CONTRIBUTION OF THORON AND PROGENY TOWARDS INHALATION DOSE IN A THORIUM ABUNDANT BEACH ENVIRONMENT

CONTRIBUTION OF THORON AND PROGENY TOWARDS INHALATION DOSE IN A THORIUM ABUNDANT BEACH ENVIRONMENT Abstract In an environment having thorium rich soil the activity concentration of thoron in soil gas and ground-level outside air is comparable to that to radon. Recent reports indicate that in terms of the energy of the alpha particle decays of thoron’s progeny, its concentration in indoor air is significant, typically about half that due to radon progeny. We made a detailed radiometric profiling of inhalation dose to the population of the high background radiation area in the west southern coastal region of India. Here we report the results obtained from the long-term time integrated passive measurements of radon, thoron and their progeny concentrations in the high background radiation areas of Chavara and Neendakara hamlets of Kollam district. The equilibrium factors of radon and thoron with their progeny were determined for the region and was consistent with a previous study. The estimated value of total annual inhalation dose in the region ranged from 0.4 ± 0.06 to 3.7 ± 0.6 mSv y−1. The annual effective dose due to thoron and thoron progeny contributes ~35% to the total inhalation dose which means that thoron and its progeny is significant in assessing the radiation dose to the public. INTRODUCTION The principal component of total radiation exposure to the general public is the exposure to natural background radiation. Every earthen material contains primordial radionuclides 40K, 232Th and 238U and their decay products. Radon (222Rn) and thoron (220Rn) are radionuclides produced in 238U and 232Th radioactive decay series, respectively. These gases are normal constituents of soil gas enter into the indoor atmosphere. When inhaled, radon, thoron and their short-lived decay products deliver radiation dose to sensitive cells in the respiratory tract, which increases the risk of lung cancer(1). The worldwide annual average dose received from the inhalation of radon gas is 1.26 mSv y−1, which is ~52.5% of the total annual average dose received from all natural sources (UNSCEAR 2008)(2). Human technological and industrial activities may create or modify pathways increasing indoor radon and thoron concentrations. Once detected or identified these pathways can be controlled by preventive and corrective measures. Indoor radon, thoron and progeny levels vary dramatically depending on ventilation, wind speed, type of house, building materials, humidity, pressure and temperature(3–5). The air inside the houses is relatively warmer than the immediate subsoil air creating a negative pressure gradient towards inside(2). This results in the diffusion of radon and thoron rich soil gas into the indoor atmosphere through the cracks in the floor resulting in a high concentration of radon and thoron. The radiological significance of radon and its decay products has been extensively studied neglecting thoron due to its relatively small half-life (55.6 s). But recent studies show that the thoron and its decay products contribute significantly to the inhalation dose to the public(6–8). The decay products of radon (218Po and 214Po) and thoron (216Po and 212Pb) are electrostatically charged by birth and tend to attach themselves to aerosols, dust particles and water droplets to form radioactive clusters, which constitutes the attached fraction of progeny. Short-lived radon and thoron progeny either attached on aerosol particles or free carries a significant amount of potential alpha-energy concentration. The unattached fraction of the progeny is more important in determining the radiation dose to the human lung(9–12). The alpha decays imparting the radiation dose of the highest impact are those of 218Po and 214Po, for 222Rn series, 212Bi and 212Po for 220Rn series. The dose to the bronchial basal cells from unattached 218Po progeny is up to 38 times that from attached radon progeny because of its efficient deposition in the upper bronchial tree(13). However, the present study does not focus on the unattached fraction of progeny. The study area is well-known for its thorium abundant monazite sand having a very high population density. It is a natural laboratory for dosimetric and epidemiological studies. Even if the radon–thoron gas concentration studies in the region have been carried out at large by many investigators, a simultaneous investigation on their decay products concentrations have not been carried out in detail. The progeny measurements require special attention as the inhalation doses are predominantly due to them(14). Recently developed and well accepted direct radon and thoron progeny sensors were used to measure the concentration of progeny and pin-hole based twin cup dosimeters were used for radon–thoron measurement(15, 16). Chougaonkar et al.(17) and Pereira et al.(18) had earlier reported the radon and thoron levels in Neendakara and Chavara villages using the membrane based passive twin cup dosimeter. The present study employs newly developed, single entry pin-hole based dosimeter and direct progeny sensors. The equilibrium factors for radon and thoron were determined experimentally and used for the assessment of inhalation dose. EXPERIMENTAL AREA AND SELECTION OF HOUSES The experimental area selected for the investigation was the Chavara and Neendakara coastal hamlets of Kollam district in Kerala where high background radiation exists. The monazite sand in this region contains uranium oxide (~0.35%) and thorium oxide (~9%) along with rare earth minerals. This high level radon–thoron coastal environment is thickly populated (Figures 1 and 2). The measurements have been carried out in 141 houses of the region under investigation. The houses selected for the study were at least 10 years old. To study the effect of building materials on radon, thoron and progeny levels, measurements were carried out in different 58 houses. These houses were classified according to the type of building materials used: mud houses (P), cemented houses with a concrete roof (Q), houses with an asbestos roof, brick walls and cement floor (R) and houses with tile on the floor (S). The survey was carried out during the months of October 2016 to January 2017. Figure 1. View largeDownload slide Map of Kerala state in India with the geographic coordinates indicating the study region. Figure 1. View largeDownload slide Map of Kerala state in India with the geographic coordinates indicating the study region. Figure 2. View largeDownload slide Map of Kollam district showing the sampling sites (Map was created with QGIS software version 2.18.3 (http://www.qgis.org/en/site/).). Figure 2. View largeDownload slide Map of Kollam district showing the sampling sites (Map was created with QGIS software version 2.18.3 (http://www.qgis.org/en/site/).). EXPERIMENTAL METHODS Measurement of radon/thoron concentrations In the present study we employed single entry pin-hole based twin cup dosimeters for the estimation of radon and thoron concentration, the details, principle of detection and calibration of which are discussed elsewhere(16). The dosimeters were suspended in the indoor environment at the height of 1.5 ± 0.2 m from the floor, which is considered as the breathing zone of a standing adult(19), and 20 ± 5 cm from walls for 100 days. Corners were avoided