simultaneous measurements of the concentrations of 222 Rn and 220 Rn progeny and their deposition in the respiratory tract.
a single 100-mesh wire screen sampling head simulates the collection characteristics of the human nose. Its back-up filter collects the activity penetrating the nasal region (Ab ). The percentage of deposition fractions in the N region is then given as 100(Aa − Ab )/Aa . PISU-C contains one 100-mesh and four 400-mesh wire screens in the sampling head, which mimics the combined collection characteristics of the N and T-B regions. Its back-up filter collects the activity penetrating the N and T-B regions (Ac ). Then, the percentage of deposition fraction in the T-B region can be expressed as 100(Ab − Ac )/Aa . The three sets of PISUs are identical cylinders made of stainless steel. The detailed construction of the PSIU has been reported by the author [17]. Four chips of CR-39 detectors are set beneath the absorbers at the sites demonstrated as four dotted circles in each PISU. Among the four CR-39 detectors, two are set beneath a thicker Al Mylar absorber (4.8 mg cm−2 ), and it was confirmed that only alpha particles (8.78 MeV) emitting from 212 Po of 220 Rn progeny collected on the filter can be detected. While the other two detectors set beneath another thinner absorber (0.29 mg cm−2 ) can detect all of the alpha particles emitting from 222 Rn/220 Rn progeny collected on the filter. After a predetermined period of sampling, the sampling heads were sealed, and the PISUs are left for 3 days to allow all 212 Bi atoms collected on the filters to decay. From the densities of etched tracks and the integrating volume of sampling flow, the PAEC of 222 Rn and 220 Rn expressed by the equilibrium-equivalent concentrations (EECRn
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and EECTn , in Bq m−3 ) were derived as follows [17]: EECRn = 4.33 × 10−2 DRn · A · V −1 ,
(3)
EECTn = 7.30 × 10−3 DTn · A · V −1 ,
(4)
where DRn and DTn are the net etched-tracks in cm−2 produced by 222 Rn and 220 Rn progeny, respectively; A is the counting area of the CR-39 in cm2 ; and V is the integrated air volume passing through the filter of each PISU in m3 .
3. Results and discussion 3.1.
222 Rn
and 220 Rn calibration factors for the new passive monitors
The calibration experiments were performed in an airtight stainless chamber with a volume of 150 L. For five 222 Rn exposure periods, the integrated exposure ranged within 367 ∼ 508 kBq h m−3 . The averaged 222 Rn calibration factors for 12 sets of 222 Rn and 220 Rn monitors were estimated to be 2.62 ± 0.20 and 2.64 ± 0.16 tracks cm−2 (kBq m−3 h−1 )−1 for the 222 Rn and 220 Rn monitors, respectively. In the 220 Rn calibration experiments, 220 Rn with minimal 222 Rn gas was continuously supplied through an airtight circulation system [18]. An interior electric fan was switched on during experiments in order to make the 220 Rn uniformly distributed in the chamber. The concentrations of 220 Rn and 222 Rn during the exposure periods were measured with scintillation cells by the grab sampling method [19]. Averaged concentrations of 220 Rn and 222 Rn were measured to be about 3200 Bq m−3 and 220 Bq m−3 during the calibration experiments which usually lasted for a week. Subtracting the track densities due to 222 Rn exposure, the derived 220 Rn calibration factors for 12 sets of 222 Rn and 220 Rn monitors are shown in Fig. 3. The mean 220 Rn calibration factors are 1.32 ± 0.14
Fig. 3. Experimental results of 220 Rn calibration factors for the 222 Rn and 220 Rn monitors. The error bars represent ±1 standard deviation from 5 time exposures.
