RADIATION RESEARCH
162, 390–396 (2004)
0033-7587/04 $15.00 q 2004 by Radiation Research Society. All rights of reproduction in any form reserved.
Radon and Thoron Exposures for Cave Residents in Shanxi and Shaanxi Provinces Shinji Tokonami,a,1 Quanfu Sun,b Suminori Akiba,c Weihai Zhuo,a Masahide Furukawa,a Tetsuo Ishikawa,a Changsong Hou,b Shouzhi Zhang,b Yukinori Narazaki,d Baku Ohji,a Hidenori Yoneharaa and Yuji Yamadaa Radon Research Group, Research Center for Radiation Safety, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-; 8555, Japan; b National Institute for Radiological Protection and Nuclear Safety, 2 Xinkang Street, Deshengmenwai, Xicheng District, Beijing 100088, China; c Faculty of Medicine, Kagoshima University, 8-35-1 Sakuragaoka, Kagoshima 890-8520, Japan; and d Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaifu, Fukuoka 818-0135, Japan a
ground radiation sources (1). In particular, radon has been recognized as a hazard to underground miners for many years. At present, the possibility that radon may be a significant hazard in living environments is a worldwide concern. Since the potential risk from residential exposures to radon is of great interest in many countries, evaluation methods to determine the risk from exposure to residential radon are needed. Epidemiological studies of underground miners provide the basis for estimating risks from residential exposures to radon and its decay products by extrapolating the results of miner studies downward to exposure levels seen in homes (2). The estimation of exposure conditions is often required in the miner studies many years back in time. Since there are few or no actual exposure measurements, however, the results from such studies may be uncertain. Many additional assumptions are needed when risks estimated in miner studies are extrapolated to residential exposure conditions (3). Taking these circumstances into account, there has been great interest in developing direct estimates of residential risk for the general population using case–control studies. The environmental conditions in mines and homes are quite different. For a precise evaluation of radon exposures, the contribution of thoron should be considered if the radon level is low in homes. Its presence often results in misleading estimation of radon concentrations. Before conducting a case–control study on residential radon exposures, it is important to understand those characteristics in the study area. There are many cave dwellings in the Chinese loess plateau in which the radon concentration appears to be high. Since the residential mobility is low, this area is suitable for conducting a case–control study on lung cancer risk and residential radon exposure. Thus radon and its associated radionuclides were measured in two provinces: Shanxi and Shaanxi. The U.S. National Cancer Institute (NCI) conducted a large-scale case–control study in Gansu Province located in the Chinese loess plateau (4, 5). The NCI concluded that the lung cancer risk increased with increasing radon level. However, thoron concentrations were not mea-
Tokonami, S., Sun, Q., Akiba, S., Zhuo, W., Furukawa, M., Ishikawa, T., Hou, C., Zhang, S., Narazaki, Y., Ohji, B., Yonehara, H. and Yamada, Y. Radon and Thoron Exposures for Cave Residents in Shanxi and Shaanxi Provinces. Radiat. Res. 162, 390–396 (2004). Measurements of natural radiation were carried out in cave dwellings distributed in the Chinese loess plateau. Those dwellings are located in Shanxi and Shaanxi provinces. Radon and thoron gas concentrations were measured using a passive integrating radon-thoron discriminative detector. Concentrations of thoron decay products were estimated from measurements of their deposition rates. A detector was placed at the center of each dwelling for 6 months and replaced with a fresh one for another 6 months. Measurements were conducted in 202 dwellings from August 2001 through August 2002. A short-term measurement was conducted during the observation period. In addition, g-ray dose rates were measured both indoors and outdoors with an electronic pocket dosimeter. Radioactivities in soil were determined by g-ray spectrometry with a pure germanium detector. Among 193 dwellings, indoor radon concentrations ranged from 19 to 195 Bq m23 with a geometric mean (GM) of 57 Bq m23, indoor thoron concentrations ranged from 10 to 865 Bq m23 with a GM of 153 Bq m23, and indoor equilibrium equivalent thoron concentrations ranged from 0.3 to 4.9 Bq m23 with a GM of 1.6 Bq m23. Arithmetic means of the g-ray dose rates were estimated to be 140 nGy h21 indoors and 110 nGy h21 outdoors. The present study revealed that the presence of thoron is not negligible for accurate radon measurements and thus that special attention should be paid to thoron and its decay products for dose assessment in such an environment. More systematic studies are necessary for a better understanding of thoron and its decay products. q 2004 by Radiation Research Society
INTRODUCTION
Radon, thoron and their decay products are well known to be significant contributors to the dose from natural back1 Address for correspondence: Radon Research Group, Research Center for Radiation Safety, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan; e-mail:
[email protected].