so that they are not exposed to two exhaling surfaces. An effort was also made to determine the spatial distribution of radon and thoron in the indoor atmosphere of three types of houses (P, Q and R). Six pin-hole dosimeters were suspended at the same vertical level (1.5 m) in a matrix with a separation of 50 cm each. Measurement of radon/thoron progeny Direct radon and thoron progeny sensors (DRPS/DTPS) equipped with LR115 type II solid state nuclear track detectors and absorbers of suitable thickness were employed for progeny measurements. In the progeny sensors, the LR115 detector selectively detects alpha-particles emitted from 214Po of radon progeny (RnP) and 212Po of thoron progeny (TnP), which is deposited on the surface of the absorber. The absorber in DTPS is 50 μm aluminum mylar, which permits only the alpha particles of energy 8.78 MeV emitted from 212Po to pass through it. In DRPS the absorber has an effective thickness of 37 μm. It is a combination of 25 μm aluminum mylar and 12 μm cellulose nitrate peeled off from LR115 detector. DRPS detects alpha particles emitted from 214Po of energy 7.69 MeV and 212Po of energy 8.78 MeV. The equilibrium equivalent decay product concentrations were estimated from the track densities using the sensitivity factors provided Mishra and Mayya(14). DTPS is not sensitive to radon progeny as 50 μm thick aluminum mylar blocks the alpha particles emitted from 214Po. Hence, the tracks registered in DTPS are entirely due to thoron progeny, and that can be directly used to calculate equilibrium equivalent thoron concentration (EETC). But in DRPS, both and radon and thoron progenies register tracks. So it is necessary to subtract the track density of thoron progeny estimated from the DTPS using the following equation(20):   TrackdensityonlyduetoRnprogeny=(TrackdensityDRPS)−ηRTηTT(TrackdensityDTPS) (1) ηRT = 0.083, track registration efficiency of thoron progeny in DRPS; and ηTT = 0.01, track registration efficiency of thoron in DTPS. The equilibrium equivalent 222Rn (EERC) and 220Rn concentrations are calculated using the following formulae:   EETC(Bqm−3)=TrackdensityDTPSKT×Exposureperiod(d) (2)  EERC(Bqm−3)=TrackdensityOnlyduetoRnprogenyKR×Exposureperiod(d) (3)where KT and KR are the calibration factors for DTPS and DRPS, respectively. The values of sensitivity factors for DTPS and DRPS in the indoor atmosphere have been calculated by Mishra et al.(21) to be 0.94 Tracks cm−2 d−1/EETC (Bq m−3) for DTPS and 0.09 Tracks cm−2 d−1/EERC (Bq m−3) for DRPS. The minimum detection limit of DTPS and DRPS are 0.1 and 1 Bq m−3, respectively(22). The dose conversion factors reported by UNSCEAR (2000)(23) have been used to estimate the indoor inhalation dose rates D (mSv y−1) due to radon, thoron and their progeny, is given by the following equation:   D(mSvy−1)=8760×0.8×10−6×[(0.17+9×FR)CR+(0.11+40×FT)CT] (4)where, FR and FT are the equilibrium factors for radon and thoron progeny, respectively, and CR and CT are the average radon and thoron concentration, respectively. In the present study the equilibrium factors were determined for dose estimation. Estimation of equilibrium factors for radon and thoron The exposed LR115 detector films were etched in an etching bath using 2.5 N NaOH solutions at 60°C temperature for 90 min without stirring. The etched films were then carefully peeled off from the 100 μm thick polyester base. The tracks recorded on the films were then scanned using spark counter with a pre-spark voltage of 890 V and an operating voltage of 535 V. The average track densities obtained were then converted into radon, thoron and progeny concentrations using calibration factors discussed above. The equilibrium factors for radon and thoron are then calculated using the following equations:   Equilibriumfactorforradon(FRn)=EERCRadonconcentration (5)  Equilibriumfactorforradon(FTh)=EETCThoronconcentration (6) RESULT AND DISCUSSION Radon, thoron and their progeny levels The descriptive statistics of the results obtained for radon and thoron in 141 dwellings is presented in Table 1. The radon and thoron concentrations in the region have been found to vary from 7.8 ± 2.1 to 89 ± 7 Bq m−3 with a mean value of 24.2 ± 12.3 and 3.7 ± 1.2 Bq m−3 to 129 ± 12 Bq m−3 with a mean value of 36.7 ± 27.9 Bq m−3, respectively. The EERC and EETC were found to vary from 1.9 ± 0.2 to 31.5 ± 1.6 Bq m−3 with a mean value of 10.9 ± 5.8 Bq m−3 and BDL to 11.4 ± 0.7 Bq m−3 with a mean value of 1.6 ± 1.5 Bq m−3. The annual average concentrations were estimated from these measurements(24). These mean values are taken as the annual average concentrations of radon, thoron, EERC and EETC, respectively. The estimated values of annual average concentration of radon (24 Bq m−3) was found to be less than the global average value of 40 Bq m−3 and national average value of 42 Bq m−3(25). But for thoron estimated annual average concentration (36.7 Bq m−3) was found to be higher than the global average value of 10 Bq m−3 and national average value of 12.2 Bq m−3(25). Table 1. The range, arithmetic mean, geometric mean and median of the indoor radon, thoron and progeny levels and the inhalation dose imparted by their progeny.   Min  Max  AM ± SD  GM (GSD)  Median  Rn concentration (Bq m−3)  7.8 ± 2.1  89 ± 7  24.2 ± 12.3  21.6 (1.6)  22.0  RnP (EERC) (Bq m−3)  1.9 ± 0.2  31.5 ± 1.6  10.9 ± 5.80  9.50 (1.7)  10.2  EF-Rn  0.17  0.87  0.47 ± 0.18  0.44 (1.5)  0.46  Tn concentration (Bq m−3)  3.7 ± 1.2  129 ± 12  36.7 ± 27.9  28.1 (2.1)  28.1  Tnp (EETC) (Bq m−3)  11.4 ± 0.7  0.1 ± 0.04  1.60 ± 1.50  1.10 (2.4)  1.20  EF Tn  0.008  0.10  0.044 ± 0.027  0.035 (2)  0.034  Tnp/RnP  0.02  3.50  0.63 ± 0.49  0.46 (2.4)  0.52  Total AEDa (mSv y−1)  0.4 ± 0.06  3.7 ± 0.6  1.1 ± 0.60  1.0 (1.6)  1.10    Min  Max  AM ± SD  GM (GSD)  Median  Rn concentration (Bq m−3)  7.8 ± 2.1  89 ± 7  24.2 ± 12.3  21.6 (1.6)  22.0  RnP (EERC) (Bq m−3)  1.9 ± 0.2  31.5 ± 1.6  10.9 ± 5.80  9.50 (1.7)  10.2  EF-Rn  0.17  0.87  0.47 ± 0.18  0.44 (1.5)  0.46  Tn concentration (Bq m−3)  3.7 ± 1.2  129 ± 12  36.7 ± 27.9  28.1 (2.1)  28.1  Tnp (EETC) (Bq m−3)  11.4 ± 0.7  0.1 ± 0.04  1.60 ± 1.50  1.10 (2.4)  1.20  EF Tn  0.008  0.10  0.044 ± 0.027  0.035 (2)  0.034  Tnp/RnP  0.02  3.50  0.63 ± 0.49  0.46 (2.4)  0.52  Total AEDa (mSv y−1)  0.4 ± 0.06  3.7 ± 0.6  1.1 ± 0.60  1.0 (1.6)  1.10  aAnnual effective inhalation dose includes radon, thoron and their progeny. Frequency distribution of radon, thoron and their progeny concentrations in the 141 houses of the study area are shown in Figure 3. In 73% of houses, the radon concentration were found to be <30 Bq m−3 and thoron concentrations were <30 Bq m−3 in 54% houses. In total, 30% houses had thoron concentration in the range 30–60 Bq m−3. Significant correlation was obtained between EERC and EETC similar to those observed in recent studies (Figure 4a)(26). Log normal tests, Kolmogorov–Smirnov (K–S), Anderson–Darling (A–D), d’Agostino-K squared (d’A), were performed at α = 0.1. The distributions of Rn, Tn and RnP passed the normality test without outlier evaluation of data showing that they follow a log normal distribution. The reason of TnP for failing the test may be because of the use of only one set of detector. There are lots of uncertainties in etching and spark counting and the data may be inconsistent with the other observations. The uncertainties in measurement of TnP will reflect in the