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tracks cm−2 (kBq m−3 h−1 )−1 and 0.10 ± 0.01 tracks cm−2 (kBq m−3 h−1 )−1 for the 220 Rn and 222 Rn monitors, respectively. The 220 Rn calibration factor of the 220 Rn monitor is more than 10 times that of the 222 Rn monitor. Therefore, it is expected that 220 Rn can be evaluated from the different etched-tracks on the detectors in the 222 Rn and 220 Rn monitors, provided measurements were simultaneously performed at the same site. The 220 Rn calibration factor being less than 4% of the 222 Rn calibration factor in the 222 Rn monitor, the influence of 220 Rn on the 222 Rn measurements may be neglected in most environments. However, in some environments, such as in some Japanese wooden houses where 220 Rn concentrations are much higher than 222 Rn concentrations, the influence of 220 Rn on the 222 Rn measurements might be significant if only the 222 Rn monitor was used for measurements. In this case, the influence of 220 Rn on the 222 Rn measurements could be quantified if the rebuilt monitor was used. Therefore, the rebuilt monitor is not only useful for 220 Rn measurements, but also helpful for more precise measurements of 222 Rn. 3.2. Main characteristics of some passive 222 Rn and 220 Rn monitors The main characteristics of the four types of passive 222 Rn and 220 Rn monitors are summarized in Table 1. Our present monitor has the highest calibration factor and lowest detection limits for 220 Rn measurements. It indicates that 220 Rn concentrations can be more precisely and easily measured with our present monitors. Because the variance of background etch-pits on the CR-39 detectors used in this work is relatively high, the lower detection limit of 222 Rn is slightly higher than for other monitors. However, it is still considered sensitive enough for indoor measurements. Besides its simple construction, the present monitor is small and light, and it is considered as the most practical for large-scale surveys among these monitors. 3.3. PAEC measurements In order to test the PAEC measurements with the PISU, measurements were performed along with a working level monitor (WLx, Pylon Electronic, Canada). Except in a radon chamber, Table 1 Main characteristics of the 4 types of passive 222 Rn and 220 Rn monitors Researcher
Doi et al. [6]
Guo et al. [7]
Iida et al. [8]
This study
Reported year Detector Polycarbonate 222 Rn calibration factora 220 Rn calibration factora LDL of 222 Rn (Bq m−3 )b LDL of 220 Rn (Bq m−3 )c Volume (cm3 ) Weight (g)
1994 CN 1.65 0.53 1.3 > 5.3d 560 200
1995 CR-39 2.15 0.51 6.4 31 260 200
1996 CR-39 4.17 0.62 2.3 35.6 260 200
2.63 1.32 3.5 11 110 20
a Unit in tracks cm−2 (kBq m−3 h−1 )−1 . b Lower detection limits estimated for an exposure of 90 d. c Estimated for an exposure of 90 d in an environment of 10 Bq m−3 of 222 Rn. d The uncertainty of 222 Rn measurements was not considered.
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Table 2 222 Rn and 220 Rn progeny concentrations measured with the WLx and PISU-A in 4 rooms Room
EECRn (Bq m−3 )
Air conditions
WLx Storage Office
Living Rn Chamber
Low air exchange Air conditioner ON Air conditioner ON/OFF Air conditioner OFF Air conditioner ON/OFF Air cleaner OFF Air cleaner OFF Air cleaner ON
47.9 ± 2.74 5.58 ± 2.86 7.72 ± 1.78 9.21 ± 2.88 4.25 ± 1.93 3610 ± 430 5540 ± 659 262 ± 31.6
EECTn (Bq m−3 ) PISU-A
43.2 ± 3.05 5.94 ± 0.45 6.89 ± 0.43 8.76 ± 0.53 4.56 ± 0.39 3360 ± 265 5280 ± 337 403 ± 32.5
WLx
PISU-A
NDa
ND 0.21 ± 0.02 0.37 ± 0.02 0.39 ± 0.02 1.21 ± 0.08 ND ND ND
0.18 ± 0.07 0.38 ± 0.08 0.34 ± 0.07 1.07 ± 0.12 ND ND ND
a Lower than the detection limits.
