Radiation Protection Dosimetry (2014), Vol. 160, No. 1–3, pp. 160 –163 Advance Access publication 17 April 2014
doi:10.1093/rpd/ncu076
THORON AND THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES S. Gierl, O. Meisenberg*, P. Feistenauer and J. Tschiersch Helmholtz Zentrum Mu¨nchen, German Research Center for Environmental Health, Institute of Radiation Protection, Ingolsta¨dter Landstr. 1, 85764 Neuherberg, Germany *Corresponding author: [email protected]
The health risks of radon (222Rn) have long been known, it is considered the second most common cause for lung cancer after smoking(1). The risk of thoron (220Rn) on the other hand has long been neglected in many cases. It was assumed that thoron cannot reach indoor air due to its short half-life of 55.6 s. However, in recent years elevated thoron concentrations were found in dwellings built from unfired clay in China(2, 3) and India(4). This additional exposure significantly increases the dose to the inhabitants of those dwellings. In Germany unfired clay has been used as a building material in half-timbered houses in the past. Half-timbered houses were the prevalent building style from ancient times up to the 19th century. Those houses are made of wooden frames and the panels between the frames are filled with unfired clay. Eco-friendly building styles have become more popular in recent years. In those modern buildings clay is mainly used as indoor plaster. About 2 million houses that use unfired earthen building material exist in Germany today(5). In this study thoron and its progeny were measured in several traditional as well as modern houses in Bavaria (South-East Germany), which use unfired clay as a building material. Emphasis of the study was the direct long-term measurement of thoron decay products. The advantage of the measurement of the progeny is that the main contributors to the dose are directly measured and no assumptions on the equilibrium factor are necessary. Furthermore, the decay products are homogeneously distributed in the indoor environment, which is not the case for thoron itself(6). The results of gas and progeny measurements in 17 houses are presented. Additionally, the dose was calculated for the inhabitants of the houses.
METHODS Thoron progeny measurement A newly developed Unattended Battery-Operated Progeny Measurement Device (UBPM) was used (Figure 1). The UBPM is a further development of the instrument first introduced by Bi et al.(7). The instrument creates an electric field in the hemisphere made of wire mesh. The wire mesh is on ground potential, whereas the solid-state nuclear track detectors (CR39), which are placed in the middle of the hemisphere and used to register alpha decay, are at a potential of þ7.0 kV. Negatively charged progeny are attracted by the electric field. The instruments were calibrated against a working level monitor to measure progeny equilibrium equivalent concentration (EEC) with contributions from 212Pb and 212Bi. The calibration was performed inside the HMGU thoron experimental house(8). Further details on the UBPM can be found in Gierl et al.(9). Radon and thoron gas measurement For gas measurements CR39 is placed at the bottom of a plastic cup, which is 3.6-cm high. A measurement for radon and thoron together is performed by using a cup with a lid, which has several holes covered by a membrane filter and conductive foam. Therefore, only the radon and thoron gas can enter the cup but not the progeny. For measuring radon only, a cup is used with a closed lid without holes. In this case the gas can only enter the cup through the screw thread. The devices were calibrated against a RAD7 in a 1 m3 calibration chamber. Taking part in intercomparison measurements(10) regularly ensures the quality of the
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Sydney on November 13, 2015
In recent years, elevated thoron concentrations were found in houses built of unfired clay. In this study experiments were carried out in 17 traditional and modern clay houses in Germany to obtain an overview of indoor thoron in such houses. Long-term measurements over an 8-week period were performed using a newly developed Unattended Battery-Operated Progeny Measurement Device (UBPM) for measuring thoron progeny. This instrument uses a high-voltage electric field to precipitate radon and thoron progeny on nuclear track detectors. Additional active and passive measurements of radon, thoron and their progeny were performed. The equilibrium equivalent thoron concentration was found to be between 2 and 10 Bq m23. Gas concentrations were found to be between 20 and 160 Bq m23 for radon and between 10 and 90 Bq m23 for thoron 20 cm from the wall. The thoron exposure contributes significantly to the inhalation dose of the dwellers (0.6– 4 mSv a21).
THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES
RESULTS AND DISCUSSION The measurements were performed during late winter and spring 2013 in 17 Bavarian houses in which unfired clay was used as a building material. Apart from this,
the houses were selected randomly. Table 1 gives an overview of the houses and shows which measurements were performed in which house. Measurements were performed in several half-timbered houses, but also in modern houses with either clay plaster or clay panels. Measurements in house no. 7 were performed under two different conditions: during the gas measurements the construction of the house was leaky (measured air exchange rate of 4.0 h – 1, condition denoted as 7a), whereas before the progeny measurements the leaks in the inner and outer walls were caulked (denoted as 7b). Progeny concentrations The thoron progeny concentration measured with the UBPM in houses 1–9 is shown in Figure 2 along with the corresponding annual contribution to the inhalation dose. The measurements were performed over an 8-week period in late winter 2013. The annual dose was calculated assuming an occupancy factor of 40 %, which means an exposure time of 10 h d21. Dose conversion coefficients published in Bi et al.(11) were used for the calculation. The thoron progeny concentrations range between 2 and 10 Bq m23 and the resulting dose from 0.6 to 4 mSv a21. Radon and thoron gas concentrations
Figure
1. Unattended Battery-Operated Measurement Device (UBPM).
Progeny
In houses 1 –6 and 8–9, radon and thoron gas concentrations were measured by one radon-plus-thorondevice and one radon-only device, which were placed at a distance from the wall of 20 cm. Figure 3 shows the results. Thoron concentrations in these houses range from below the detection limit to 90 Bq m23. Radon concentrations were found in a range of 20 –160 Bq m23.
Table 1. Overview of the houses in which measurements were performed. No.
House type
Usage
Gas measurement
Progeny measurement
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Half-timbered Clay plaster Half-timbered Half-timbered Half-timbered Clay plaster Half-timbered Clay plaster Clay plaster Clay panels Clay panels Clay panels Clay panels Clay panels Clay panels Mixed (adobe, plaster, panels) Adobe
Residential Residential Residential Townhall Residential Residential Residential (unoccupied) Residential Rescue station Residential Residential Residential Residential Office Residential Residential Office
20 cm 20 cm 20 cm 20 cm 20 cm 20 cm Profile 20 cm 20 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm Profile Profile
x x x x x x x x x — — — — — — — —
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calibration. For thoron measurements with these devices at least one radon-only device has to be kept in the room with devices which measure radon and thoron. Measurements were performed in different distances from clay walls. When gas measurements were performed alongside progeny measurements, one radon plus thoron and one radon device were kept at a distance of 20 cm from the clay wall. When only gas measurements were performed, radon-plus-thoron devices were kept at distances of 10 and 50 cm from the wall, respectively. Additionally, in three houses distance profile measurements with several devices were performed.
S. GIERL ET AL.
In houses 7 and 10– 17, measurements were performed at distances of 10 and 50 cm from the clay wall. The results are shown in Figure 4. Thoron concentrations at a distance from the clay wall of 10 cm are slightly higher than thoron concentrations in houses 1–6 and 8–9 measured at a distance of 20 cm as expected. Therefore, it can be assumed that thoron progeny concentrations in houses 7 and 10– 17 are in the same range as thoron progeny concentrations in houses 1–6 and 8–9. Profile measurements
cðdÞ ¼ c0 expðd=d1=e Þ þ cm
ð1Þ
Figure 3. Radon and thoron gas concentrations in houses 1 –6 and 8– 9. The thoron concentrations in houses 1 and 5 were below the detection limit.