determination of equilibrium factor of thoron. It is recommended that at least three sets of dosimeters should be used to avoid this problem. Figure 3. View largeDownload slide Frequency distribution of radon and its progeny (a, b), thoron and its progeny (c and d) in 141 houses. Figure 3. View largeDownload slide Frequency distribution of radon and its progeny (a, b), thoron and its progeny (c and d) in 141 houses. Figure 4. View largeDownload slide Distribution of equilibrium factors of radon ad thoron in 141 houses (a) and the correlation between EERC and EETC (b). Figure 4. View largeDownload slide Distribution of equilibrium factors of radon ad thoron in 141 houses (a) and the correlation between EERC and EETC (b). The annual average equilibrium factor for radon and its progeny and thoron and its progeny were estimated from the measurements made in the winter season. It was found to vary from 0.17 to 0.87 with a mean value of 0.47 ± 0.18 for radon and 0.008 to 0.101 with a mean value of 0.043 ± 0.028 for thoron. The average value of equilibrium factor of radon and its progeny has been found to be in good agreement with the previously reported value (0.51 ± 0.16) for the region(27) and the value (0.4) recommended by UNSCEAR(28). The average value of the equilibrium factor for thoron and its progeny is found to be lower than the globally assumed value (0.1) as reported in UNSCEAR and in good agreement with the value reported for the region(27, 28). The distribution of equilibrium factors for radon and thoron is shown in Figure 4b. The total annual inhalation dose due to radon, thoron and their progeny have been found to vary from 0.4 to 3.7 mSv y−1 with a mean value of 1.1 ± 0.6 mSv y−1 and is presented in Table 2. The average ratio of dose due to TnP to RnP was obtained to be 0.63 ± 0.49(27). This strongly suggests that the dose contribution from TnP is highly significant. The annual effective dose due to the exposure of radon and its progeny in the study area has been found to vary from 0.13 to 2.04 mSv y−1 with an average of 0.71 ± 0.37 mSv y−1. The annual effective dose from the exposure to thoron and its progeny in the study area has been found to vary from 0.07 to 2.1 mSv y−1 with an average of 0.41 ± 0.33 mSv y−1. The geometric mean of total annual inhalation dose, annual effective dose due to radon and its progeny and annual effective dose due to thoron and its progeny was found to be 1.0 , 0.62 and 0.3 mSv y−1, respectively. Also the annual effective dose due to thoron and TnP ranges from 8 to 77% with a mean value of 35%. The frequency distribution of annual effective dose due to radon and its progeny, thoron and its progeny and annual inhalation dose in the study area is shown in Figure 5a–c. Table 2. Total annual inhalation dose due to radon, thoron and their progeny. House type    Rn (Bq m−3)  Tn (Bq m−3)  RnP (Bq m−3)  TnP (Bq m−3)  AED (mSv y−1)  P  Min  33 ± 4  58 ± 6  9.1 ± 1  3.7 ± 1  0.93 ± 0.1  Max  89 ± 7  129 ± 12  31.5 ± 2  11.4 ± 1  3.6 ± 0.3  AM ± SD  48 ± 18  95 ± 24  19 ± 7  4.7 ± 3  2.4 ± 1.0  Q  Min  13 ± 3  36 ± 4  4 ± 1  0.35 ± 0.1  0.42 ± 0.05  Max  51 ± 5  116 ± 11  18 ± 1  2.9 ± 0.3  1.5 ± 0.1  AM ± SD  26 ± 12  58 ± 21  11 ± 5  1.5 ± 0.7  1.1 ± 0.34  R  Min  11 ± 2  35 ± 4  4 ± 0.5  0.4 ± 0.1  0.4 ± 0.03  Max  46 ± 4  88 ± 9  25 ± 1  4.5 ± 0.4  2.7 ± 0.2  AM ± SD  20 ± 9  51 ± 16  10 ± 6  1.8 ± 1.0  1.3 ± 0.6  S  Min  10 ± 1  3.7 ± 0.2  4 ± 0.4  BDL  0.4 ± 0.1  Max  33 ± 3  19.4 ± 2  15 ± 1  1.6 ± 0.1  1.4 ± 0.1  AM ± SD  14 ± 6  14.7 ± 6  9.4 ± 4  0.7 ± 0.5  0.8 ± 0.3  House type    Rn (Bq m−3)  Tn (Bq m−3)  RnP (Bq m−3)  TnP (Bq m−3)  AED (mSv y−1)  P  Min  33 ± 4  58 ± 6  9.1 ± 1  3.7 ± 1  0.93 ± 0.1  Max  89 ± 7  129 ± 12  31.5 ± 2  11.4 ± 1  3.6 ± 0.3  AM ± SD  48 ± 18  95 ± 24  19 ± 7  4.7 ± 3  2.4 ± 1.0  Q  Min  13 ± 3  36 ± 4  4 ± 1  0.35 ± 0.1  0.42 ± 0.05  Max  51 ± 5  116 ± 11  18 ± 1  2.9 ± 0.3  1.5 ± 0.1  AM ± SD  26 ± 12  58 ± 21  11 ± 5  1.5 ± 0.7  1.1 ± 0.34  R  Min  11 ± 2  35 ± 4  4 ± 0.5  0.4 ± 0.1  0.4 ± 0.03  Max  46 ± 4  88 ± 9  25 ± 1  4.5 ± 0.4  2.7 ± 0.2  AM ± SD  20 ± 9  51 ± 16  10 ± 6  1.8 ± 1.0  1.3 ± 0.6  S  Min  10 ± 1  3.7 ± 0.2  4 ± 0.4  BDL  0.4 ± 0.1  Max  33 ± 3  19.4 ± 2  15 ± 1  1.6 ± 0.1  1.4 ± 0.1  AM ± SD  14 ± 6  14.7 ± 6  9.4 ± 4  0.7 ± 0.5  0.8 ± 0.3  Figure 5. View largeDownload slide (a) Annual effective dose due to radon and its progeny in 141 houses. (b) Annual effective dose due to thoron and its progeny in 141 houses. (c) Total annual inhalation dose in 141 houses. (d) Levels of radon, thoron and their progenies in different house types. Figure 5. View largeDownload slide (a) Annual effective dose due to radon and its progeny in 141 houses. (b) Annual effective dose due to thoron and its progeny in 141 houses. (c) Total annual inhalation dose in 141 houses. (d) Levels of radon, thoron and their progenies in different house types. Comparison among the types of houses A total of 58 houses were selected for the house type comparison and they were characterized into four categories, namely: mud houses (P), brick walled houses with a concrete roof (Q), houses with an asbestos roof brick walls and floor (R) and houses with tile on the floor (S). The average values of radon, thoron, EERC, EETC were found highest in mud block houses and lowest in houses with tile on floor. The results are displayed in Figure 5d. This may be attributed to the fact that in the houses with tiles on floor, the ‘source term’ is sealed, which restricts the entry of radon and thoron into the indoor atmosphere. For mud houses, the ‘source term’ is exposed to the indoor environment, resulting in a high concentration. In some houses, locally available sand and soil was used to fill the basement. Higher radon concentrations were reported in these houses irrespective of building materials. The spatial distribution of radon and thoron was studied in three houses of type P, Q and R. The radon concentration was found to be nearly uniform in the room(19), but the thoron concentration was fund to decrease as we move from the wall to the center of the room(29). This means that the thoron concentration at the center of the room could be considerably less than what we have obtained in this study. The spatial distribution of thoron and radon in different house types is shown in Figure 6. Figure 6. View largeDownload slide Spatial distribution of radon concentration (a) and thoron concentration (b) in different house types. Figure 6. View largeDownload slide Spatial distribution of radon concentration (a) and thoron concentration (b) in different house types. CONCLUSION The average value of thoron concentration was found to be higher than the global average of 10 Bq m−3 and national average of 12.2 Bq m−3, which is expected in a thorium rich environment. The radon, thoron, radon progeny concentrations in the study area were found to follow a log normal distribution. The EERC and EETC are positively correlated with R2 = 0.38.The average value of EERC and EETC in the study area have been found to be 10.9 ± 5.8 and 1.6 ± 1.5 Bq m−3, respectively. Also, the annual effective dose due to thoron and TnP contributes ~35% to the total inhalation dose which means that thoron and its progeny is significant in assessing the radiation dose to the public. Higher thoron concentrations were found mostly in dwellings with asbestos roofing cemented walls and mud houses. Unsealed thoron