measurements ran continuously for about 2 days in order to obtain a large number of etched tracks on the CR-39 detectors. The results of the 222 Rn and 220 Rn progeny concentrations measured in different rooms and air conditions are summarized in Table 2. Both EECRn and EECTn measured with the two devices in the same room and air conditions agreed well with each other in most cases. In the condition that the air cleaner and the dehumidifier were switched on, the unattached fraction of PAEC was about 80% in the radon chamber, and the EECRn measured with the PISU-A was much higher than that measured by the WLx. It is considered that a large amount of sampling loss might occur in the sampling system of the WLx, because the gap between the filter and detector is very narrow in its filter holder assembly. To examine this possibility, samples were also taken with open-faced filters at 5 min intervals in the same period. The individual concentrations of 222 Rn progeny were calculated using the Raabe–Wrenn method [20], and the average EECRn was 434 Bq m−3 , similar to the results measured with PISU-A. From the measurements, it was also observed that the concentrations of both 222 Rn and 220 Rn progeny changed with the operating status of the air conditioning. The concentrations were generally higher when the air conditioning was off, and lower when it was on. 3.4. Deposition fraction measurements Measurements of deposition fractions of 222 Rn and 220 Rn progeny in the N and T-B regions were performed in the same 4 rooms as the previous measurements. The results including total particle concentrations measured by PORTACOUNT Plus (Model 8020, TSI, USA) are summarized in Table 3. In ordinary room air conditions, the deposition fractions of 222 Rn and 220 Rn progeny in the N region averaged 5.3 and 3.9%, respectively. The fractions of 222 Rn and 220 Rn progeny in the T-B region were 4.5 and 4.0%, respectively. For the deposition fractions of 222 Rn progeny, our results are similar to the values reported by other researchers [12,16]. For these measurements, the deposition fractions of 222 Rn progeny in the N and T-B regions generally decrease with increasing aerosol concentration. However, variations of the 220 Rn progeny deposition are not clear. The fp of 222 Rn progeny is generally known to be inversely proportional to the aerosol concentration indoors, and most of the unattached progeny are
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
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Table 3 Deposition of 222 Rn and 202 Rn progeny in the nasal and tracheobronchial regions in 4 roomsa Room
Aerosol concentration (×103 cm−3 ) 5.8 ± 2.0 12.0 ± 4.6 21.3 ± 6.1 32.5 ± 8.9 12.2 ± 5.8 2.4 ± 0.7 3.2 ± 0.6 0.4 ± 0.2
Storage Office
Living Rn chamber
222 Rn progeny deposition
220 Rn progeny deposition
N (%)
N (%)
T-B (%)
7.3 5.8 4.3 3.1 5.9 7.9 8.4 65.3
T-B (%)
–b 4.4 2.8 3.9 4.3 – – –
6.0 4.6 3.6 3.1 5.2 6.5 7.0 14.8
– 4.5 3.5 3.3 4.6 – – –
a All data are calculated based on the PAEC. b No data.
Table 4 PAEC weighted size distribution of 222 Rn progeny in a radon chamber Run
1 2 3
Unattached progeny
Attached progeny
%
GMD (nm)
GSD
%
GMD (nm)
GSD
77.3 84.7 79.1
1.05 1.00 0.97
1.51 1.49 1.50
22.7 15.3 20.9
172.14 147.56 123.43
2.80 3.23 3.65
likely to deposit in the N and T-B regions according to the diffusion mechanism. For 220 Rn progeny, its first decay product 216 Po has an extremely short half-life (0.15 s), but the half-life of the second decay product 212 Pb is relatively long (10.6 h). It is expected that the unattached fractions of 220 Rn progeny are negligible in the general living environment. Therefore, the variations of the 220 Rn progeny deposited in the N and T-B regions may not be significant for different aerosol concentrations. Also shown in Table 3, the deposition fractions of 220 Rn progeny in both N and T-B regions are somewhat different from those of 222 Rn progeny. This may be due to the different fp and size distributions of 222 Rn and 220 Rn progeny [21]. The results above suggest that, for precise dose assessment, not only the concentrations but also the deposition fractions in the T-B region should be measured for both 222 Rn and 220 Rn progeny. In order to compare the measured results with theoretical estimations, the PAEC-weighted particle size distributions of 222 Rn progeny were also measured in the radon chamber when the air cleaner and the dehumidifier were on. The graded screen array (GSA) including five graded screens (30, 145, 200, 400 and 635 mesh) and a back-up glass fiber filter (GF/F) was used in the measurements. The results calculated with the code developed by the Environmental Measurements Laboratory [22] are shown in Table 4. Based on the equation derived by Ramamurthi and Hopke [23], the deposition fractions in the N and T-B regions were calculated
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to be about 63 and 16%, respectively. The theoretical values are in good agreement with our observed values of 65.3 and 14.8%, respectively.
4. Conclusion A commercially available passive 222 Rn monitor was redesigned and rebuilt for both 222 Rn and 220 Rn measurements. The new monitor is not only useful for 220 Rn measurements, but also useful for more precise measurements of 222 Rn. Using the conceptual designs of multiple wire screen samplers proposed by previous researchers, a portable integrating bronchial dosimeter for both 222 Rn and 220 Rn progeny was developed. Its measuring results are in general agreement with other reported values in ordinary room conditions. The main merits of the new bronchial dosimeter and the associated passive integrating monitors are their compactness, portability and low-cost. These devices are expected to be useful for large-scale dose surveys.