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FIG. 2. Interior of a typical cave dwelling.
FIG. 1. Study areas in the present survey in the Chinese loess plateau.
sured in the study, though some studies have pointed out the presence of thoron in cave dwellings (6, 7). The present study illustrates the characteristics of radon and thoron exposures for cave residents in the Chinese loess plateau. MATERIALS AND METHODS The present survey was conducted in Luliang Prefecture, Shanxi Province and Yan’an Prefecture, Shaanxi Province. The study area is shown in Fig. 1. It can be seen that the NCI study area is geographically close to ours. The size of the entire Chinese loess plateau is about 700 km north to south and 1000 km west to east. In the present study, 20 villages were first selected from Shanxi and Shaanxi Provinces. After the type of housing structures (cave dwelling, brick flat, stone flat and detached brick house) was taken into account, 10 house samples were selected from each village. A total of 202 houses were investigated for radiation measurements. Figure 2 illustrates the interior of a typical cave dwelling. The cave dwelling consists of one room with a single entrance and two windows at the front side. The length is 8–10 m with a width of 3–3.5 m and a height of 3–3.5 m. The cave dwelling is equipped with a traditional bed formed from a loess cube, which is called Kang in Chinese. To determine the annual concentrations of radon and thoron, a newly designed passive integrating radon-thoron discriminative detector was used in this study (8). A CR-39 detector was used as the detecting material. This detector consists of two sets of diffusion chambers with different air exchange rates. The Radopot,2 which is available commercially and is made in Hungary, was used for radon detection. The performance of this detector was described by Tokonami et al. (9). The other Radopot was modified to detect thoron more effectively. The lowest detection limit was estimated to be 3.5 Bq m23 for the radon concentration and 13 Bq m23 for the thoron concentration for an exposure period of 90 days. The concentrations of thoron decay products were estimated using the deposition rate measurement developed by Zhuo and Iida (10). A CR-39 detector was also used as measuring device. This CR-39 detector was covered with an aluminum-vaporized Mylar film of 7.15 cm air-equivalent thickness so as to selectively detect 8.8 MeV a particles emitted only from 212Po. The lowest detection limit of this device was estimated to be 0.08 Bq m23 for the equilibrium equivalent thoron concentration (EETC)
for an exposure period of 90 days. These devices were generally suspended on the wall or ceiling at the middle of the cave. The distances from the wall and ground were estimated to be 5–30 cm and 150–300 cm, respectively. These passive devices were placed in 202 dwellings (101 dwellings in each province) for 6 months and were replaced with fresh ones for another 6 months. The long-term measurement was made from August 2001 through August 2002. Grab sampling measurements were made during the observation period in some dwellings. For determination of the concentrations of individual radon decay products, the Trembley method was used with a silicon semiconductor detector and its associated equipment as well as a portable multi-channel analyzer (11). The timetable consists of a sampling period of 20 min with a flow rate of 4 liters min21, waiting period of 5 min, and measuring period of 15 min. The geometric detection efficiency of this system was 0.134, and consequently the detection limit of the equilibrium equivalent radon concentration (EERC) was estimated to be 1 Bq m23 after the above conditions were taken into account. The EETC