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Natural radiation levels in Tamil Nadu and Kerala, India
554
Natural radiation levels in Tamil Nadu and Kerala, India S. Tokonami a , H. Yonehara a , S. Akiba b , M.V. Thampi c , W. Zhuo a , Y. Narazaki d , Y. Yamada a a Radon Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage,
Chiba 263-8555, Japan b Department of Public Health, Kagoshima University, 8-35-1 Sakuragaoka, Kagoshima 890-8520, Japan c Low Level Radiation Research Laboratory, Bio-Medical Group, Bhabha Atomic Research Centre, IRE Campus,
Beach Road, Quilon, Kerala 691 001, India d Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaihu, Fukuoka 818-0135, Japan
Natural radiation measurements were preliminarily carried out in high background areas, in India, so as to understand the biological effects on human beings due to natural radiation exposures. The following parameters were taken into account for dose assessment: (1) 222 Rn (radon) concentration, (2) 220 Rn (thoron) concentration, (3) equilibrium equivalent thoron concentration (EETC), and (4) indoor/outdoor gamma dose rates. These measurements were made at 15 sites in Tamil Nadu and 5 sites in Kerala, India. The sites include houses and schools. The radon concentration, thoron concentration and EETCs ranged between 2–70, 6–690 and 0.1–1.6 Bq m−3 , respectively. The indoor and outdoor gamma dose rates ranged from 0.3 to 3.9 and from 0.4 to 6.2 µGy h−1 for radon and thoron, respectively. After classifying the dose by exposure, the annual effective dose at each site for radon and thoron was calculated with several assumptions. From the sampled data, the annual effective dose ranged from 4 to 22 mSv with an arithmetic mean of 9.3 mSv and the dose contribution was significantly due to external exposure.
1. Introduction It is well known that high natural radiation areas are located in the southwest coast of the Indian peninsula [1]. Such situations of high radiation levels result from the presence of heavy minerals containing monazite, which have high levels of thorium. The monazite sands are RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07066-4
© 2005 Elsevier Ltd. All rights reserved.
Natural radiation levels in Tamil Nadu and Kerala, India
555
specifically distributed along several beaches, where people are mining out. In addition, many people are living near those beaches, and are exposed to natural radiation sources. In order to understand the biological effect due to natural radiation, information on the relevant dosimetric quantities is indispensable. If accurate dose assessment is successful, people in these areas will be suitable for cases and controls in an epidemiological study. Although several studies have been carried out already [2–4], natural radiation measurements using our up-to-date instruments were preliminarily but systematically carried out in Tamil Nadu and Kerala, India in the present study. The following parameters were taken into account for dose assessment: radon, thoron and thoron decay products as internal exposures and gamma dose rates indoors and outdoors as external exposures. The present paper describes the preliminary results of the radiation survey program.
2. Materials and methods In total, twenty houses or schools were chosen in our radiation survey program: 15 sites in Tamil Nadu (Manavalakurichy) and 5 sites in Kerala (Needendakara), India. For the determination of radon and thoron concentrations, two types of alpha-track detector (CR-39) were used. One detector was designed to detect radon effectively. It is called RADOPOT, is made in Hungary and is commercially available. The other detector is a modified RADOPOT. The air exchange rate of the detector has been enhanced so as to detect thoron as well as radon. Using two readings, both radon and thoron concentrations can be evaluated. On the other hand, a deposition rate measurement of the thoron decay products was applied for the EETC determination [5]. A CR-39 detector was incorporated into the measuring system. The CR-39 detector was covered with an energy absorber such as aluminium-evaporated Mylar® film to detect alpha particles emitted from 212 Po (8.78 MeV) only. The thickness of the film was set to about 71.5 mm as an air-equivalent value. The EETC (Bq m−3 ) can be given by the following equation: EETC = 0.87 × N/T
(1) cm−2 )
where N is the track density (tracks and T is the exposure period (day). Note that the above three devices measured indoor concentrations and no measurements were made outdoors in the present study. These three devices were placed indoors for 3–5 months. Information on housing structure, exposure period and position of the device is tabulated in Table 1. The gamma dose rate is measured at 1 m height above ground with a 1 × 2 NaI (Tl) scintillation spectrometer (commercial name: SS-γ) indoors and outdoors. The energy resolution of the scintillation spectrometer is 11% for the 662 keV gammas of 137 Cs. Readings on the spectrometer were corrected with another well-calibrated instrument with an empirical equation [6]. Soil samples were taken in mined beaches to measure their radioactivities. They were determined by gamma spectrometry using a pure Ge detector. A HPGe detector manufactured by ORTEC was used. The relative efficiency and FWHM resolution were 36% and 1.76 keV, respectively, for the 1.33 MeV gammas of 60 Co. Counting time for each sample was generally 8 × 104 s or more.