References [1] NCRP, Evaluation of occupational and environmental exposures to radon and radon daughters in the United States, NCRP report No. 78, National Council on Radiation Protection and Measurements, Bethesda, MD, 1984. [2] W.W. Nazaroff, A.V. Nero (Eds.), Radon and Its Decay Products in Indoor Air, Wiley, New York, 1988. [3] F. Steinhäusler, Environ. Int. 22 (1996) S1111. [4] W. Zhuo, T. Iida, X. Yang, Radiat. Prot. Dosim. 87 (2000) 137. [5] Q. Guo, J. Sun, W. Zhuo, J. Nucl. Sci. Technol. 37 (2000) 716. [6] M. Doi, S. Kobayashi, Hoken Butsuri 29 (1994) 155. [7] Q. Guo, T. Iida, K. Okamoto, T. Yamasaki, J. Nucl. Sci. Technol. 32 (1995) 794. [8] T. Iida, R. Nurishi, K. Okamoto, Environ. Int. 22 (1996) S641. [9] G. Batterweck, G. Vezzù, Ch. Schuler, et al., Radiat. Prot. Dosim. 94 (2001) 247. [10] K.W. Tu, E.O. Knutson, A.C. George, Aerosol Sci. Technol. 15 (1991) 170. [11] N. Jonassen, B. Jensen, Radiat. Prot. Dosim. 45 (1992) 669. [12] A.C. George, E.O. Knutson, Radiat. Prot. Dosim. 45 (1992) 689. [13] S. Oberstedt, H. Vanmarcke, Radiat. Prot. Dosim. 59 (1995) 285. [14] S.B. Solomon, Radiat. Prot. Dosim. 72 (1997) 31. [15] K.N. Yu, Z.J. Guan, Health Phys. 75 (1998) 147. [16] K.N. Yu, Z.J. Guan, E.C.M. Young, M.J. Stokes, Health Phys. 75 (1998) 153. [17] W. Zhuo, T. Iida, Health Phys. 77 (1999) 584. [18] S. Tokonami, M. Yang, T. Sanada, Health Phys. 80 (2001) 612. [19] S. Tokonami, M. Yang, H. Yonehara, Y. Yamada, Rev. Sci. Instrum. 73 (2001) 69. [20] O.G. Raabe, M.E. Wrenn, Health Phys. 17 (1969) 593. [21] K.W. Tu, E.O. Knutson, A.C. George, Aerosol Sci. Technol. 20 (1994) 266. [22] E.O. Knutson, EML Report 517, USDOE, New York, 1989. [23] M. Ramamurthi, P.K. Hopke, Health Phys. 56 (1989) 189.
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract W. Zhuo, S. Tokonami, H. Yonehara, Y. Yamada Radon Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan
For simultaneous measurements of indoor 222 Rn and 220 Rn and estimation of their deposition fractions in the respiratory tract, a new type of passive integrating 222 Rn and 220 Rn monitor and a portable bronchial dosimeter were developed. The passive 222 Rn and 220 Rn monitor was rebuilt from a commercially available passive 222 Rn monitor. Besides its simple construction, the volume and weight of the new monitor are only 110 cm3 and 20 g. Calibration factors of 222 Rn and 220 Rn for the new monitor were systematically studied through calibration experiments. The results indicated that indoor 220 Rn could be discriminated from 222 Rn by using the new passive monitor. The bronchial dosimeter consists of three sets of progeny integrating sampling units (PISUs) with different configurations of sampling heads. Multiple metal screens are used to mimic the penetrating and deposition behaviour of 222 Rn/220 Rn progeny in the nasal and tracheo-bronchial (T-B) regions of the human respiratory tract. The potential alpha energy concentrations (PAEC) of 222 Rn/220 Rn progeny are directly measured with the allyl diglycol carbonate (CR-39) detectors inside the PISUs. The deposition fractions of 222 Rn and 220 Rn progeny in the T-B region were measured, with averages of 4.5 and 4.0% for ordinary room conditions, in general agreement with other reported values. Both the new monitor and device are simple and compact as well as of low cost, and they are considered to be practical for large-scale and long-term surveys of indoor 222 Rn and 220 Rn.
1. Introduction Public exposure to 222 Rn and its progeny has been of worldwide concern in the past decades. It is well known that the health detriment associated with radon (222 Rn) mainly arises from the inhalation of its progeny and the resultant alpha particle exposure to the epithelium of the respiratory tract [1]. Previous studies have shown that the tracheo-bronchial (T-B) region is the most radiosensitive in the respiratory tract, and the radiation dose in the region due to 222 Rn progeny constitutes a dominant fraction of the total lung dose [2]. Recent surveys revealed that the exposure to thoron (220 Rn) and its progeny could equal or even exceed that RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07040-8
© 2005 Elsevier Ltd. All rights reserved.