source term is the probable reason for higher thoron concentration in mud houses. Thoron spreads into the room not only by molecular diffusion but by many kinds of driving forces like convection. In the present case the ventilation enhances the convective flow of thoron and hence an increased diffusion length. The equilibrium factor of radon and its progeny for the region (0.47 ± 0.18) has been found to be in good agreement with the previously determined value (0.51 ± 0.16) for the region and the value (0.4) recommended by UNSCEAR. The equilibrium factor for thoron and its progeny is found to be lower than the globally assumed value (0.1) as reported in UNSCEAR and in good agreement with the value calculated for the region recently. ACKNOWLEDGMENTS We thankfully acknowledge the Technical support and advice extended by Dr Mohan P Chougaonkar, Former Head, PM & PDS, BARC, Mumbai. The cooperation of the residents in the study area is also acknowledged thankfully. FUNDING This work was supported by Board of Research in Nuclear Sciences [Grant no. 36(4)/14/45/2014-BRNS/1180], Department of Atomic Energy, Government of India. REFERENCES 1 Lubin, J. H. and Boice, J. D. Lung cancer risk from residential radon: meta-analysis of eight epidemiological studies. J. Natl. Cancer Inst.  89, 49– 57 ( 1997). Google Scholar CrossRef Search ADS PubMed  2 United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR 2008). ANNEXURE-E, 203 ( 2008). 3 Jonassen, N. On the effect of atmospheric pressure variations on the radon-222 concentration in unventilated rooms. Health Phys.  29, 216 ( 1975). Google Scholar PubMed  4 Martz, D. E., Rood, A. S., Gorge, J. L., Pearson, M. D. and Langer, H., Jr. Year to year variations in annual average indoor radon concentrations. Health Phys.  61, 413 ( 1991). 5 Nazaroff, W. W. and Doyle, S. M. Radon entry in the houses having crawl space. Health Phys.  48, 265 ( 1985). Google Scholar CrossRef Search ADS PubMed  6 Ramola, R. C., Prasad, M., Kandari, T., Pant, P., Bossew, P., Mishra, R. and Tokonami, S. Dose estimation derived from the exposure to radon, thoron and their progeny in the indoor environment. Sci. Rep.  6, 31061 ( 2016). Google Scholar CrossRef Search ADS PubMed  7 Mayya, Y. S. et al.  . Deposition based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies. Radiat. Prot. Dosimetry  152( 1–3), 18– 24 ( 2012). Google Scholar CrossRef Search ADS PubMed  8 Haque, A. K. M. M. and Collinson, A. J. L. Radiation dose to respiratory system due to radon and its daughter products. Health Phys.  13, 431 ( 1967). Google Scholar CrossRef Search ADS PubMed  9 Altshuler, B., Nelson, N. and Kuschner, M. Estimation of lung tissue dose from the inhalation of radon and its daughters. Health Phys.  10, 1137 ( 1964). Google Scholar CrossRef Search ADS PubMed  10 Jacobi, W. The dose to human respiratory tract by inhalation of short lived 222Rn decay products. Health Phys.  10, 1163 ( 1964). Google Scholar CrossRef Search ADS PubMed  11 Harley, N. H. and Pasternack, B. S. Experimental absorption applied to lung dose from thoron daughters. Health Phys.  17, 115 ( 1973). 12 NCRP Report no. 78. Evaluation of occupational and environmental exposures to radon and radon daughters in the United States. National Council on Radiation Protection and Measurements, Bethesda, MD, 1984. 13 Chamberlain, A. C. and Dyson, E. D. The dose to the trachea and bronchi from the decay products of radon and thoron. Br. J. Radiol.  29, 317 ( 1956). Google Scholar CrossRef Search ADS PubMed  14 Mishra, R. and Mayya, Y. S. Study of a deposition-based direct thoron progeny sensors (DTPS) technique for estimating equilibrium equivalent thoron concentration (EETC) in indoor environment. Radiat. Meas.  43, 1408– 1416 ( 2008). Google Scholar CrossRef Search ADS   15 Mishra, R., Sapra, B. K. and Mayya, Y. S. Multi parametric approach towards the assessment of radon and thoron progeny exposure. Rev. Sci. Instrum.  85( 2), 022105 ( 2014). Google Scholar CrossRef Search ADS PubMed  16 Sahoo, B. K., Sapra, B. K., Kanse, S. D., Gawarre, J. J. and Mayya, Y. S. A new pin-hole discriminated 222Rn/220Rn passive measurement device with single entry face. Radiat. Meas.  58, 52– 60 ( 2013). Google Scholar CrossRef Search ADS   17 Chougaonkar, M. P., Eappen, K. P., Ramachandran, T. V., Shetty, P. G., Mayya, Y. S., Sadashivan, S. and venkatraj, V. Profiles of dose to population living in the high background radiation areas in Kerala, India. J. Environ. Radioact.  71, 275– 297 ( 2004). Google Scholar CrossRef Search ADS   18 Pereira, C. E., Vaidyan, V. K., Chougaonkar, M. P., Mayya, Y. S., Sahoo, B. K. and Jojo, P. J. Indoor radon and thoron levels in Neendakara and Chavara regions of southern coastal Kerala, India. Radiat. Prot. Dosim.  150( 3), 385– 390 ( 2012). Google Scholar CrossRef Search ADS   19 Zhou, W., Iida, T., Moriizum, J., Aoyagi, T. and Takahashi, I. Simulation of concentrations and distributions of indoor radon and thoron. Radiat. Prot. Dosimetry  93( 4), 357– 368 ( 2001). Google Scholar CrossRef Search ADS PubMed  20 Prasad, M., Rawat, M., Dangwal, A., Yadav, M., Guasain, G. S., Mishra, R. and ramola, R. C. Measurement of radon and thoron progeny concentrations in dwellings of Tehri Garhwal, India, using LR-115 deposition-based DTPS/DRPS technique. Radiat. Prot. Dosimetry  167( 1–3), 102– 106 ( 2015). Google Scholar CrossRef Search ADS PubMed  21 Mishra, R., Prajith, R., Sapra, B. K. and Mayya, Y. S. Response of direct thoron progeny sensors (DTPS) to various aerosol concentrations and ventilation rates. Nucl. Instrum. Methods Phys. Res. B  268, 671– 675 ( 2010). Google Scholar CrossRef Search ADS   22 Mishra, R., Sapra, B. K. and Mayya, Y. S. Multi-parametric approach towards the assessment of radon and thoron progeny exposures. Rev. Sci. Instrum.  85( 2), 022105 ( 2014). Google Scholar CrossRef Search ADS PubMed  23 United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR 2008). ANNEXURE-B. Exposures from natural Radiation Sources, United Nations, p. 104 ( 2000). 24 Stojanovska, Z., Januseski, J., Bossew, P., Zunich, Z. S., Tollefsen, T. and Ristova, M. Seasonal indoor radon concentration in FYR Macedonia. Radiat. Meas.  46, 602– 610 ( 2011). Google Scholar CrossRef Search ADS   25 Ramola, R. C., Prasad, M., Kandari, T., Pant, P., Bossew, P., Mishra, R. and Tokonami, S. Dose estimation derived from the exposure of radon, thoron and their progeny in the indoor environment. Sci. Rep.  6, 31061 ( 2016). Google Scholar CrossRef Search ADS PubMed  26 Mishra, R. et al.  . An evaluation of thoron (and radon) equilibrium factor close to walls based on long-term measurements in dwellings. Radiat. Prot. Dosimetry  160, 164– 168 ( 2014). Google Scholar CrossRef Search ADS PubMed  27 Mayya, Y. S. et al.  . Deposition-based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies. Radiat. Prot. Dosimetry  152( 1–3), 18– 24 ( 2012). Google Scholar CrossRef Search ADS PubMed  28 UNSCEAR, United Nation Scientific Committee on the Effect of Atomic Radiation. Effects and Risks of Ionizing Radiation. Annex—B; Exposure from Natural Sources of Radiation. United Nations, New York ( 1993). 29 Chauhan, N., Chauhan, R. P., Joshi, M., Agarwal, T. K. and Sapra, B. K. Measurements and CFD modeling of indoor thoron distribution. Atmos. Environ.  105, 7– 13 ( 2015). Google Scholar CrossRef Search ADS   © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