556
S. Tokonami et al.
Table 1 Information on measurement sites Site ID
Exposure period beginning, end
Housing structure (roof/wall/floor)
Positions∗ of the device location and remarks Gas & DP (both): on the table. Gas: 30 cm, DP: 0 cm, H = 2.5 m Both: 0 cm, H = 2 m Gas: 20 cm, DP: 0 cm. H = 2 m; Near mining beach Both: 0 cm, H = 2 m; Near mining beach Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m; Near mining beach Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m; School Both: 0 cm, H = 2.2 m Middle school Both: 0 cm, H = 2.2 m; Secondary school Both: 0 cm, H = 2.2 m; High school Both: 0 cm, H = 2.5 m; Elementary school Both: 0 cm, H = 2.2 m Both: 0 cm, Height = 2 m for gas, H = 1.9 m for DP Gas: center of room, DP: 0 cm, H = 2 m for gas, Height = 1.9 m for DP Both from ceiling, Height = 2 m; Soil sample taken Both: 0 cm, H = 1.9 m Both: 0 cm, H = 2.1 m
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2/18/01, 7/24/01 2/19/01, 7/25/01 2/19/01, 7/25/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/25/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/23/01, 5/24/01 2/23/01, 5/24/01
Concrete/Granite/Cement Concrete/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Tile/Brick/Cement Tile/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Thatch/Coconut leaves/Sand Concrete/Brick/Cement Asbestos/Wood/Cement
18 19 20
2/23/01, 5/24/01 N.D. 2/23/01, 5/24/01
Asbestos/Brick/Cement Concrete/Brick/Cement Asbestos/Wood/Cement
∗ In most cases, the distance from wall and height (H ) from the floor.
3. Results and discussion Results of the radiation survey are listed in Table 2. Only the data from one site (No. 19) was invalid because the detector was lost. The radon concentration ranged from 2 to 70 Bq m−3 , and an arithmetic mean (AM) was estimated to be 17 Bq m−3 , as low as expected due to open housing structures. On the other hand, thoron concentration ranged widely from 6 to 690 Bq m−3 , and the AM was 168 Bq m−3 . Our thoron detectors were generally suspended on walls and were close to the wall. It is well known that the thoron concentration varies greatly with distance from the source. However, our thoron gas measurements were justified on the following grounds: The result of thoron concentrations would be useful for dose assessment from thoron gas itself due to inhalation because the position of the device was clarified at the site. Since there is little information on thoron in the environment as the UNSCEAR 2000 report has pointed out, such data should also be accumulated wherever possible. From the thoron decay product measurements using their deposition rate measurements, the AM of EETC was estimated to be 0.5 Bq m−3 , which was fairly low (range: 0.1–1.6 Bq m−3 ). The reason can be also explained as being because of open housing structures, as for radon. If an equilibrium factor (F ) between thoron and thoron decay products is evaluated using two relevant data, though meaningless in a sense, an average F is obtained as 0.007. Since the thoron concentration varies greatly in space, the equilibrium factor will subsequently change according to the position of those devices. Using the data of Table 2, an average thoron to
Natural radiation levels in Tamil Nadu and Kerala, India
557
Table 2 Results of the radiation survey Site ID
Radon (Bq m−3 )
Thoron (Bq m−3 )
EETC (Bq m−3 )
Indoor dose rate (µGy h−1 )
Outdoor dose rate (µGy h−1 )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
56 3 8 9 33 20 19 10 19 8 7 5 12 70 2 9 18 9 N.D. 5
45 16 143 13 690 411 480 31 441 38 83 34 94 480 76 30 6 67 N.D. 17
0.4 0.2 0.6 0.2 1.6 0.8 0.9 0.1 0.7 0.2 0.4 0.4 0.2 0.9 0.3 0.1 0.03 1.4 N.D. 0.5
1.7 1.3 0.6 3.9 2.3 0.7 1.3 0.4 0.8 0.8 0.9 0.4 0.4 0.9 1.6 0.3 1.1 3.8 0.8 1.8
1.1 1.71 0.4 2.5 1.8 0.4 0.5 0.6 0.4 0.9 2.0 1.6 1.7 1.5 2.8 2.4 2.0 6.2 1.6 4.6