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
353
of 222 Rn and its progeny in some areas of the world [3–5]. Therefore, measurements of 220 Rn are indispensable for more precise assessment of public exposure to 222 Rn and 220 Rn. For simultaneous measurements of indoor 222 Rn and 220 Rn, several types of 222 Rn and 220 Rn discriminative monitors have been developed. Doi et al. [6] developed a unified 222 Rn and 220 Rn discriminative monitor with polycarbonate film as detectors. Besides its large volume (about 560 cm3 ), the construction of monitor and the etching processes of detectors were complicated. The 222 Rn and 220 Rn monitors developed by Guo et al. [7] and Iida et al. [8] were somewhat inconvenient for field applications because they were two separated hemispherical cups. Moreover, it was also complicated and time-consuming to exchange the detectors. The traditional methods for measuring the deposition of 222 Rn progeny in the T-B region are tedious and difficult, and generally require complex equipment [9,10]. For simulating and measuring the deposition of 222 Rn progeny in the respiratory tract, several type of sampling systems consisting of multiple layers of wire screens with appropriate wire factors and air flow velocity have been developed, and the deposition fractions of 222 Rn progeny in the N and TB regions have been measured under certain conditions [11–16]. However, the inadequacy of the grab sampling method used in the measuring systems may cause large measurement uncertainties [12]. Moreover, the whole measuring systems were generally complicated, largesized and expensive, and so they are unpractical for field surveys. In this paper, a new type of passive 222 Rn and 220 Rn monitor and a portable device for measuring the deposition fractions of 222 Rn and 220 Rn progeny in the respiratory tract are described. Because of their simple construction and low cost, they are considered to be suitable for large-scale and long-term field surveys.
2. Materials and methods 2.1. Passive 222 Rn and 220 Rn monitor Figure 1 shows the construction of the passive 222 Rn and 220 Rn monitor. The 222 Rn monitor made in Hungary is commercially available. The diffusion chamber is a small cylindrical pot (φ = 35 mm, H = 55 mm) made of anti-statically treated plastic. The allyl diglycol carbonate (CR-39) detector is fixed on the middle of the pot cover with its sensitive side towards the pot interior. The cover is screwed to match the pot bottom firmly during measurements. 222 Rn in air can penetrate into the pot through the non-visible air gaps between its cover and bottom through diffusion. The mean 222 Rn concentration during the exposure period can be derived from the track density on the CR-39, the 222 Rn calibration factor and the exposure time. Because of its simple construction and low cost, the 222 Rn monitor has been widely used for 222 Rn surveys worldwide. For this study, in order to measure 220 Rn concentrations, the air exchange rate in the original pot needed to be enhanced. Therefore, four holes (φ = 12 mm) were evenly made on the wall of the pot bottom and covered with the cellulose fiber filter (Whatman® No. 41) to form a 220 Rn monitor for this study. The filter paper used here was chosen for its low cost, high mechanical strength and high permeability. Filtration prevents particulate 222 Rn/220 Rn progeny from entering the pot, thus ensuring the establishment of radioactive equilibrium between 222 Rn/220 Rn and their short-lived progeny. In order to discriminate 222 Rn from the 220 Rn monitor, the 222 Rn monitor is attached to the 220 Rn monitor
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Fig. 1. Construction diagram of the remodelled passive 222 Rn and 220 Rn monitors.
using a piece of double-sided tape. The rebuilt 220 Rn and 222 Rn monitor has a total volume of only 110 cm3 and a weight of as low as 20 g. The etched-track densities on the CR-39 detectors set in the 222 Rn and 220 Rn monitors (NRn and NTn ) can be expressed by the following equations: NRn = QRn CF Rn1 T + QTn CF Tn1 T + B,
(1)
NTn = QRn CF Rn2 T + QTn CF Tn2 T + B,
(2)
where QRn and QTn are the mean concentrations of 222 Rn and 220 Rn in the exposed period in Bq m−3 , CF Rn1 and CF Tn1 are the 222 Rn and 220 Rn calibration factors for the 222 Rn monitor in tracks cm−2 (Bq m−3 h−1 )−1 , CF Rn2 and CF Tn2 are the 222 Rn and 220 Rn calibration factors for the 220 Rn monitor in tracks cm−2 (Bq m−3 h−1 )−1 , T is the exposure time in h, and B is the background track density of the CR-39 detector in cm−2 . Therefore, provided the etched-track densities, calibration factors and the exposure time were known, 222 Rn and 220 Rn concentrations could be derived from the simultaneous equations. 2.2. Bronchial dosimeter A conceptional diagram of the measuring system is shown in Fig. 2. The whole system consists of three sets of progeny integrating sampling units (PISUs) and flow meters as well as a mini air pump. The single mini pump with combined flows was used for air sampling. The sampling flow rate is adjusted to be 13.6 cm3 s−1 for each PSIU by the flow meters, resulting in a face velocity of 12.0 cm s−1 on the wire screens. PISU-A collects the total airborne activity (Aa ) on an AA type MF membrane filter with a pore size of 0.8 µm. PISU-B containing
Integrating measurements of radon and thoron and their deposition fractions in the respiratory tract
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Fig. 2. Schematic diagram for simultaneous measurements of the concentrations of 222 Rn and 220 Rn progeny and their deposition in the respiratory tract.