was estimated by grab sampling and a-particle track registration on a CR-39 detector. After an air sample was taken on a filter, the filter was left for several hours so that the radon decay products decayed completely. Subsequently the filter was placed on the CR-39 detector through an energy absorber and was for left over a day for the a-particle track registration. More detailed information can be found in ref. (12). The lowest detection limit of the system was estimated to be 0.02 Bq m23 for the EETC for a sampling period of 8 h with a flow rate of 1.0 liter min21, elapsed time after sampling of 6 h, measuring period of 24 h, and background track density of 0.65 mm22. To characterize the effectiveness of residential radon exposures, an equilibrium factor is also needed. Therefore, continuous and simultaneous measurements of the concentrations of radon and its decay products were made using commercial instruments. The radon concentration was measured using an AlphaGUARD3 with a 60-min interval and diffusion mode. The device was operated with the 3D a-particle spectroscopy and current mode. The sensitivity is 1 cpm at 20 Bq m23. The concentrations of the radon decay products expressed by the EERC were measured continuously for 60 min. The EERC was determined with a Model WLx Working Level Measurement System.4 Continuous sampling and counting were carried out at 60-min intervals using a-particle spectroscopy. Using these two data sets, the equilibrium factor could be obtained. As relevant information on radon and thoron potentials, g-ray dose rates indoors and 3
2
Radosys Co., Ltd., Budapest, Hungary.
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4
Genitron Instruments GmbH, Frankfurt/Main, Germany. Pylon Electronics Inc., Ottawa, Ontario, Canada.
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FIG. 3. Example for time variation of the radon concentration in a cave dwelling.
outdoors were measured using a PDR-1015 electronic pocket dosimeter (fluctuation coefficient: 5.0, measurable energy: 60 keV to 3 MeV, calibration coefficient for standard g radiation for 1 MeV g rays: 0.87 6 0.01 Gy Sv21 in air) in all houses and a 10; 3 20 NaI(Tl) scintillation spectrometer SS-g6 (energy resolution: 11% for 662 keV 137Cs g rays) at several sites. In addition to the above measurements, soil samples were taken to measure their radioactivities. They were determined with g-ray spectrometry using a pure germanium detector. The hyper-pure germanium detector manufactured by ORTEC7 was used. The relative efficiency and FWHM resolution were 0.36 and 1.76 keV, respectively, for the 1.33 MeV g rays of 60Co. The measuring period for each sample was generally 8 3 104 s or more.
RESULTS AND DISCUSSION
Data on the concentrations of individual radon decay products were obtained from 21 houses with the EERC; they ranged from 1 to 94 Bq m23 with an arithmetic mean of 18 Bq m23. In this measurement, concentrations of unattached radon decay products were also measured with a single wire screen technique in 10 caves; data were available for only three. The unattached fraction was estimated to be 0.023 6 0.01, 0.033 6 0.008 and 0.035 6 0.01 and was lower than that in the general indoor environment. Based on the results of the continuous measurements using the AlphaGUARD, the radon concentration varied with time over the range of 40 to 320 Bq m23 with an arithmetic mean of 188 Bq m23 in these caves in the Shaanxi Province. Figure 3 shows the time variations of radon concentrations in a cave dwelling. The radon concentration was high during the night and low during the day. In addition, the EERC was measured together with the radon concentration in some cave dwellings to obtain typical equilibrium factors. Aloka Co., Ltd., Tokyo, Japan. Hamamatsu Photonics K.K., Shizuoka, Japan. 7 ORTEC, Oak Ridge, TN. 5 6