radon ratio was estimated to be 11.0. Special attention should be paid to selection of the device when radon measurements are accurately made [7]. The indoor and outdoor gamma dose rates ranged from 0.3 to 3.9 and 0.4 to 6.2 µGy h−1 , respectively. Their AM were estimated to be 1.3 and 1.8 µGy h−1 , respectively. The indoor to outdoor ratios ranged from 0.1 to 2.4, with an AM of 0.97. As far as the sampled data are concerned, there is little correlation amongst them and they were independent. When an epidemiological study has to be carried out, individual doses should be considered together with general information on their living activities. Natural radioactivities in soil at some locations (four samples on the mining beach in Tamil Nadu and one sample in Kerala, India) were measured by gamma-ray spectrometry using the Ge detector. In the site where the highest gamma doses were given, 238 U and 232 Th activities in soil were 2.4 and 12.0 kBq kg−1 , respectively, with some assumptions of radioactive equilibrium between those nuclides and their decay products. Regarding the activities in beach sands, they are widely distributed in the range of 0.03–7.0 kBq kg−1 for 238 U and 0.14–50.8 kBq kg−1 for 232 Th, respectively, even on the same beach. To understand the variation of individual doses among the sampled data, the preliminary dose assessment was made with our limited data based on the UNSCEAR 2000 Report approach. Several assumptions were made as follows: outdoor radon concentration was equal to 2 Bq m−3 because all the sites were near beaches where the outdoor radon concentration seems to be low; the equilibrium factors indoors and outdoors were given as 0.4 and 0.6, respectively, because no measurements were made but they were reasonable values after taking their surrounding conditions into account; outdoor thoron concentrations and EETCs were equal to 10 and 0.1 Bq m−3 , respectively, because there were no means to assign these values
558
S. Tokonami et al. Table 3 Summary of the annual effective dose (mSv y−1 ) under several assumptions based on the UNSCEAR 2000 Report approach Items
Radon and its decay products
Thoron and its decay products
Internal dose (subtotal)
External dose
Total
Min Max Mean
0.07 1.9 0.5
0.02 1.0 0.3
0.2 2.5 0.8
3.7 21.6 8.6
4.0 21.9 9.3
at the moment. Occupancy factors were assumed to be the same at all the sites, as for outdoor gamma dose rates, a mean value of 1.8 µGy h−1 being assigned. After classifying the exposure doses, the annual effective dose at each site was calculated via the several assumptions mentioned above. The results are summarised in Table 3. When the dose from radon and its decay products is compared with that from thoron and its decay products, the dose from the former is more significant than that from thoron as long as the dose was assessed based on the UNSCEAR 2000 Report methodology. From the sampled data, the annual effective dose ranges from 4 to 22 mSv with an AM of 9.3 mSv and the dose contribution is significantly due to the external exposure. The external dose accounts for 90% of the total dose in the present study.
4. Conclusion There are a lot of problems to be solved in order to understand the biological effects due to very low-dose radiations. If an epidemiological study is effectively conducted with a small population in the future, high background radiation areas will be suitable for the purpose. The present radiation survey was preliminarily conducted so as to understand the relevant dosimetric quantities taking this future project into account. There was little correlation amongst the data and they seemed to be independent. The most significant dose arises from gamma radiation because the other dosimetric quantities are much less than the gamma dose. Since there are no epidemiological data on thoron exposure, many problems remain unsolved. The dose assessment for thoron and its decay products has not yet been established as well as that for radon. It can be pointed out that the dose contribution from thoron gas itself should be considered because residents lie almost directly on/near the ground/floor (main source of thoron) when they sleep. Note that there is a large variation in radiation dose even in such high background radiation areas. Therefore, individual doses should be accurately evaluated in order to understand the biological effects on human beings of natural radiation exposures.
References [1] Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes, United Nations, New York, 2000. [2] A.C. Paul, et al., in: Proc. 2nd National Symposium on Environment, 1993, p. 33. [3] A.C. Paul, et al., J. Environ. Radioact. 22 (1994) 243.
Natural radiation levels in Tamil Nadu and Kerala, India [4] [5] [6] [7]
A.C. Paul, et al., J. Environ. Radioact. 40 (1998) 251. W. Zhuo, T. Iida, Jpn. Health Phys. 35 (2000) 365. M. Furukawa, S. Tokonami, Jpn. Health Phys. 36 (2001) 195. S. Tokonami, et al., Health Phys. 80 (2001) 612.