a single 100-mesh wire screen sampling head simulates the collection characteristics of the human nose. Its back-up filter collects the activity penetrating the nasal region (Ab ). The percentage of deposition fractions in the N region is then given as 100(Aa − Ab )/Aa . PISU-C contains one 100-mesh and four 400-mesh wire screens in the sampling head, which mimics the combined collection characteristics of the N and T-B regions. Its back-up filter collects the activity penetrating the N and T-B regions (Ac ). Then, the percentage of deposition fraction in the T-B region can be expressed as 100(Ab − Ac )/Aa . The three sets of PISUs are identical cylinders made of stainless steel. The detailed construction of the PSIU has been reported by the author [17]. Four chips of CR-39 detectors are set beneath the absorbers at the sites demonstrated as four dotted circles in each PISU. Among the four CR-39 detectors, two are set beneath a thicker Al Mylar absorber (4.8 mg cm−2 ), and it was confirmed that only alpha particles (8.78 MeV) emitting from 212 Po of 220 Rn progeny collected on the filter can be detected. While the other two detectors set beneath another thinner absorber (0.29 mg cm−2 ) can detect all of the alpha particles emitting from 222 Rn/220 Rn progeny collected on the filter. After a predetermined period of sampling, the sampling heads were sealed, and the PISUs are left for 3 days to allow all 212 Bi atoms collected on the filters to decay. From the densities of etched tracks and the integrating volume of sampling flow, the PAEC of 222 Rn and 220 Rn expressed by the equilibrium-equivalent concentrations (EECRn
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and EECTn , in Bq m−3 ) were derived as follows [17]: EECRn = 4.33 × 10−2 DRn · A · V −1 ,
(3)
EECTn = 7.30 × 10−3 DTn · A · V −1 ,
(4)
where DRn and DTn are the net etched-tracks in cm−2 produced by 222 Rn and 220 Rn progeny, respectively; A is the counting area of the CR-39 in cm2 ; and V is the integrated air volume passing through the filter of each PISU in m3 .
3. Results and discussion 3.1.
222 Rn
and 220 Rn calibration factors for the new passive monitors
The calibration experiments were performed in an airtight stainless chamber with a volume of 150 L. For five 222 Rn exposure periods, the integrated exposure ranged within 367 ∼ 508 kBq h m−3 . The averaged 222 Rn calibration factors for 12 sets of 222 Rn and 220 Rn monitors were estimated to be 2.62 ± 0.20 and 2.64 ± 0.16 tracks cm−2 (kBq m−3 h−1 )−1 for the 222 Rn and 220 Rn monitors, respectively. In the 220 Rn calibration experiments, 220 Rn with minimal 222 Rn gas was continuously supplied through an airtight circulation system [18]. An interior electric fan was switched on during experiments in order to make the 220 Rn uniformly distributed in the chamber. The concentrations of 220 Rn and 222 Rn during the exposure periods were measured with scintillation cells by the grab sampling method [19]. Averaged concentrations of 220 Rn and 222 Rn were measured to be about 3200 Bq m−3 and 220 Bq m−3 during the calibration experiments which usually lasted for a week. Subtracting the track densities due to 222 Rn exposure, the derived 220 Rn calibration factors for 12 sets of 222 Rn and 220 Rn monitors are shown in Fig. 3. The mean 220 Rn calibration factors are 1.32 ± 0.14
Fig. 3. Experimental results of 220 Rn calibration factors for the 222 Rn and 220 Rn monitors. The error bars represent ±1 standard deviation from 5 time exposures.