CONTRIBUTION OF THORON AND PROGENY TOWARDS INHALATION DOSE IN A THORIUM ABUNDANT BEACH ENVIRONMENT

Loading next page...
 
/lp/ou_press/contribution-of-thoron-and-progeny-towards-inhalation-dose-in-a-ksIijHKZsf
Publisher
Oxford University Press
Copyright
© The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com
ISSN
0144-8420
eISSN
1742-3406
D.O.I.
10.1093/rpd/ncx126
Publisher site
See Article on Publisher Site

Abstract

Abstract In an environment having thorium rich soil the activity concentration of thoron in soil gas and ground-level outside air is comparable to that to radon. Recent reports indicate that in terms of the energy of the alpha particle decays of thoron’s progeny, its concentration in indoor air is significant, typically about half that due to radon progeny. We made a detailed radiometric profiling of inhalation dose to the population of the high background radiation area in the west southern coastal region of India. Here we report the results obtained from the long-term time integrated passive measurements of radon, thoron and their progeny concentrations in the high background radiation areas of Chavara and Neendakara hamlets of Kollam district. The equilibrium factors of radon and thoron with their progeny were determined for the region and was consistent with a previous study. The estimated value of total annual inhalation dose in the region ranged from 0.4 ± 0.06 to 3.7 ± 0.6 mSv y−1. The annual effective dose due to thoron and thoron progeny contributes ~35% to the total inhalation dose which means that thoron and its progeny is significant in assessing the radiation dose to the public. INTRODUCTION The principal component of total radiation exposure to the general public is the exposure to natural background radiation. Every earthen material contains primordial radionuclides 40K, 232Th and 238U and their decay products. Radon (222Rn) and thoron (220Rn) are radionuclides produced in 238U and 232Th radioactive decay series, respectively. These gases are normal constituents of soil gas enter into the indoor atmosphere. When inhaled, radon, thoron and their short-lived decay products deliver radiation dose to sensitive cells in the respiratory tract, which increases the risk of lung cancer(1). The worldwide annual average dose received from the inhalation of radon gas is 1.26 mSv y−1, which is ~52.5% of the total annual average dose received from all natural sources (UNSCEAR 2008)(2). Human technological and industrial activities may create or modify pathways increasing indoor radon and thoron concentrations. Once detected or identified these pathways can be controlled by preventive and corrective measures. Indoor radon, thoron and progeny levels vary dramatically depending on ventilation, wind speed, type of house, building materials, humidity, pressure and temperature(3–5). The air inside the houses is relatively warmer than the immediate subsoil air creating a negative pressure gradient towards inside(2). This results in the diffusion of radon and thoron rich soil gas into the indoor atmosphere through the cracks in the floor resulting in a high concentration of radon and thoron. The radiological significance of radon and its decay products has been extensively studied neglecting thoron due to its relatively small half-life (55.6 s). But recent studies show that the thoron and its decay products contribute significantly to the inhalation dose to the public(6–8). The decay products of radon (218Po and 214Po) and thoron (216Po and 212Pb) are electrostatically charged by birth and tend to attach themselves to aerosols, dust particles and water droplets to form radioactive clusters, which constitutes the attached fraction of progeny. Short-lived radon and thoron progeny either attached on aerosol particles or free carries a significant amount of potential alpha-energy concentration. The unattached fraction of the progeny is more important in determining the radiation dose to the human lung(9–12). The alpha decays imparting the radiation dose of the highest impact are those of 218Po and 214Po, for 222Rn series, 212Bi and 212Po for 220Rn series. The dose to the bronchial basal cells from unattached 218Po progeny is up to 38 times that from attached radon progeny because of its efficient deposition in the upper bronchial tree(13). However, the present study does not focus on the unattached fraction of progeny. The study area is well-known for its thorium abundant monazite sand having a very high population density. It is a natural laboratory for dosimetric and epidemiological studies. Even if the radon–thoron gas concentration studies in the region have been carried out at large by many investigators, a simultaneous investigation on their decay products concentrations have not been carried out in detail. The progeny measurements require special attention as the inhalation doses are predominantly due to them(14). Recently developed and well accepted direct radon and thoron progeny sensors were used to measure the concentration of progeny and pin-hole based twin cup dosimeters were used for radon–thoron measurement(15, 16). Chougaonkar et al.(17) and Pereira et al.(18) had earlier reported the radon and thoron levels in Neendakara and Chavara villages using the membrane based passive twin cup dosimeter. The present study employs newly developed, single entry pin-hole based dosimeter and direct progeny sensors. The equilibrium factors for radon and thoron were determined experimentally and used for the assessment of inhalation dose. EXPERIMENTAL AREA AND SELECTION OF HOUSES The experimental area selected for the investigation was the Chavara and Neendakara coastal hamlets of Kollam district in Kerala where high background radiation exists. The monazite sand in this region contains uranium oxide (~0.35%) and thorium oxide (~9%) along with rare earth minerals. This high level radon–thoron coastal environment is thickly populated (Figures 1 and 2). The measurements have been carried out in 141 houses of the region under investigation. The houses selected for the study were at least 10 years old. To study the effect of building materials on radon, thoron and progeny levels, measurements were carried out in different 58 houses. These houses were classified according to the type of building materials used: mud houses (P), cemented houses with a concrete roof (Q), houses with an asbestos roof, brick walls and cement floor (R) and houses with tile on the floor (S). The survey was carried out during the months of October 2016 to January 2017. Figure 1. View largeDownload slide Map of Kerala state in India with the geographic coordinates indicating the study region. Figure 1. View largeDownload slide Map of Kerala state in India with the geographic coordinates indicating the study region. Figure 2. View largeDownload slide Map of Kollam district showing the sampling sites (Map was created with QGIS software version 2.18.3 (http://www.qgis.org/en/site/).). Figure 2. View largeDownload slide Map of Kollam district showing the sampling sites (Map was created with QGIS software version 2.18.3 (http://www.qgis.org/en/site/).). EXPERIMENTAL METHODS Measurement of radon/thoron concentrations In the present study we employed single entry pin-hole based twin cup dosimeters for the estimation of radon and thoron concentration, the details, principle of detection and calibration of which are discussed elsewhere(16). The dosimeters were suspended in the indoor environment at the height of 1.5 ± 0.2 m from the floor, which is considered as the breathing zone of a standing adult(19), and 20 ± 5 cm from walls for 100 days. Corners were avoided so that they are not exposed to two exhaling surfaces. An effort was