559
Natural radiation levels in Tamil Nadu and Kerala, India S. Tokonami a , H. Yonehara a , S. Akiba b , M.V. Thampi c , W. Zhuo a , Y. Narazaki d , Y. Yamada a a Radon Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage,
Chiba 263-8555, Japan b Department of Public Health, Kagoshima University, 8-35-1 Sakuragaoka, Kagoshima 890-8520, Japan c Low Level Radiation Research Laboratory, Bio-Medical Group, Bhabha Atomic Research Centre, IRE Campus,
Beach Road, Quilon, Kerala 691 001, India d Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaihu, Fukuoka 818-0135, Japan
Natural radiation measurements were preliminarily carried out in high background areas, in India, so as to understand the biological effects on human beings due to natural radiation exposures. The following parameters were taken into account for dose assessment: (1) 222 Rn (radon) concentration, (2) 220 Rn (thoron) concentration, (3) equilibrium equivalent thoron concentration (EETC), and (4) indoor/outdoor gamma dose rates. These measurements were made at 15 sites in Tamil Nadu and 5 sites in Kerala, India. The sites include houses and schools. The radon concentration, thoron concentration and EETCs ranged between 2–70, 6–690 and 0.1–1.6 Bq m−3 , respectively. The indoor and outdoor gamma dose rates ranged from 0.3 to 3.9 and from 0.4 to 6.2 µGy h−1 for radon and thoron, respectively. After classifying the dose by exposure, the annual effective dose at each site for radon and thoron was calculated with several assumptions. From the sampled data, the annual effective dose ranged from 4 to 22 mSv with an arithmetic mean of 9.3 mSv and the dose contribution was significantly due to external exposure.
1. Introduction It is well known that high natural radiation areas are located in the southwest coast of the Indian peninsula [1]. Such situations of high radiation levels result from the presence of heavy minerals containing monazite, which have high levels of thorium. The monazite sands are RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07066-4
© 2005 Elsevier Ltd. All rights reserved.
Natural radiation levels in Tamil Nadu and Kerala, India
555
specifically distributed along several beaches, where people are mining out. In addition, many people are living near those beaches, and are exposed to natural radiation sources. In order to understand the biological effect due to natural radiation, information on the relevant dosimetric quantities is indispensable. If accurate dose assessment is successful, people in these areas will be suitable for cases and controls in an epidemiological study. Although several studies have been carried out already [2–4], natural radiation measurements using our up-to-date instruments were preliminarily but systematically carried out in Tamil Nadu and Kerala, India in the present study. The following parameters were taken into account for dose assessment: radon, thoron and thoron decay products as internal exposures and gamma dose rates indoors and outdoors as external exposures. The present paper describes the preliminary results of the radiation survey program.
2. Materials and methods In total, twenty houses or schools were chosen in our radiation survey program: 15 sites in Tamil Nadu (Manavalakurichy) and 5 sites in Kerala (Needendakara), India. For the determination of radon and thoron concentrations, two types of alpha-track detector (CR-39) were used. One detector was designed to detect radon effectively. It is called RADOPOT, is made in Hungary and is commercially available. The other detector is a modified RADOPOT. The air exchange rate of the detector has been enhanced so as to detect thoron as well as radon. Using two readings, both radon and thoron concentrations can be evaluated. On the other hand, a deposition rate measurement of the thoron decay products was applied for the EETC determination [5]. A CR-39 detector was incorporated into the measuring system. The CR-39 detector was covered with an energy absorber such as aluminium-evaporated Mylar® film to detect alpha particles emitted from 212 Po (8.78 MeV) only. The thickness of the film was set to about 71.5 mm as an air-equivalent value. The EETC (Bq m−3 ) can be given by the following equation: EETC = 0.87 × N/T
(1) cm−2 )
where N is the track density (tracks and T is the exposure period (day). Note that the above three devices measured indoor concentrations and no measurements were made outdoors in the present study. These three devices were placed indoors for 3–5 months. Information on housing structure, exposure period and position of the device is tabulated in Table 1. The gamma dose rate is measured at 1 m height above ground with a 1 × 2 NaI (Tl) scintillation spectrometer (commercial name: SS-γ) indoors and outdoors. The energy resolution of the scintillation spectrometer is 11% for the 662 keV gammas of 137 Cs. Readings on the spectrometer were corrected with another well-calibrated instrument with an empirical equation [6]. Soil samples were taken in mined beaches to measure their radioactivities. They were determined by gamma spectrometry using a pure Ge detector. A HPGe detector manufactured by ORTEC was used. The relative efficiency and FWHM resolution were 36% and 1.76 keV, respectively, for the 1.33 MeV gammas of 60 Co. Counting time for each sample was generally 8 × 104 s or more.