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tracks cm−2 (kBq m−3 h−1 )−1 and 0.10 ± 0.01 tracks cm−2 (kBq m−3 h−1 )−1 for the 220 Rn and 222 Rn monitors, respectively. The 220 Rn calibration factor of the 220 Rn monitor is more than 10 times that of the 222 Rn monitor. Therefore, it is expected that 220 Rn can be evaluated from the different etched-tracks on the detectors in the 222 Rn and 220 Rn monitors, provided measurements were simultaneously performed at the same site. The 220 Rn calibration factor being less than 4% of the 222 Rn calibration factor in the 222 Rn monitor, the influence of 220 Rn on the 222 Rn measurements may be neglected in most environments. However, in some environments, such as in some Japanese wooden houses where 220 Rn concentrations are much higher than 222 Rn concentrations, the influence of 220 Rn on the 222 Rn measurements might be significant if only the 222 Rn monitor was used for measurements. In this case, the influence of 220 Rn on the 222 Rn measurements could be quantified if the rebuilt monitor was used. Therefore, the rebuilt monitor is not only useful for 220 Rn measurements, but also helpful for more precise measurements of 222 Rn. 3.2. Main characteristics of some passive 222 Rn and 220 Rn monitors The main characteristics of the four types of passive 222 Rn and 220 Rn monitors are summarized in Table 1. Our present monitor has the highest calibration factor and lowest detection limits for 220 Rn measurements. It indicates that 220 Rn concentrations can be more precisely and easily measured with our present monitors. Because the variance of background etch-pits on the CR-39 detectors used in this work is relatively high, the lower detection limit of 222 Rn is slightly higher than for other monitors. However, it is still considered sensitive enough for indoor measurements. Besides its simple construction, the present monitor is small and light, and it is considered as the most practical for large-scale surveys among these monitors. 3.3. PAEC measurements In order to test the PAEC measurements with the PISU, measurements were performed along with a working level monitor (WLx, Pylon Electronic, Canada). Except in a radon chamber, Table 1 Main characteristics of the 4 types of passive 222 Rn and 220 Rn monitors Researcher
Doi et al. [6]
Guo et al. [7]
Iida et al. [8]
This study
Reported year Detector Polycarbonate 222 Rn calibration factora 220 Rn calibration factora LDL of 222 Rn (Bq m−3 )b LDL of 220 Rn (Bq m−3 )c Volume (cm3 ) Weight (g)
1994 CN 1.65 0.53 1.3 > 5.3d 560 200
1995 CR-39 2.15 0.51 6.4 31 260 200
1996 CR-39 4.17 0.62 2.3 35.6 260 200
2.63 1.32 3.5 11 110 20
a Unit in tracks cm−2 (kBq m−3 h−1 )−1 . b Lower detection limits estimated for an exposure of 90 d. c Estimated for an exposure of 90 d in an environment of 10 Bq m−3 of 222 Rn. d The uncertainty of 222 Rn measurements was not considered.
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Table 2 222 Rn and 220 Rn progeny concentrations measured with the WLx and PISU-A in 4 rooms Room
EECRn (Bq m−3 )
Air conditions
WLx Storage Office
Living Rn Chamber
Low air exchange Air conditioner ON Air conditioner ON/OFF Air conditioner OFF Air conditioner ON/OFF Air cleaner OFF Air cleaner OFF Air cleaner ON
47.9 ± 2.74 5.58 ± 2.86 7.72 ± 1.78 9.21 ± 2.88 4.25 ± 1.93 3610 ± 430 5540 ± 659 262 ± 31.6
EECTn (Bq m−3 ) PISU-A
43.2 ± 3.05 5.94 ± 0.45 6.89 ± 0.43 8.76 ± 0.53 4.56 ± 0.39 3360 ± 265 5280 ± 337 403 ± 32.5
WLx
PISU-A
NDa
ND 0.21 ± 0.02 0.37 ± 0.02 0.39 ± 0.02 1.21 ± 0.08 ND ND ND
0.18 ± 0.07 0.38 ± 0.08 0.34 ± 0.07 1.07 ± 0.12 ND ND ND
a Lower than the detection limits.