cm is the concentration in the middle of the room and c0 the concentration which is added to cm directly at the wall. d1/e is the distance in which the concentration decreases by a factor of 1/e. This exponential function is suitable to describe contributions of advection (exactly) and diffusion (approximately) to the transport of thoron(12). By integrating the function for all clay walls, the total thoron activity in the rooms could be calculated. Dividing by the volume of the room leads to the average thoron concentration. Multiplying this result with the equilibrium factor leads to the progeny concentration. Calculations of the equilibrium factor were done according to Meisenberg et al.(6). The air exchange rate that was used for the calculations was measured to be 4.0 h21 for house 7, whereas the air exchange rate in houses 16 and 17 was assumed to be 0.6 h21, which is a typical value for residential buildings with closed windows. The annual dose was calculated from the progeny concentration. The results can be found in Table 2. The difference between the results shown in Figure 2 and the calculated concentration and dose
Figure 4. Radon and thoron gas concentrations in houses 7 and 10–15. The error bars show the range from minimum to maximum value.
Figure 5. Concentration profiles of thoron in front of different walls in houses 7, 16 and 17. Inlay: scaled extract.
Figure 2. Thoron progeny concentration and annual dose in houses 1– 9. The bars indicate the combined uncertainty with contributions from counting statistics and the calibration factor at a coverage factor k ¼ 1.
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In houses 7, 16 and 17, profile measurements were performed. For each profile, seven measurement devices were set up with small lateral offset at different distances from 0 to 40 cm measured from the respective
wall. The thoron concentration profile from houses 7 and 16 (measured in front of two walls within one room each) and 17 showed different slopes: In house 7, in which relatively strong air turbulence prevailed because of the leaky construction, concentrations which were significantly increased compared with those in the middle of the room could still be found at distances of up to 15 cm (Figure 5). In the other two houses, the concentrations strongly decreased within only 5 cm. Very high concentrations near the walls were found in house no. 17, which is a one-room office building with massive adobe walls. With concentrations of up to 200 Bq m23 at distances of 10 and 20 cm, also in these houses progeny concentrations that are similar to those in houses 1–6 and 8 can be expected. From the thoron profiles the total thoron activity in the rooms can be calculated. Using the data analysis software SciDAVis, an exponential function was fitted to the data with a least-squares method:
THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES Table 2. Annual dose calculated from thoron gas concentration in rooms where profile measurements have been performed. No
Volume of the room [m3]
Thoron exhaling area [m2]
Average thoron concentration [Bq m23]
Equilibrium factor (%)
Annual dose [mSv a21]
7a 16 17
42.2 16.9 72.8
18.6 25.6 88.4
49.9+12.5 62.1+26.0 140.8+67.1
1.5 11 11
0.32+0.08 2.92+1.23 6.63+3.12
in Table 2 for house 7 result from the high air exchange rate in the house during the gas measurements.
Thoron progeny concentrations were measured in nine houses. Radon and thoron gas concentrations were measured in 17 houses and in 3 of those profile measurements were performed. In all houses unfired clay was used as a building material. The results show that increased thoron gas concentrations as well as thoron progeny concentrations can be found in these types of houses. The significant increase in the inhalation dose to the inhabitants shows that thoron is an issue of radiation protection in these houses and should be taken into account in further studies. The results suggest that thoron gas measurements are not enough to evaluate the dose accurately. Especially, the profile measurements show that there is no certain distance from the wall which can be used to estimate an average concentration. Gas measurements are suitable for a first assessment of the situation, but for thorough evaluations progeny concentrations should be measured. FUNDING This study was supported partly by the Federal Ministry of Education and Research (BMBF, contract 02NUK015B) and partly by the Bavarian State Ministry of the Environment and Consumer Protection (BayStMUV). Its contents are solely the responsibility of the authors. REFERENCES 1. World Health Organization. WHO handbook on indoor radon: a public health perspective, WHO, pp. 1– 94 (2009).