556
S. Tokonami et al.
Table 1 Information on measurement sites Site ID
Exposure period beginning, end
Housing structure (roof/wall/floor)
Positions∗ of the device location and remarks Gas & DP (both): on the table. Gas: 30 cm, DP: 0 cm, H = 2.5 m Both: 0 cm, H = 2 m Gas: 20 cm, DP: 0 cm. H = 2 m; Near mining beach Both: 0 cm, H = 2 m; Near mining beach Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m; Near mining beach Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m Both: 0 cm, H = 2 m; School Both: 0 cm, H = 2.2 m Middle school Both: 0 cm, H = 2.2 m; Secondary school Both: 0 cm, H = 2.2 m; High school Both: 0 cm, H = 2.5 m; Elementary school Both: 0 cm, H = 2.2 m Both: 0 cm, Height = 2 m for gas, H = 1.9 m for DP Gas: center of room, DP: 0 cm, H = 2 m for gas, Height = 1.9 m for DP Both from ceiling, Height = 2 m; Soil sample taken Both: 0 cm, H = 1.9 m Both: 0 cm, H = 2.1 m
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
2/18/01, 7/24/01 2/19/01, 7/25/01 2/19/01, 7/25/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/25/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/24/01 2/19/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/20/01, 7/25/01 2/23/01, 5/24/01 2/23/01, 5/24/01
Concrete/Granite/Cement Concrete/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Tile/Brick/Cement Tile/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Tile/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Concrete/Brick/Cement Thatch/Coconut leaves/Sand Concrete/Brick/Cement Asbestos/Wood/Cement
18 19 20
2/23/01, 5/24/01 N.D. 2/23/01, 5/24/01
Asbestos/Brick/Cement Concrete/Brick/Cement Asbestos/Wood/Cement
∗ In most cases, the distance from wall and height (H ) from the floor.
3. Results and discussion Results of the radiation survey are listed in Table 2. Only the data from one site (No. 19) was invalid because the detector was lost. The radon concentration ranged from 2 to 70 Bq m−3 , and an arithmetic mean (AM) was estimated to be 17 Bq m−3 , as low as expected due to open housing structures. On the other hand, thoron concentration ranged widely from 6 to 690 Bq m−3 , and the AM was 168 Bq m−3 . Our thoron detectors were generally suspended on walls and were close to the wall. It is well known that the thoron concentration varies greatly with distance from the source. However, our thoron gas measurements were justified on the following grounds: The result of thoron concentrations would be useful for dose assessment from thoron gas itself due to inhalation because the position of the device was clarified at the site. Since there is little information on thoron in the environment as the UNSCEAR 2000 report has pointed out, such data should also be accumulated wherever possible. From the thoron decay product measurements using their deposition rate measurements, the AM of EETC was estimated to be 0.5 Bq m−3 , which was fairly low (range: 0.1–1.6 Bq m−3 ). The reason can be also explained as being because of open housing structures, as for radon. If an equilibrium factor (F ) between thoron and thoron decay products is evaluated using two relevant data, though meaningless in a sense, an average F is obtained as 0.007. Since the thoron concentration varies greatly in space, the equilibrium factor will subsequently change according to the position of those devices. Using the data of Table 2, an average thoron to
Natural radiation levels in Tamil Nadu and Kerala, India
557
Table 2 Results of the radiation survey Site ID
Radon (Bq m−3 )
Thoron (Bq m−3 )
EETC (Bq m−3 )
Indoor dose rate (µGy h−1 )
Outdoor dose rate (µGy h−1 )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
56 3 8 9 33 20 19 10 19 8 7 5 12 70 2 9 18 9 N.D. 5
45 16 143 13 690 411 480 31 441 38 83 34 94 480 76 30 6 67 N.D. 17
0.4 0.2 0.6 0.2 1.6 0.8 0.9 0.1 0.7 0.2 0.4 0.4 0.2 0.9 0.3 0.1 0.03 1.4 N.D. 0.5
1.7 1.3 0.6 3.9 2.3 0.7 1.3 0.4 0.8 0.8 0.9 0.4 0.4 0.9 1.6 0.3 1.1 3.8 0.8 1.8
1.1 1.71 0.4 2.5 1.8 0.4 0.5 0.6 0.4 0.9 2.0 1.6 1.7 1.5 2.8 2.4 2.0 6.2 1.6 4.6