also made to determine the spatial distribution of radon and thoron in the indoor atmosphere of three types of houses (P, Q and R). Six pin-hole dosimeters were suspended at the same vertical level (1.5 m) in a matrix with a separation of 50 cm each. Measurement of radon/thoron progeny Direct radon and thoron progeny sensors (DRPS/DTPS) equipped with LR115 type II solid state nuclear track detectors and absorbers of suitable thickness were employed for progeny measurements. In the progeny sensors, the LR115 detector selectively detects alpha-particles emitted from 214Po of radon progeny (RnP) and 212Po of thoron progeny (TnP), which is deposited on the surface of the absorber. The absorber in DTPS is 50 μm aluminum mylar, which permits only the alpha particles of energy 8.78 MeV emitted from 212Po to pass through it. In DRPS the absorber has an effective thickness of 37 μm. It is a combination of 25 μm aluminum mylar and 12 μm cellulose nitrate peeled off from LR115 detector. DRPS detects alpha particles emitted from 214Po of energy 7.69 MeV and 212Po of energy 8.78 MeV. The equilibrium equivalent decay product concentrations were estimated from the track densities using the sensitivity factors provided Mishra and Mayya(14). DTPS is not sensitive to radon progeny as 50 μm thick aluminum mylar blocks the alpha particles emitted from 214Po. Hence, the tracks registered in DTPS are entirely due to thoron progeny, and that can be directly used to calculate equilibrium equivalent thoron concentration (EETC). But in DRPS, both and radon and thoron progenies register tracks. So it is necessary to subtract the track density of thoron progeny estimated from the DTPS using the following equation(20):   TrackdensityonlyduetoRnprogeny=(TrackdensityDRPS)−ηRTηTT(TrackdensityDTPS) (1) ηRT = 0.083, track registration efficiency of thoron progeny in DRPS; and ηTT = 0.01, track registration efficiency of thoron in DTPS. The equilibrium equivalent 222Rn (EERC) and 220Rn concentrations are calculated using the following formulae:   EETC(Bqm−3)=TrackdensityDTPSKT×Exposureperiod(d) (2)  EERC(Bqm−3)=TrackdensityOnlyduetoRnprogenyKR×Exposureperiod(d) (3)where KT and KR are the calibration factors for DTPS and DRPS, respectively. The values of sensitivity factors for DTPS and DRPS in the indoor atmosphere have been calculated by Mishra et al.(21) to be 0.94 Tracks cm−2 d−1/EETC (Bq m−3) for DTPS and 0.09 Tracks cm−2 d−1/EERC (Bq m−3) for DRPS. The minimum detection limit of DTPS and DRPS are 0.1 and 1 Bq m−3, respectively(22). The dose conversion factors reported by UNSCEAR (2000)(23) have been used to estimate the indoor inhalation dose rates D (mSv y−1) due to radon, thoron and their progeny, is given by the following equation:   D(mSvy−1)=8760×0.8×10−6×[(0.17+9×FR)CR+(0.11+40×FT)CT] (4)where, FR and FT are the equilibrium factors for radon and thoron progeny, respectively, and CR and CT are the average radon and thoron concentration, respectively. In the present study the equilibrium factors were determined for dose estimation. Estimation of equilibrium factors for radon and thoron The exposed LR115 detector films were etched in an etching bath using 2.5 N NaOH solutions at 60°C temperature for 90 min without stirring. The etched films were then carefully peeled off from the 100 μm thick polyester base. The tracks recorded on the films were then scanned using spark counter with a pre-spark voltage of 890 V and an operating voltage of 535 V. The average track densities obtained were then converted into radon, thoron and progeny concentrations using calibration factors discussed above. The equilibrium factors for radon and thoron are then calculated using the following equations:   Equilibriumfactorforradon(FRn)=EERCRadonconcentration (5)  Equilibriumfactorforradon(FTh)=EETCThoronconcentration (6) RESULT AND DISCUSSION Radon, thoron and their progeny levels The descriptive statistics of the results obtained for radon and thoron in 141 dwellings is presented in Table 1. The radon and thoron concentrations in the region have been found to vary from 7.8 ± 2.1 to 89 ± 7 Bq m−3 with a mean value of 24.2 ± 12.3 and 3.7 ± 1.2 Bq m−3 to 129 ± 12 Bq m−3 with a mean value of 36.7 ± 27.9 Bq m−3, respectively. The EERC and EETC were found to vary from 1.9 ± 0.2 to 31.5 ± 1.6 Bq m−3 with a mean value of 10.9 ± 5.8 Bq m−3 and BDL to 11.4 ± 0.7 Bq m−3 with a mean value of 1.6 ± 1.5 Bq m−3. The annual average concentrations were estimated from these measurements(24). These mean values are taken as the annual average concentrations of radon, thoron, EERC and EETC, respectively. The estimated values of annual average concentration of radon (24 Bq m−3) was found to be less than the global average value of 40 Bq m−3 and national average value of 42 Bq m−3(25). But for thoron estimated annual average concentration (36.7 Bq m−3) was found to be higher than the global average value of 10 Bq m−3 and national average value of 12.2 Bq m−3(25). Table 1. The range, arithmetic mean, geometric mean and median of the indoor radon, thoron and progeny levels and the inhalation dose imparted by their progeny.   Min  Max  AM ± SD  GM (GSD)  Median  Rn concentration (Bq m−3)  7.8 ± 2.1  89 ± 7  24.2 ± 12.3  21.6 (1.6)  22.0  RnP (EERC) (Bq m−3)  1.9 ± 0.2  31.5 ± 1.6  10.9 ± 5.80  9.50 (1.7)  10.2  EF-Rn  0.17  0.87  0.47 ± 0.18  0.44 (1.5)  0.46  Tn concentration (Bq m−3)  3.7 ± 1.2  129 ± 12  36.7 ± 27.9  28.1 (2.1)  28.1  Tnp (EETC) (Bq m−3)  11.4 ± 0.7  0.1 ± 0.04  1.60 ± 1.50  1.10 (2.4)  1.20  EF Tn  0.008  0.10  0.044 ± 0.027  0.035 (2)  0.034  Tnp/RnP  0.02  3.50  0.63 ± 0.49  0.46 (2.4)  0.52  Total AEDa (mSv y−1)  0.4 ± 0.06  3.7 ± 0.6  1.1 ± 0.60  1.0 (1.6)  1.10    Min  Max  AM ± SD  GM (GSD)  Median  Rn concentration (Bq m−3)  7.8 ± 2.1  89 ± 7  24.2 ± 12.3  21.6 (1.6)  22.0  RnP (EERC) (Bq m−3)  1.9 ± 0.2  31.5 ± 1.6  10.9 ± 5.80  9.50 (1.7)  10.2  EF-Rn  0.17  0.87  0.47 ± 0.18  0.44 (1.5)  0.46  Tn concentration (Bq m−3)  3.7 ± 1.2  129 ± 12  36.7 ± 27.9  28.1 (2.1)  28.1  Tnp (EETC) (Bq m−3)  11.4 ± 0.7  0.1 ± 0.04  1.60 ± 1.50  1.10 (2.4)  1.20  EF Tn  0.008  0.10  0.044 ± 0.027  0.035 (2)  0.034  Tnp/RnP  0.02  3.50  0.63 ± 0.49  0.46 (2.4)  0.52  Total AEDa (mSv y−1)  0.4 ± 0.06  3.7 ± 0.6  1.1 ± 0.60  1.0 (1.6)  1.10  aAnnual effective inhalation dose includes radon, thoron and their progeny. Frequency distribution of radon, thoron and their progeny concentrations in the 141 houses of the study area are shown in Figure 3. In 73% of houses, the radon concentration were found to be <30 Bq m−3 and thoron concentrations were <30 Bq m−3 in 54% houses. In total, 30% houses had thoron concentration in the range 30–60 Bq m−3. Significant correlation was obtained between EERC and EETC similar to those observed in recent studies (Figure 4a)(26). Log normal tests, Kolmogorov–Smirnov (K–S), Anderson–Darling (A–D), d’Agostino-K squared (d’A), were performed at α = 0.1. The distributions of Rn, Tn and RnP passed the normality test without outlier evaluation of data showing that they follow a log normal distribution. The reason of TnP for failing the test may be because of the use of only one set of detector. There are lots of uncertainties in etching and spark counting and the data may be inconsistent with the other observations. The uncertainties in measurement of TnP will reflect in the determination of equilibrium factor of thoron. It is recommended that at least three sets of dosimeters should be used to avoid this problem. Figure 3. View largeDownload slide Frequency distribution of radon and its progeny (a, b), thoron and its progeny (c and d) in 141 houses. Figure 3. View largeDownload slide Frequency distribution of radon and its progeny (a, b), thoron