163
Downloaded from http://rpd.oxfordjournals.org/ at University of Sydney on November 13, 2015
CONCLUSION
2. Wiegand, J., Feige, S., Quingling, X., Schreiber, U., Wieditz, K., Wittman, C. H. and Xiarong, L. Radon and thoron in cave dwellings (Yan’an, China). Health Phys. 78, 438– 444 (2000). 3. Shang, B., Chen, B., Gao, Y., Cui, H. and Li, Z. Thoron levels in traditional Chinese residential dwellings. Radiat. Environ Biophys. 44, 193– 199 (2005). 4. Sreenath Reddy, M., Yadagiri Reddy, P., Rama Reddy, K., Eappen, K. P., Ramachandran, T. V. and Mayya, Y. S. Thoron levels in the dwellings of Hyderabad city, Andhra Pradesh, India. J. Environ. Radioact. 73, 21–28 (2004). 5. Schreckenbach, H. Building with earth—consumer information. German Association for Building with Earth. (2004). 6. Meisenberg, O. and Tschiersch, J. Thoron in indoor air: modeling for a better exposure estimate. Indoor Air 21, 240–252 (2011). 7. Bi, L., Tschiersch, J., Meisenberg, O., Wielunski, M., Li, J. L. and Shang, B. Development of a new thoron progeny detector based on SSNTD and the collection by an electric field. Radiat. Prot. Dosim. 145, 288– 294 (2011). 8. Tschiersch, J. and Meisenberg, O. The HMGU thoron house: a new tool for exposure assessment. Radiat. Prot. Dosim. 141, 395– 399 (2010). 9. Gierl, S., Meisenberg, O., Wielunski, M. and Tschiersch, J. An unattended device for high-voltage sampling of radon and thoron progeny. Rev. Sci. Instrum. 85, (2014). doi:10.1063/1.4865163. 10. Janik, M., Tokonami, S., Kranrod, C., Sorimachi, A., Ishikawa, T. and Hassan, N. M. International intercomparisons of integrating radon/thoron detectors with the NIRS radon/thoron chambers. Radiat. Prot. Dosim. 141, 436–439 (2010). 11. Bi, L., Li, W. B., Tschiersch, J. and Li, J. L. Age and sex dependent inhalation doses to members of the public from indoor thoron progeny. J. Radiol. Prot. 30, 639– 658 (2010). 12. Meisenberg, O. and Tschiersch, J. Specific properties of a model of thoron and its decay products in indoor atmospheres. Nukleonika 55, 463– 469 (2010).
doi:10.1093/rpd/ncu076
THORON AND THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES S. Gierl, O. Meisenberg*, P. Feistenauer and J. Tschiersch Helmholtz Zentrum Mu¨nchen, German Research Center for Environmental Health, Institute of Radiation Protection, Ingolsta¨dter Landstr. 1, 85764 Neuherberg, Germany *Corresponding author: [email protected]
The health risks of radon (222Rn) have long been known, it is considered the second most common cause for lung cancer after smoking(1). The risk of thoron (220Rn) on the other hand has long been neglected in many cases. It was assumed that thoron cannot reach indoor air due to its short half-life of 55.6 s. However, in recent years elevated thoron concentrations were found in dwellings built from unfired clay in China(2, 3) and India(4). This additional exposure significantly increases the dose to the inhabitants of those dwellings. In Germany unfired clay has been used as a building material in half-timbered houses in the past. Half-timbered houses were the prevalent building style from ancient times up to the 19th century. Those houses are made of wooden frames and the panels between the frames are filled with unfired clay. Eco-friendly building styles have become more popular in recent years. In those modern buildings clay is mainly used as indoor plaster. About 2 million houses that use unfired earthen building material exist in Germany today(5). In this study thoron and its progeny were measured in several traditional as well as modern houses in Bavaria (South-East Germany), which use unfired clay as a building material. Emphasis of the study was the direct long-term measurement of thoron decay products. The advantage of the measurement of the progeny is that the main contributors to the dose are directly measured and no assumptions on the equilibrium factor are necessary. Furthermore, the decay products are homogeneously distributed in the indoor environment, which is not the case for thoron itself(6). The results of gas and progeny measurements in 17 houses are presented. Additionally, the dose was calculated for the inhabitants of the houses.
