Abstract Indoor measurements of radon and thoron in Kuwait were conducted during the years 2015 and 2016. In this study, 65 dwellings were selected for the long-term radon–thoron survey using passive nuclear track monitors. The monitors (at least one) were used at various locations in the dwellings for 83–306 days. Some measurements were also repeated at the same locations in different seasons. This current study is a preliminary thoron survey with relatively small sample size. The results showed that the range of thoron concentration was from below the lower limit of detection to 35 Bq m−3, whereas the range of radon concentration was within 10–202 Bq m−3. Furthermore, 22% of the radon results exceeded the WHO radon reference level of 100 Bq m−3. The analysis of variance showed a correlation between indoor radon concentration and the season. However, the thoron measurements were rather limited and the values were low. In addition, the relationship was investigated between radon and thoron concentrations involving the floor levels and the type of ventilation systems used. INTRODUCTION The earth’s crust contains the primordial radionuclides 238U and 232Th. They decay through 226Ra (uranium chain) and 224Ra (thorium chain) to stable isotopes of lead. Most of the decay products are isotopes of solid elements but two are gases: 222Rn (radon) from the uranium decay chain and 220Rn (thoron) from the thorium decay chain. Radon formed in the underlying rock or soil can migrate to the building through cracks in the foundation slab, floor drains, sump pumps, construction joints and pores in hollow-block walls. Building materials are another possible source of indoor radon.(1, 2) Because of such properties, radon can be accumulated in the lowest level of the buildings and the highest radon concentration levels can be expected at such locations.(3, 4) Within buildings, radon can migrate to the upper floors, being driven by internal ventilation or diffusion processes. There is a concern that elevated radon concentration levels may contribute to an increased risk of lung cancer. According to the UNSCEAR Report,(5) about a half of the world mean annual effective dose (2.4 mSv) from natural radiation can be attributed to the inhalation of radon, thoron and their progenies. The latest investigation in Europe showed that radon in dwellings accounted for ~9% of the deaths from lung cancer and hence 2% of all cancer deaths.(6) In contrast to radon (half-life 3.82 d), the short half-life (55.6 s) of thoron severely limits its migration distance from its source, and as a consequence, thoron in indoor air almost exclusively comes from the materials of the internal surfaces of rooms in buildings. Due to this behaviour thoron gas is unevenly distributed in indoor environments compared to uniformly distributed radon. For example, the measurement of spatial distribution of radon and thoron from gypsum board showed that thoron concentration decreased exponentially with a distance from the wall, whereas radon concentration was almost constant regardless to the distance.(7) In order to protect the general public against elevated radon concentration levels, the concept of a reference level was introduced. The definition of ‘reference level’ given in EU COUNCIL DIRECTIVE 2013/59/EURATOM (definition no. 84) is: ‘reference level means in an emergency exposure situation or in an existing exposure situation, the level of effective dose or equivalent dose or activity concentration above which it is judged inappropriate to allow exposures to occur as a result of that exposure situation, even though it is not a limit that may not be exceeded’.(8) Such activity concentration (for radon) reference levels differ from one country to another. As examples, in 2007 the WHO(9) reported that the reference levels set at 400 Bq m−3 in Czech Republic for existing dwellings and at 200 Bq m−3 for new dwellings whereas in Switzerland, the levels were 400 Bq m−3 for existing dwellings and 1000 Bq m−3 for new ones. In the USA, the levels recommended by the Environmental Protection Agency(10) for both existing and new dwellings were 148 Bq m−3, but in Germany, the non-binding recommendation levels for both were only 100 Bq m−3. Moreover, in some countries, e.g. Japan and Italy, the reference levels were still not established yet. However, the European Union is imposing an obligation on its member states to establish national reference levels for indoor radon concentrations(8); specifically, that the annual average concentration in indoor air should not be higher than 300 Bq m−3. This is in contrast with the WHO’s recommended reference level of 100 Bq m−3. Moreover, the IAEA BSS (Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards), cosponsored by WHO, postulate the 300 Bq m−3 as reference level.(11) In many countries, surveys of radon and thoron levels in different environments (dwellings, workplaces, hospitals, schools, kindergartens and caves) have been carried out to identify the locations where radon and thoron levels are elevated and to determine the radon and thoron exposures that the population may receive.(12–18) In Kuwait, the number of radon investigations that have been made is limited. Measurements of indoor radon concentration has been carried out in houses(3) using PicoRad dosimeters and in schools(19) using etch-track detectors. However, no measurements of thoron concentration levels in Kuwait dwellings were made before this study. This article presents the results of the first indoor thoron survey performed in Kuwait during 2015 and 2016 using passive radon–thoron discrimination monitors. METHODS Site description Kuwait is located in the north-eastern corner of the Arabian Peninsula; on the northern coastal area of the Arabian Gulf between latitudes 28 and 31 north and longitudes 46 and 49 east. It is a flat country with altitudes reaching ~300 m in the west. The geological structure of Kuwait consists of relatively uniform desert-type surface soil. Top-soil samples from Kuwait had concentrations of 40K in the soil an order of magnitude higher than the concentrations of 232Th and 238U; and the measured concentrations for 40K, 238U and 232Th had average values of 288, 14 and 11 Bq kg−1, respectively.