measurements ran continuously for about 2 days in order to obtain a large number of etched tracks on the CR-39 detectors. The results of the 222 Rn and 220 Rn progeny concentrations measured in different rooms and air conditions are summarized in Table 2. Both EECRn and EECTn measured with the two devices in the same room and air conditions agreed well with each other in most cases. In the condition that the air cleaner and the dehumidifier were switched on, the unattached fraction of PAEC was about 80% in the radon chamber, and the EECRn measured with the PISU-A was much higher than that measured by the WLx. It is considered that a large amount of sampling loss might occur in the sampling system of the WLx, because the gap between the filter and detector is very narrow in its filter holder assembly. To examine this possibility, samples were also taken with open-faced filters at 5 min intervals in the same period. The individual concentrations of 222 Rn progeny were calculated using the Raabe–Wrenn method [20], and the average EECRn was 434 Bq m−3 , similar to the results measured with PISU-A. From the measurements, it was also observed that the concentrations of both 222 Rn and 220 Rn progeny changed with the operating status of the air conditioning. The concentrations were generally higher when the air conditioning was off, and lower when it was on. 3.4. Deposition fraction measurements Measurements of deposition fractions of 222 Rn and 220 Rn progeny in the N and T-B regions were performed in the same 4 rooms as the previous measurements. The results including total particle concentrations measured by PORTACOUNT Plus (Model 8020, TSI, USA) are summarized in Table 3. In ordinary room air conditions, the deposition fractions of 222 Rn and 220 Rn progeny in the N region averaged 5.3 and 3.9%, respectively. The fractions of 222 Rn and 220 Rn progeny in the T-B region were 4.5 and 4.0%, respectively. For the deposition fractions of 222 Rn progeny, our results are similar to the values reported by other researchers [12,16]. For these measurements, the deposition fractions of 222 Rn progeny in the N and T-B regions generally decrease with increasing aerosol concentration. However, variations of the 220 Rn progeny deposition are not clear. The fp of 222 Rn progeny is generally known to be inversely proportional to the aerosol concentration indoors, and most of the unattached progeny are
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Table 3 Deposition of 222 Rn and 202 Rn progeny in the nasal and tracheobronchial regions in 4 roomsa Room
Aerosol concentration (×103 cm−3 ) 5.8 ± 2.0 12.0 ± 4.6 21.3 ± 6.1 32.5 ± 8.9 12.2 ± 5.8 2.4 ± 0.7 3.2 ± 0.6 0.4 ± 0.2
Storage Office
Living Rn chamber
222 Rn progeny deposition
220 Rn progeny deposition
N (%)
N (%)
T-B (%)
7.3 5.8 4.3 3.1 5.9 7.9 8.4 65.3
T-B (%)
–b 4.4 2.8 3.9 4.3 – – –
6.0 4.6 3.6 3.1 5.2 6.5 7.0 14.8
– 4.5 3.5 3.3 4.6 – – –
a All data are calculated based on the PAEC. b No data.
Table 4 PAEC weighted size distribution of 222 Rn progeny in a radon chamber Run
1 2 3
Unattached progeny
Attached progeny
%
GMD (nm)
GSD
%
GMD (nm)
GSD
77.3 84.7 79.1
1.05 1.00 0.97
1.51 1.49 1.50
22.7 15.3 20.9
172.14 147.56 123.43
2.80 3.23 3.65
likely to deposit in the N and T-B regions according to the diffusion mechanism. For 220 Rn progeny, its first decay product 216 Po has an extremely short half-life (0.15 s), but the half-life of the second decay product 212 Pb is relatively long (10.6 h). It is expected that the unattached fractions of 220 Rn progeny are negligible in the general living environment. Therefore, the variations of the 220 Rn progeny deposited in the N and T-B regions may not be significant for different aerosol concentrations. Also shown in Table 3, the deposition fractions of 220 Rn progeny in both N and T-B regions are somewhat different from those of 222 Rn progeny. This may be due to the different fp and size distributions of 222 Rn and 220 Rn progeny [21]. The results above suggest that, for precise dose assessment, not only the concentrations but also the deposition fractions in the T-B region should be measured for both 222 Rn and 220 Rn progeny. In order to compare the measured results with theoretical estimations, the PAEC-weighted particle size distributions of 222 Rn progeny were also measured in the radon chamber when the air cleaner and the dehumidifier were on. The graded screen array (GSA) including five graded screens (30, 145, 200, 400 and 635 mesh) and a back-up glass fiber filter (GF/F) was used in the measurements. The results calculated with the code developed by the Environmental Measurements Laboratory [22] are shown in Table 4. Based on the equation derived by Ramamurthi and Hopke [23], the deposition fractions in the N and T-B regions were calculated
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to be about 63 and 16%, respectively. The theoretical values are in good agreement with our observed values of 65.3 and 14.8%, respectively.
4. Conclusion A commercially available passive 222 Rn monitor was redesigned and rebuilt for both 222 Rn and 220 Rn measurements. The new monitor is not only useful for 220 Rn measurements, but also useful for more precise measurements of 222 Rn. Using the conceptual designs of multiple wire screen samplers proposed by previous researchers, a portable integrating bronchial dosimeter for both 222 Rn and 220 Rn progeny was developed. Its measuring results are in general agreement with other reported values in ordinary room conditions. The main merits of the new bronchial dosimeter and the associated passive integrating monitors are their compactness, portability and low-cost. These devices are expected to be useful for large-scale dose surveys.
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