METHODS Thoron progeny measurement A newly developed Unattended Battery-Operated Progeny Measurement Device (UBPM) was used (Figure 1). The UBPM is a further development of the instrument first introduced by Bi et al.(7). The instrument creates an electric field in the hemisphere made of wire mesh. The wire mesh is on ground potential, whereas the solid-state nuclear track detectors (CR39), which are placed in the middle of the hemisphere and used to register alpha decay, are at a potential of þ7.0 kV. Negatively charged progeny are attracted by the electric field. The instruments were calibrated against a working level monitor to measure progeny equilibrium equivalent concentration (EEC) with contributions from 212Pb and 212Bi. The calibration was performed inside the HMGU thoron experimental house(8). Further details on the UBPM can be found in Gierl et al.(9). Radon and thoron gas measurement For gas measurements CR39 is placed at the bottom of a plastic cup, which is 3.6-cm high. A measurement for radon and thoron together is performed by using a cup with a lid, which has several holes covered by a membrane filter and conductive foam. Therefore, only the radon and thoron gas can enter the cup but not the progeny. For measuring radon only, a cup is used with a closed lid without holes. In this case the gas can only enter the cup through the screw thread. The devices were calibrated against a RAD7 in a 1 m3 calibration chamber. Taking part in intercomparison measurements(10) regularly ensures the quality of the
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Sydney on November 13, 2015
In recent years, elevated thoron concentrations were found in houses built of unfired clay. In this study experiments were carried out in 17 traditional and modern clay houses in Germany to obtain an overview of indoor thoron in such houses. Long-term measurements over an 8-week period were performed using a newly developed Unattended Battery-Operated Progeny Measurement Device (UBPM) for measuring thoron progeny. This instrument uses a high-voltage electric field to precipitate radon and thoron progeny on nuclear track detectors. Additional active and passive measurements of radon, thoron and their progeny were performed. The equilibrium equivalent thoron concentration was found to be between 2 and 10 Bq m23. Gas concentrations were found to be between 20 and 160 Bq m23 for radon and between 10 and 90 Bq m23 for thoron 20 cm from the wall. The thoron exposure contributes significantly to the inhalation dose of the dwellers (0.6– 4 mSv a21).
THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES
RESULTS AND DISCUSSION The measurements were performed during late winter and spring 2013 in 17 Bavarian houses in which unfired clay was used as a building material. Apart from this,
the houses were selected randomly. Table 1 gives an overview of the houses and shows which measurements were performed in which house. Measurements were performed in several half-timbered houses, but also in modern houses with either clay plaster or clay panels. Measurements in house no. 7 were performed under two different conditions: during the gas measurements the construction of the house was leaky (measured air exchange rate of 4.0 h – 1, condition denoted as 7a), whereas before the progeny measurements the leaks in the inner and outer walls were caulked (denoted as 7b). Progeny concentrations The thoron progeny concentration measured with the UBPM in houses 1–9 is shown in Figure 2 along with the corresponding annual contribution to the inhalation dose. The measurements were performed over an 8-week period in late winter 2013. The annual dose was calculated assuming an occupancy factor of 40 %, which means an exposure time of 10 h d21. Dose conversion coefficients published in Bi et al.(11) were used for the calculation. The thoron progeny concentrations range between 2 and 10 Bq m23 and the resulting dose from 0.6 to 4 mSv a21. Radon and thoron gas concentrations
Figure
1. Unattended Battery-Operated Measurement Device (UBPM).
Progeny
In houses 1 –6 and 8–9, radon and thoron gas concentrations were measured by one radon-plus-thorondevice and one radon-only device, which were placed at a distance from the wall of 20 cm. Figure 3 shows the results. Thoron concentrations in these houses range from below the detection limit to 90 Bq m23. Radon concentrations were found in a range of 20 –160 Bq m23.
Table 1. Overview of the houses in which measurements were performed. No.