A total of 44 measurements of the equilibrium factor were made during the entire investigation. The equilibrium factor ranged from less than 0.1 to 0.55 with an arithmetic mean of 0.28. The average equilibrium factor was lower than expected. Thirty-two grab samples were also taken to measure the concentration of thoron decay products with the a-particle track registration technique on the CR-39 detector. The EETC eventually ranged from 0.4 to 3.5 Bq m23 with an arithmetic mean of 1.3 Bq m23. When the short-term measurement was carried out, the g-ray dose rate was measured with the 10 3 20 NaI(Tl) scintillation spectrometer indoors and outdoors at 32 sites. The scintillation spectrometer reading ranged from 0.12 to 0.18 mSv h21 indoors and 0.1 to 0.14 mSv h21 outdoors. These readings in mSv h21 were corrected with another well-calibrated instrument using an empirical equation (13). The arithmetic means were eventually estimated to be 140 nGy h21 indoors and 110 nGy h21 outdoors. Soil samples were also taken at 12 sites in this short-term survey. The radioactivities of the 238U series, 232 Th series and 40K in loess soils ranged from 30 to 37, 41 to 47, and 578–670 Bq kg21 with arithmetic means of 34, 44 and 614 Bq kg21, respectively. Compared to those in the UNSCEAR report, the radioactivity level appears to be normal and may be distributed homogeneously in two provinces located at the Chinese loess plateau. The housing structures in the present survey eventually consisted of 114 loess cave dwellings, 55 brick flats, 21 stone flats, and three detached brick houses after some houses with incomplete data were excluded. The results for the annual average concentrations of radon, thoron and its decay products were obtained from for the 193-house investigation, as shown in Table 1. Analysis of the data showed that indoor radon concentrations ranged from 19 to 195 Bq m23 with a geometric mean of 57 Bq m23, indoor thoron concentrations ranged from 10 to 865 Bq m23 with a geometric mean of 153 Bq m23, and concentrations of thoron decay products expressed by the EETC ranged from 0.3 to 4.9 Bq m23 with a geometric mean of 1.6 Bq m23. These data were comparable to those obtained by the active measurement. The radon concentrations were subsequently characterized for the four types of housing structures. The loess cave had the highest radon concentration while the detached brick house had the lowest concentration. There was little difference between brick and stone flats. Comparing Shanxi data with Shaanxi data, all the concentrations in Shaanxi were higher than those in Shanxi. The difference can be explained by the housing structures in the two provinces. In Shaanxi, about 90% of the houses were loess caves, while about 50% of the houses in Shanxi were stone flats in which indoor radon concentrations were relatively low. The presence of relatively high thoron levels was confirmed in all the sites. Most of the measurement devices were placed near the wall or ceiling. This indicates that special attention must be paid to the location of the radon detector if a single device is used. Figure 4 shows the distributions of radon, thoron and EETC. After Shapiro-Wilk
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TABLE 1 Results for Radon, Thoron and Thoron Decay Product Concentrations (2001–2002) Number of houses
Radon (Bq m23)
Thoron (Bq m23)
EETC (Bq m23)
Shanxi
97
Shaanxi
96
46 (1.7) 53 70 (1.5) 76 57 (1.7) 64
130 (2.0) 160 181 (1.6) 202 153 (1.9) 181
1.2 (1.8) 1.4 2.1 (1.6) 2.3 1.6 (1.8) 1.9
All
193
Notes. Results are given as the geometric (upper) and arithmetic (lower) means. Geometric standard deviations are in parentheses.