radon ratio was estimated to be 11.0. Special attention should be paid to selection of the device when radon measurements are accurately made [7]. The indoor and outdoor gamma dose rates ranged from 0.3 to 3.9 and 0.4 to 6.2 µGy h−1 , respectively. Their AM were estimated to be 1.3 and 1.8 µGy h−1 , respectively. The indoor to outdoor ratios ranged from 0.1 to 2.4, with an AM of 0.97. As far as the sampled data are concerned, there is little correlation amongst them and they were independent. When an epidemiological study has to be carried out, individual doses should be considered together with general information on their living activities. Natural radioactivities in soil at some locations (four samples on the mining beach in Tamil Nadu and one sample in Kerala, India) were measured by gamma-ray spectrometry using the Ge detector. In the site where the highest gamma doses were given, 238 U and 232 Th activities in soil were 2.4 and 12.0 kBq kg−1 , respectively, with some assumptions of radioactive equilibrium between those nuclides and their decay products. Regarding the activities in beach sands, they are widely distributed in the range of 0.03–7.0 kBq kg−1 for 238 U and 0.14–50.8 kBq kg−1 for 232 Th, respectively, even on the same beach. To understand the variation of individual doses among the sampled data, the preliminary dose assessment was made with our limited data based on the UNSCEAR 2000 Report approach. Several assumptions were made as follows: outdoor radon concentration was equal to 2 Bq m−3 because all the sites were near beaches where the outdoor radon concentration seems to be low; the equilibrium factors indoors and outdoors were given as 0.4 and 0.6, respectively, because no measurements were made but they were reasonable values after taking their surrounding conditions into account; outdoor thoron concentrations and EETCs were equal to 10 and 0.1 Bq m−3 , respectively, because there were no means to assign these values
558
S. Tokonami et al. Table 3 Summary of the annual effective dose (mSv y−1 ) under several assumptions based on the UNSCEAR 2000 Report approach Items
Radon and its decay products
Thoron and its decay products
Internal dose (subtotal)
External dose
Total
Min Max Mean
0.07 1.9 0.5
0.02 1.0 0.3
0.2 2.5 0.8
3.7 21.6 8.6
4.0 21.9 9.3
at the moment. Occupancy factors were assumed to be the same at all the sites, as for outdoor gamma dose rates, a mean value of 1.8 µGy h−1 being assigned. After classifying the exposure doses, the annual effective dose at each site was calculated via the several assumptions mentioned above. The results are summarised in Table 3. When the dose from radon and its decay products is compared with that from thoron and its decay products, the dose from the former is more significant than that from thoron as long as the dose was assessed based on the UNSCEAR 2000 Report methodology. From the sampled data, the annual effective dose ranges from 4 to 22 mSv with an AM of 9.3 mSv and the dose contribution is significantly due to the external exposure. The external dose accounts for 90% of the total dose in the present study.
4. Conclusion There are a lot of problems to be solved in order to understand the biological effects due to very low-dose radiations. If an epidemiological study is effectively conducted with a small population in the future, high background radiation areas will be suitable for the purpose. The present radiation survey was preliminarily conducted so as to understand the relevant dosimetric quantities taking this future project into account. There was little correlation amongst the data and they seemed to be independent. The most significant dose arises from gamma radiation because the other dosimetric quantities are much less than the gamma dose. Since there are no epidemiological data on thoron exposure, many problems remain unsolved. The dose assessment for thoron and its decay products has not yet been established as well as that for radon. It can be pointed out that the dose contribution from thoron gas itself should be considered because residents lie almost directly on/near the ground/floor (main source of thoron) when they sleep. Note that there is a large variation in radiation dose even in such high background radiation areas. Therefore, individual doses should be accurately evaluated in order to understand the biological effects on human beings of natural radiation exposures.
References [1] Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes, United Nations, New York, 2000. [2] A.C. Paul, et al., in: Proc. 2nd National Symposium on Environment, 1993, p. 33. [3] A.C. Paul, et al., J. Environ. Radioact. 22 (1994) 243.
Natural radiation levels in Tamil Nadu and Kerala, India [4] [5] [6] [7]
A.C. Paul, et al., J. Environ. Radioact. 40 (1998) 251. W. Zhuo, T. Iida, Jpn. Health Phys. 35 (2000) 365. M. Furukawa, S. Tokonami, Jpn. Health Phys. 36 (2001) 195. S. Tokonami, et al., Health Phys. 80 (2001) 612.
559