House type
Usage
Gas measurement
Progeny measurement
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Half-timbered Clay plaster Half-timbered Half-timbered Half-timbered Clay plaster Half-timbered Clay plaster Clay plaster Clay panels Clay panels Clay panels Clay panels Clay panels Clay panels Mixed (adobe, plaster, panels) Adobe
Residential Residential Residential Townhall Residential Residential Residential (unoccupied) Residential Rescue station Residential Residential Residential Residential Office Residential Residential Office
20 cm 20 cm 20 cm 20 cm 20 cm 20 cm Profile 20 cm 20 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm 10 cm 50 cm Profile Profile
x x x x x x x x x — — — — — — — —
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calibration. For thoron measurements with these devices at least one radon-only device has to be kept in the room with devices which measure radon and thoron. Measurements were performed in different distances from clay walls. When gas measurements were performed alongside progeny measurements, one radon plus thoron and one radon device were kept at a distance of 20 cm from the clay wall. When only gas measurements were performed, radon-plus-thoron devices were kept at distances of 10 and 50 cm from the wall, respectively. Additionally, in three houses distance profile measurements with several devices were performed.
S. GIERL ET AL.
In houses 7 and 10– 17, measurements were performed at distances of 10 and 50 cm from the clay wall. The results are shown in Figure 4. Thoron concentrations at a distance from the clay wall of 10 cm are slightly higher than thoron concentrations in houses 1–6 and 8–9 measured at a distance of 20 cm as expected. Therefore, it can be assumed that thoron progeny concentrations in houses 7 and 10– 17 are in the same range as thoron progeny concentrations in houses 1–6 and 8–9. Profile measurements
cðdÞ ¼ c0 expðd=d1=e Þ þ cm
ð1Þ
Figure 3. Radon and thoron gas concentrations in houses 1 –6 and 8– 9. The thoron concentrations in houses 1 and 5 were below the detection limit.
cm is the concentration in the middle of the room and c0 the concentration which is added to cm directly at the wall. d1/e is the distance in which the concentration decreases by a factor of 1/e. This exponential function is suitable to describe contributions of advection (exactly) and diffusion (approximately) to the transport of thoron(12). By integrating the function for all clay walls, the total thoron activity in the rooms could be calculated. Dividing by the volume of the room leads to the average thoron concentration. Multiplying this result with the equilibrium factor leads to the progeny concentration. Calculations of the equilibrium factor were done according to Meisenberg et al.(6). The air exchange rate that was used for the calculations was measured to be 4.0 h21 for house 7, whereas the air exchange rate in houses 16 and 17 was assumed to be 0.6 h21, which is a typical value for residential buildings with closed windows. The annual dose was calculated from the progeny concentration. The results can be found in Table 2. The difference between the results shown in Figure 2 and the calculated concentration and dose
Figure 4. Radon and thoron gas concentrations in houses 7 and 10–15. The error bars show the range from minimum to maximum value.
Figure 5. Concentration profiles of thoron in front of different walls in houses 7, 16 and 17. Inlay: scaled extract.
Figure 2. Thoron progeny concentration and annual dose in houses 1– 9. The bars indicate the combined uncertainty with contributions from counting statistics and the calibration factor at a coverage factor k ¼ 1.
162
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In houses 7, 16 and 17, profile measurements were performed. For each profile, seven measurement devices were set up with small lateral offset at different distances from 0 to 40 cm measured from the respective
wall. The thoron concentration profile from houses 7 and 16 (measured in front of two walls within one room each) and 17 showed different slopes: In house 7, in which relatively strong air turbulence prevailed because of the leaky construction, concentrations which were significantly increased compared with those in the middle of the room could still be found at distances of up to 15 cm (Figure 5). In the other two houses, the concentrations strongly decreased within only 5 cm. Very high concentrations near the walls were found in house no. 17, which is a one-room office building with massive adobe walls. With concentrations of up to 200 Bq m23 at distances of 10 and 20 cm, also in these houses progeny concentrations that are similar to those in houses 1–6 and 8 can be expected. From the thoron profiles the total thoron activity in the rooms can be calculated. Using the data analysis software SciDAVis, an exponential function was fitted to the data with a least-squares method:
THORON PROGENY MEASUREMENTS IN GERMAN CLAY HOUSES Table 2. Annual dose calculated from thoron gas concentration in rooms where profile measurements have been performed. No
Volume of the room [m3]
Thoron exhaling area [m2]
Average thoron concentration [Bq m23]
Equilibrium factor (%)
Annual dose [mSv a21]
7a 16 17
42.2 16.9 72.8
18.6 25.6 88.4
49.9+12.5 62.1+26.0 140.8+67.1
1.5 11 11
0.32+0.08 2.92+1.23 6.63+3.12
in Table 2 for house 7 result from the high air exchange rate in the house during the gas measurements.