tests for normality were performed, radon, thoron and EETC clearly followed a lognormal distribution. Figure 5 shows correlations among the three concentrations. Although the correlation coefficient was calculated, the analysis provided a weak positive correlation between radon and thoron concentrations. Those concentrations appear to be independent of each other. The data analysis demonstrated that the relationship between the thoron concentration and EETC is positive, with a correlation coefficient of 0.57. Table 2 shows the g-ray dose rates measured when passive devices were placed for long-term measurements. The data were obtained from 199 sites. The arithmetic means of the g-ray dose rates were estimated to be 140 nGy h21 indoors and 110 nGy h21 outdoors. The indoor and outdoor g-ray data were examined for normality using Shapiro-Wilk tests; they did not follow a normal or lognormal distribution (P , 0.0001). Since the loess cave dwelling had the highest g-ray dose rate, the indoor g-ray dose rate in Shaanxi was higher than that in Shanxi. These two provinces had comparable outdoor g-ray dose rates. This may suggest that the soil radioactivity is distributed homogeneously in this area. Table 3 shows a comparison of the present study with other studies. The NCI study area is close to our study area and is located in the Chinese loess plateau. The Yan’an data in the present study have shown that relatively high thoron concentrations were observed and were similar to the results of the study of Wiegand et al. (7). Only the NCI study found high radon concentrations, though the study areas were not quite the same. When comparing the EETC results of the present study with those of Wiegand et al., there is a major difference between the two. In fact, their EETC was estimated with an assumption of as equilibrium factor for thoron decay products. Although the equilibrium factor was assigned to be 0.1 as adopted by UNSCEAR, the actual EETC was quite small. Therefore, the dose contribution from thoron decay products will not be significant. However, the dose contribution ratio should be shown if a lung cancer study is conducted. Cave residents lie on the Kang formed by a loess cube while they are sleeping. Since the thoron concentration may be fairly high near the loess soil surface, they may be exposed to fairly high thoron gas concentrations. Therefore, the dose from thoron gas itself should also be considered in such circumstances even
though the current dose conversion factor for thoron gas is quite small. Since the radon and thoron concentrations were determined at the same location, the detection response of a passive radon detector was investigated. If a single passive detector is used in such caves, attention should be paid to its location. Figure 6 shows a histogram of the thoron/radon concentration ratio at the same detector location in cave dwellings. This thoron/radon concentration ratio depends on the detector placement. The arithmetic mean was estimated to be 3.4 and individual measurements ranged from 0.2 to 16.9. Our previous study examined the thoron sensitivity of some widely used passive radon detectors (14). For instance, if the Radtrak8 detector were used in this study area, it is possible that the radon concentration would be overestimated at the same detector location. The performance of the Radtrak has also been examined by Pearson and Spangler (15). The observed concentration (Cob) with a single passive radon detector can be expressed by the following equation: Cob 5 CRn 1 (CFTn /CFRn) · CTn,
(1)
where CRn is the discriminatively measured radon concentration (Bq m23), CTn is the discriminatively measured thoron concentration (Bq m23), CFRn is the conversion factor for the radon concentration (track cm22 kBq21 m3 h), and CFTn is the conversion factor for the thoron concentration (track cm22 kBq21 m3 h). Since our study area in Shaanxi is very close to the NCI study area, the Shaanxi data were used for further estimations. Using the data obtained in the present study, for example, the observed concentration can be given with a single use of Radtrak as follows: Cob 5 76 1 (1.88/2.81) · 202 5 211.
(2)
The observed radon concentration was then estimated to be 211 Bq m23 from the Radtrak reading. This value is comparable with the NCI result for the radon concentration (223 Bq m23), because the Radtrak was used for determination of the radon concentration in their case–control study, al8
Landauer, Glenwood, IL.
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FIG. 5. Correlation relationships for radon and thoron (panel a) and thoron and EETC (panel b).
TABLE 2 Gamma-Ray Dose Rates Measured Indoors and Outdoors
FIG. 4. Lognormal distribution probability plots for radon (panel a), thoron (panel b) and EETC (panel c).
Number of houses
Indoor dose rate (nGy h21)
Outdoor dose rate (nGy h21)
Shanxi
100
Shaanxi
99
130 (80–170) 150 (90–190) 140 (80–190)
110 (70–140) 110 (90–140) 110 (70–140)
All
199
Notes. Results are given as arithmetic means. Ranges are in parentheses.