and its progeny (c and d) in 141 houses. Figure 4. View largeDownload slide Distribution of equilibrium factors of radon ad thoron in 141 houses (a) and the correlation between EERC and EETC (b). Figure 4. View largeDownload slide Distribution of equilibrium factors of radon ad thoron in 141 houses (a) and the correlation between EERC and EETC (b). The annual average equilibrium factor for radon and its progeny and thoron and its progeny were estimated from the measurements made in the winter season. It was found to vary from 0.17 to 0.87 with a mean value of 0.47 ± 0.18 for radon and 0.008 to 0.101 with a mean value of 0.043 ± 0.028 for thoron. The average value of equilibrium factor of radon and its progeny has been found to be in good agreement with the previously reported value (0.51 ± 0.16) for the region(27) and the value (0.4) recommended by UNSCEAR(28). The average value of the equilibrium factor for thoron and its progeny is found to be lower than the globally assumed value (0.1) as reported in UNSCEAR and in good agreement with the value reported for the region(27, 28). The distribution of equilibrium factors for radon and thoron is shown in Figure 4b. The total annual inhalation dose due to radon, thoron and their progeny have been found to vary from 0.4 to 3.7 mSv y−1 with a mean value of 1.1 ± 0.6 mSv y−1 and is presented in Table 2. The average ratio of dose due to TnP to RnP was obtained to be 0.63 ± 0.49(27). This strongly suggests that the dose contribution from TnP is highly significant. The annual effective dose due to the exposure of radon and its progeny in the study area has been found to vary from 0.13 to 2.04 mSv y−1 with an average of 0.71 ± 0.37 mSv y−1. The annual effective dose from the exposure to thoron and its progeny in the study area has been found to vary from 0.07 to 2.1 mSv y−1 with an average of 0.41 ± 0.33 mSv y−1. The geometric mean of total annual inhalation dose, annual effective dose due to radon and its progeny and annual effective dose due to thoron and its progeny was found to be 1.0 , 0.62 and 0.3 mSv y−1, respectively. Also the annual effective dose due to thoron and TnP ranges from 8 to 77% with a mean value of 35%. The frequency distribution of annual effective dose due to radon and its progeny, thoron and its progeny and annual inhalation dose in the study area is shown in Figure 5a–c. Table 2. Total annual inhalation dose due to radon, thoron and their progeny. House type    Rn (Bq m−3)  Tn (Bq m−3)  RnP (Bq m−3)  TnP (Bq m−3)  AED (mSv y−1)  P  Min  33 ± 4  58 ± 6  9.1 ± 1  3.7 ± 1  0.93 ± 0.1  Max  89 ± 7  129 ± 12  31.5 ± 2  11.4 ± 1  3.6 ± 0.3  AM ± SD  48 ± 18  95 ± 24  19 ± 7  4.7 ± 3  2.4 ± 1.0  Q  Min  13 ± 3  36 ± 4  4 ± 1  0.35 ± 0.1  0.42 ± 0.05  Max  51 ± 5  116 ± 11  18 ± 1  2.9 ± 0.3  1.5 ± 0.1  AM ± SD  26 ± 12  58 ± 21  11 ± 5  1.5 ± 0.7  1.1 ± 0.34  R  Min  11 ± 2  35 ± 4  4 ± 0.5  0.4 ± 0.1  0.4 ± 0.03  Max  46 ± 4  88 ± 9  25 ± 1  4.5 ± 0.4  2.7 ± 0.2  AM ± SD  20 ± 9  51 ± 16  10 ± 6  1.8 ± 1.0  1.3 ± 0.6  S  Min  10 ± 1  3.7 ± 0.2  4 ± 0.4  BDL  0.4 ± 0.1  Max  33 ± 3  19.4 ± 2  15 ± 1  1.6 ± 0.1  1.4 ± 0.1  AM ± SD  14 ± 6  14.7 ± 6  9.4 ± 4  0.7 ± 0.5  0.8 ± 0.3  House type    Rn (Bq m−3)  Tn (Bq m−3)  RnP (Bq m−3)  TnP (Bq m−3)  AED (mSv y−1)  P  Min  33 ± 4  58 ± 6  9.1 ± 1  3.7 ± 1  0.93 ± 0.1  Max  89 ± 7  129 ± 12  31.5 ± 2  11.4 ± 1  3.6 ± 0.3  AM ± SD  48 ± 18  95 ± 24  19 ± 7  4.7 ± 3  2.4 ± 1.0  Q  Min  13 ± 3  36 ± 4  4 ± 1  0.35 ± 0.1  0.42 ± 0.05  Max  51 ± 5  116 ± 11  18 ± 1  2.9 ± 0.3  1.5 ± 0.1  AM ± SD  26 ± 12  58 ± 21  11 ± 5  1.5 ± 0.7  1.1 ± 0.34  R  Min  11 ± 2  35 ± 4  4 ± 0.5  0.4 ± 0.1  0.4 ± 0.03  Max  46 ± 4  88 ± 9  25 ± 1  4.5 ± 0.4  2.7 ± 0.2  AM ± SD  20 ± 9  51 ± 16  10 ± 6  1.8 ± 1.0  1.3 ± 0.6  S  Min  10 ± 1  3.7 ± 0.2  4 ± 0.4  BDL  0.4 ± 0.1  Max  33 ± 3  19.4 ± 2  15 ± 1  1.6 ± 0.1  1.4 ± 0.1  AM ± SD  14 ± 6  14.7 ± 6  9.4 ± 4  0.7 ± 0.5  0.8 ± 0.3  Figure 5. View largeDownload slide (a) Annual effective dose due to radon and its progeny in 141 houses. (b) Annual effective dose due to thoron and its progeny in 141 houses. (c) Total annual inhalation dose in 141 houses. (d) Levels of radon, thoron and their progenies in different house types. Figure 5. View largeDownload slide (a) Annual effective dose due to radon and its progeny in 141 houses. (b) Annual effective dose due to thoron and its progeny in 141 houses. (c) Total annual inhalation dose in 141 houses. (d) Levels of radon, thoron and their progenies in different house types. Comparison among the types of houses A total of 58 houses were selected for the house type comparison and they were characterized into four categories, namely: mud houses (P), brick walled houses with a concrete roof (Q), houses with an asbestos roof brick walls and floor (R) and houses with tile on the floor (S). The average values of radon, thoron, EERC, EETC were found highest in mud block houses and lowest in houses with tile on floor. The results are displayed in Figure 5d. This may be attributed to the fact that in the houses with tiles on floor, the ‘source term’ is sealed, which restricts the entry of radon and thoron into the indoor atmosphere. For mud houses, the ‘source term’ is exposed to the indoor environment, resulting in a high concentration. In some houses, locally available sand and soil was used to fill the basement. Higher radon concentrations were reported in these houses irrespective of building materials. The spatial distribution of radon and thoron was studied in three houses of type P, Q and R. The radon concentration was found to be nearly uniform in the room(19), but the thoron concentration was fund to decrease as we move from the wall to the center of the room(29). This means that the thoron concentration at the center of the room could be considerably less than what we have obtained in this study. The spatial distribution of thoron and radon in different house types is shown in Figure 6. Figure 6. View largeDownload slide Spatial distribution of radon concentration (a) and thoron concentration (b) in different house types. Figure 6. View largeDownload slide Spatial distribution of radon concentration (a) and thoron concentration (b) in different house types. CONCLUSION The average value of thoron concentration was found to be higher than the global average of 10 Bq m−3 and national average of 12.2 Bq m−3, which is expected in a thorium rich environment. The radon, thoron, radon progeny concentrations in the study area were found to follow a log normal distribution. The EERC and EETC are positively correlated with R2 = 0.38.The average value of EERC and EETC in the study area have been found to be 10.9 ± 5.8 and 1.6 ± 1.5 Bq m−3, respectively. Also, the annual effective dose due to thoron and TnP contributes ~35% to the total inhalation dose which means that thoron and its progeny is significant in assessing the radiation dose to the public. Higher thoron concentrations were found mostly in dwellings with asbestos roofing cemented walls and mud houses. Unsealed thoron source term is the probable reason for higher thoron concentration in mud houses. Thoron spreads into the room not only by molecular diffusion but by many kinds of driving forces like convection. In the present case the ventilation enhances the convective flow of thoron and hence an increased diffusion length. The equilibrium factor of radon and its progeny for the region (0.47 ± 0.18) has been found to be in good agreement with the previously determined value (0.51 ± 0.16) for the region and the value (0.4) recommended by UNSCEAR. The equilibrium factor for thoron and its progeny is found to be lower than the globally assumed value (0.1) as reported in UNSCEAR and in good agreement with the value calculated for the region recently. ACKNOWLEDGMENTS We thankfully acknowledge the Technical support and advice extended by Dr Mohan P Chougaonkar, Former Head, PM & PDS, BARC, Mumbai. The cooperation of the residents in the study area is also acknowledged thankfully. FUNDING This work was supported by Board of Research in Nuclear Sciences [Grant no. 36(4)/14/45/2014-BRNS/1180], Department of Atomic Energy, Government of India. REFERENCES 1 Lubin, J. H. and Boice, J. D. Lung cancer risk from residential radon: meta-analysis of eight epidemiological studies. J. Natl. Cancer Inst.  89, 49– 57 ( 1997). Google Scholar CrossRef Search ADS PubMed  2 United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR 2008). ANNEXURE-E, 203 ( 2008). 