Thoron progeny concentrations were measured in nine houses. Radon and thoron gas concentrations were measured in 17 houses and in 3 of those profile measurements were performed. In all houses unfired clay was used as a building material. The results show that increased thoron gas concentrations as well as thoron progeny concentrations can be found in these types of houses. The significant increase in the inhalation dose to the inhabitants shows that thoron is an issue of radiation protection in these houses and should be taken into account in further studies. The results suggest that thoron gas measurements are not enough to evaluate the dose accurately. Especially, the profile measurements show that there is no certain distance from the wall which can be used to estimate an average concentration. Gas measurements are suitable for a first assessment of the situation, but for thorough evaluations progeny concentrations should be measured. FUNDING This study was supported partly by the Federal Ministry of Education and Research (BMBF, contract 02NUK015B) and partly by the Bavarian State Ministry of the Environment and Consumer Protection (BayStMUV). Its contents are solely the responsibility of the authors. REFERENCES 1. World Health Organization. WHO handbook on indoor radon: a public health perspective, WHO, pp. 1– 94 (2009).
163
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CONCLUSION
2. Wiegand, J., Feige, S., Quingling, X., Schreiber, U., Wieditz, K., Wittman, C. H. and Xiarong, L. Radon and thoron in cave dwellings (Yan’an, China). Health Phys. 78, 438– 444 (2000). 3. Shang, B., Chen, B., Gao, Y., Cui, H. and Li, Z. Thoron levels in traditional Chinese residential dwellings. Radiat. Environ Biophys. 44, 193– 199 (2005). 4. Sreenath Reddy, M., Yadagiri Reddy, P., Rama Reddy, K., Eappen, K. P., Ramachandran, T. V. and Mayya, Y. S. Thoron levels in the dwellings of Hyderabad city, Andhra Pradesh, India. J. Environ. Radioact. 73, 21–28 (2004). 5. Schreckenbach, H. Building with earth—consumer information. German Association for Building with Earth. (2004). 6. Meisenberg, O. and Tschiersch, J. Thoron in indoor air: modeling for a better exposure estimate. Indoor Air 21, 240–252 (2011). 7. Bi, L., Tschiersch, J., Meisenberg, O., Wielunski, M., Li, J. L. and Shang, B. Development of a new thoron progeny detector based on SSNTD and the collection by an electric field. Radiat. Prot. Dosim. 145, 288– 294 (2011). 8. Tschiersch, J. and Meisenberg, O. The HMGU thoron house: a new tool for exposure assessment. Radiat. Prot. Dosim. 141, 395– 399 (2010). 9. Gierl, S., Meisenberg, O., Wielunski, M. and Tschiersch, J. An unattended device for high-voltage sampling of radon and thoron progeny. Rev. Sci. Instrum. 85, (2014). doi:10.1063/1.4865163. 10. Janik, M., Tokonami, S., Kranrod, C., Sorimachi, A., Ishikawa, T. and Hassan, N. M. International intercomparisons of integrating radon/thoron detectors with the NIRS radon/thoron chambers. Radiat. Prot. Dosim. 141, 436–439 (2010). 11. Bi, L., Li, W. B., Tschiersch, J. and Li, J. L. Age and sex dependent inhalation doses to members of the public from indoor thoron progeny. J. Radiol. Prot. 30, 639– 658 (2010). 12. Meisenberg, O. and Tschiersch, J. Specific properties of a model of thoron and its decay products in indoor atmospheres. Nukleonika 55, 463– 469 (2010).