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TABLE 3 Comparison of the Present Survey with Other Studies Items
NCI (5)
Study area (prefecture and province) Number of rooms measured Duration of measurement Radon (Bq m23) Thoron (Bq m23) EETC (Bq m23) Excess odds ratio (Lung cancer risk)
Pingliang and Qingyang Gansu 3188 1 year 223 None None 0.19 at 100 Bq m23 (95% CI: 0.05, 0.47)
a b
Wiegand et al. (7)
Present study
Yan’an Shaanxi 23 30 min (grab sampling) 92 215a 21.5b (F of 0.1 used) None
Yan’an Shaanxi 96 1 year 76 202 2.3 To be studied
Present study Luiliang Shanxi 97 1 year 53 160 1.4 To be studied
Median. The equilibrium factor of 0.1 was assigned to obtain the thoron decay products concentration based on the UNSCEAR approach.
though the detector location was not known. Further detailed investigations will be needed. Using the data shown in Table 3, the annual effective dose was compared among the three studies. The dose from radon and thoron gases is negligible, so it was not considered in the dose comparison. The annual effective dose caused by radon decay products (ERn mSv year21) can be obtained by the following equation:
the EETC, because the equilibrium factor was much less than that in the UNSCEAR report and its use was not reasonable since the thoron concentration varied with the detector location. Therefore, the annual effective dose caused by thoron decay products (ETn mSv year21) can be obtained with the following equation:
ERn 5 CRn2222 · F · t · DRn,
where DTn is the dose conversion factor for thoron (40 nSv Bq21 m3 h21) (1). Using the NCI data, the annual effective dose due to radon was estimated to be 4.2 mSv year21. The study of Wiegand et al. found a total effective dose of 8.3 mSv year21 with the typical equilibrium factors for radon and thoron decay products (0.4 and 0.1). The effective doses of radon and thoron progeny were 2.3 and 6.0 mSv year21, respectively. On the other hand, the total effective dose in the present study was eventually estimated to be 2.1 mSv year21 (1.5 for radon decay products and 0.6 mSv for thoron decay products). When comparing the study of Wiegand et al. with the present study, there is a four-times difference in the annual effective dose. This is derived from the estimate of thoron decay product dose because radon concentrations were comparable. The annual effective dose from g radiation in the present study was estimated to be 1.2 mSv year21 on the assumption that the occupancy factor is 0.8 indoors and 0.2 outdoors. The results of the comparison suggest that any measurements for evaluations of health effects without discriminative detection of radon isotopes may result in highly uncertain risk estimates and that thoron decay products should be measured directly.
(3)
where CRn-222 is the radon concentration (Bq m23), F is the equilibrium factor (0.3 used from the actual measurement), t is the time of exposure in a year (7000 h), and DRn is the dose conversion factor for radon (9 nSv Bq21 m3 h21) (1). UNSCEAR recommended an equilibrium factor of 0.4, but a value of 0.3 was used for this calculation. Although the annual dose of thoron can be obtained with the thoron concentration and equilibrium factor as the typical value of 0.1 as done by UNSCEAR, it was assessed directly with the concentration of thoron decay products, expressed by
ETn 5 EETC · t · DTn,
(4)
CONCLUSION
FIG. 6. Histogram of the thoron/radon concentration ratio at the same location in cave dwellings.
Natural radiation measurements were carried out in cave dwellings in Shanxi and Shaanxi Provinces, located in the area of the Chinese loess plateau. Since residential mobility is low, this area may be suitable for conducting an epidemiological study on lung cancer risk and residential radon exposure. According to the results of the present study, the radon concentration was lower than that in the NCI study
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but the thoron concentration was higher. Since the significance of thoron has been underestimated in the past, radon measurements and assessments may be influenced by the presence of thoron (16, 17). ACKNOWLEDGMENTS We are grateful to many local health officers and technicians, especially Dr. Runxi Wang of Shanxi Institute of Health Supervision, Mrs. Xuewang Chen, Yuejin Suo, Yonghong Liu, and Feng Gao in Luliang Prefecture, Shanxi Province, and Dr. Guifang Zheng, Lian Yu of Shaanxi Institute of Health Supervision, and Mrs. Huaijie Lin, Yulong Zong in Yan’an City, Shaanxi Province, for their excellent organizational work and assistance in the field survey and field measurements. Received: October 13, 2003; accepted: May 21, 2004
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