3 Jonassen, N. On the effect of atmospheric pressure variations on the radon-222 concentration in unventilated rooms. Health Phys.  29, 216 ( 1975). Google Scholar PubMed  4 Martz, D. E., Rood, A. S., Gorge, J. L., Pearson, M. D. and Langer, H., Jr. Year to year variations in annual average indoor radon concentrations. Health Phys.  61, 413 ( 1991). 5 Nazaroff, W. W. and Doyle, S. M. Radon entry in the houses having crawl space. Health Phys.  48, 265 ( 1985). Google Scholar CrossRef Search ADS PubMed  6 Ramola, R. C., Prasad, M., Kandari, T., Pant, P., Bossew, P., Mishra, R. and Tokonami, S. Dose estimation derived from the exposure to radon, thoron and their progeny in the indoor environment. Sci. Rep.  6, 31061 ( 2016). Google Scholar CrossRef Search ADS PubMed  7 Mayya, Y. S. et al.  . Deposition based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies. Radiat. Prot. Dosimetry  152( 1–3), 18– 24 ( 2012). Google Scholar CrossRef Search ADS PubMed  8 Haque, A. K. M. M. and Collinson, A. J. L. Radiation dose to respiratory system due to radon and its daughter products. Health Phys.  13, 431 ( 1967). Google Scholar CrossRef Search ADS PubMed  9 Altshuler, B., Nelson, N. and Kuschner, M. Estimation of lung tissue dose from the inhalation of radon and its daughters. Health Phys.  10, 1137 ( 1964). Google Scholar CrossRef Search ADS PubMed  10 Jacobi, W. The dose to human respiratory tract by inhalation of short lived 222Rn decay products. Health Phys.  10, 1163 ( 1964). Google Scholar CrossRef Search ADS PubMed  11 Harley, N. H. and Pasternack, B. S. Experimental absorption applied to lung dose from thoron daughters. Health Phys.  17, 115 ( 1973). 12 NCRP Report no. 78. Evaluation of occupational and environmental exposures to radon and radon daughters in the United States. National Council on Radiation Protection and Measurements, Bethesda, MD, 1984. 13 Chamberlain, A. C. and Dyson, E. D. The dose to the trachea and bronchi from the decay products of radon and thoron. Br. J. Radiol.  29, 317 ( 1956). Google Scholar CrossRef Search ADS PubMed  14 Mishra, R. and Mayya, Y. S. Study of a deposition-based direct thoron progeny sensors (DTPS) technique for estimating equilibrium equivalent thoron concentration (EETC) in indoor environment. Radiat. Meas.  43, 1408– 1416 ( 2008). Google Scholar CrossRef Search ADS   15 Mishra, R., Sapra, B. K. and Mayya, Y. S. Multi parametric approach towards the assessment of radon and thoron progeny exposure. Rev. Sci. Instrum.  85( 2), 022105 ( 2014). Google Scholar CrossRef Search ADS PubMed  16 Sahoo, B. K., Sapra, B. K., Kanse, S. D., Gawarre, J. J. and Mayya, Y. S. A new pin-hole discriminated 222Rn/220Rn passive measurement device with single entry face. Radiat. Meas.  58, 52– 60 ( 2013). Google Scholar CrossRef Search ADS   17 Chougaonkar, M. P., Eappen, K. P., Ramachandran, T. V., Shetty, P. G., Mayya, Y. S., Sadashivan, S. and venkatraj, V. Profiles of dose to population living in the high background radiation areas in Kerala, India. J. Environ. Radioact.  71, 275– 297 ( 2004). Google Scholar CrossRef Search ADS   18 Pereira, C. E., Vaidyan, V. K., Chougaonkar, M. P., Mayya, Y. S., Sahoo, B. K. and Jojo, P. J. Indoor radon and thoron levels in Neendakara and Chavara regions of southern coastal Kerala, India. Radiat. Prot. Dosim.  150( 3), 385– 390 ( 2012). Google Scholar CrossRef Search ADS   19 Zhou, W., Iida, T., Moriizum, J., Aoyagi, T. and Takahashi, I. Simulation of concentrations and distributions of indoor radon and thoron. Radiat. Prot. Dosimetry  93( 4), 357– 368 ( 2001). Google Scholar CrossRef Search ADS PubMed  20 Prasad, M., Rawat, M., Dangwal, A., Yadav, M., Guasain, G. S., Mishra, R. and ramola, R. C. Measurement of radon and thoron progeny concentrations in dwellings of Tehri Garhwal, India, using LR-115 deposition-based DTPS/DRPS technique. Radiat. Prot. Dosimetry  167( 1–3), 102– 106 ( 2015). Google Scholar CrossRef Search ADS PubMed  21 Mishra, R., Prajith, R., Sapra, B. K. and Mayya, Y. S. Response of direct thoron progeny sensors (DTPS) to various aerosol concentrations and ventilation rates. Nucl. Instrum. Methods Phys. Res. B  268, 671– 675 ( 2010). Google Scholar CrossRef Search ADS   22 Mishra, R., Sapra, B. K. and Mayya, Y. S. Multi-parametric approach towards the assessment of radon and thoron progeny exposures. Rev. Sci. Instrum.  85( 2), 022105 ( 2014). Google Scholar CrossRef Search ADS PubMed  23 United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR 2008). ANNEXURE-B. Exposures from natural Radiation Sources, United Nations, p. 104 ( 2000). 24 Stojanovska, Z., Januseski, J., Bossew, P., Zunich, Z. S., Tollefsen, T. and Ristova, M. Seasonal indoor radon concentration in FYR Macedonia. Radiat. Meas.  46, 602– 610 ( 2011). Google Scholar CrossRef Search ADS   25 Ramola, R. C., Prasad, M., Kandari, T., Pant, P., Bossew, P., Mishra, R. and Tokonami, S. Dose estimation derived from the exposure of radon, thoron and their progeny in the indoor environment. Sci. Rep.  6, 31061 ( 2016). Google Scholar CrossRef Search ADS PubMed  26 Mishra, R. et al.  . An evaluation of thoron (and radon) equilibrium factor close to walls based on long-term measurements in dwellings. Radiat. Prot. Dosimetry  160, 164– 168 ( 2014). Google Scholar CrossRef Search ADS PubMed  27 Mayya, Y. S. et al.  . Deposition-based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies. Radiat. Prot. Dosimetry  152( 1–3), 18– 24 ( 2012). Google Scholar CrossRef Search ADS PubMed  28 UNSCEAR, United Nation Scientific Committee on the Effect of Atomic Radiation. Effects and Risks of Ionizing Radiation. Annex—B; Exposure from Natural Sources of Radiation. United Nations, New York ( 1993). 29 Chauhan, N., Chauhan, R. P., Joshi, M., Agarwal, T. K. and Sapra, B. K. Measurements and CFD modeling of indoor thoron distribution. Atmos. Environ.  105, 7– 13 ( 2015). Google Scholar CrossRef Search ADS   © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

Journal

Radiation Protection DosimetryOxford University Press

Published: Mar 1, 2018

There are no references for this article.

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 12 million articles from more than
10,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Unlimited reading

Read as many articles as you need. Full articles with original layout, charts and figures. Read online, from anywhere.

Stay up to date

Keep up with your field with Personalized Recommendations and Follow Journals to get automatic updates.

Organize your research

It’s easy to organize your research with our built-in tools.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve Freelancer

DeepDyve Pro

Price
FREE
$49/month

$360/year
Save searches from
Google Scholar,
PubMed
Create lists to
organize your research
Export lists, citations
Read DeepDyve articles
Abstract access only
Unlimited access to over
18 million full-text articles
Print
20 pages/month
PDF Discount
20% off