(20) Such values are lower than world’s average of 400 Bq kg−1 for 40K, 35 Bq kg−1 for 238U and 30 Bq kg−1 for 232Th.(5) The climate in Kuwait is mainly a long dry summer and a short wet winter. According to the Kuwait Meteorological Centre, the climate in Kuwait has four main defined seasons with sub-seasons each year.(21) The calendar winter starts in early December and finishes in the middle of February during which time the weather is cold and windy; temperatures sometimes reach below 0°C, from January to the middle of February. Spring runs from the middle of February to the end of May and the temperature starts increasing from February and reaches an average daily temperature of ~31°C in May. This season has many thunderstorms, accompanied by dust storms and sometimes the visibility may drop to zero. Summer lasts from the end of May to early November and the average maximum temperature may reach 44°C. Weather changes in these summer months from clear sky to dry days with Semoom winds (dry north-westerly winds), to humid days with winds from the Gulf carrying huge amounts of water vapour, before temperatures drop with the arrival of autumn. Temperatures in autumn (November and December) range from 20 to 30°C. However, because it is difficult to distinguish a clear change between the four seasons in Kuwait, only two seasons, i.e. a long summer (middle of February to November) and a short winter (December to the middle of February) are considered in practice. As mentioned earlier, due to the high temperature during the summer, air conditioning systems are heavily used in Kuwait from the middle of March until the middle of November. Mainly, three different types of air conditioning systems are in use: central air conditioning systems are commonly found in new buildings; and split air conditioners and window air conditioners are used in older buildings. On the other hand, heaters are used infrequently since the winter is not very cold and it is short. Unlike countries with cold winters (e.g. European countries), windows are usually opened for fresh-air ventilation during the day time in winter. Windows are closed in summer to provide better cooling by the air conditioners and save energy, and also to avoid dust entering the dwellings. Most houses constructed in recent years in Kuwait are made of solid reinforced concrete with cement bricks used for internal partitions. Ceramic tiles, marble and granite are commonly used as decorative materials and for the floors (in rooms and corridors) instead of carpets (used in earlier days) for cleaner and more hygienic interior environments. There is a concern that such decorative materials may result in higher indoor radon levels.(22, 23) Many houses in Kuwait are constructed with basements that are large halls, and some precautionary measures are made to prevent the entering of groundwater and humidity to the house through the basement, which is in contact with the ground. Before constructing a house, waterproof protective layers are installed underneath the basement of the house and around the walls of the house that are in contact with the surrounding ground. Therefore, with regard to the issue of the radon gas, the soil underneath the house is expected to have no effect on the indoor radon concentration levels. Therefore, in the present work, measurements of thoron and radon levels were performed in the basements of houses as well as in other rooms located at the distance of ~20 cm from the wall or the ceiling. Radon and thoron Long-term measurements were carried out using the discriminative radon and thoron (Rn–Tn) monitor known as RADUET (Radosys Co. Ltd, Hungary). The monitor contains two diffusion chambers with different air exchange rates,(24) therefore, radon and thoron can be measured simultaneously. One of the RADUET monitor chamber is sensitive to radon activity, whereas the other chamber is sensitive to both radon and thoron. Alpha particles from radon and thoron emitted during the radioactive decay are registered in a CR-39 chip attached at the bottom of each of the diffusion chambers. After chemical treatment of the CR-39 chips and analysing the difference of track numbers on both of them, the concentrations of radon and thoron can be calculated separately; the detailed procedure is described elsewhere.(25) The lower limit of detection (LLD) is obtained for each detector individually because it depends on concentration of each gas (radon and thoron) and hence the numbers of tracks for each CR-39 chip as well as the exposure period.(26) In the present study, the average detection limit was assumed to 6 Bq m−3 for radon and 10 Bq m−3 for thoron. The monitors are periodically tested and calibrated in the NIRS radon and thoron chambers.(27, 28) RESULTS AND DISCUSSION Long-term radon and thoron measurements were carried out in three campaigns in Kuwait. In the first campaign (winter), Rn–Tn monitors were exposed for periods between 89 and 158 days, with a median exposure period of 115 days. The second campaign (winter–summer) covered the periods from 167 to 306 days with a median exposure period of 204 days. The last campaign (summer) lasted between 83 and 194 days with a median exposure period of 93 days. Moreover, in ten selected houses, repeated measurements were also carried out for winter and the following summer to investigate the radon and thoron concentration levels in different seasons. The results of both indoor radon and thoron concentration measurements in the three campaigns are summarised in Table 1. Table 1. Descriptive statisticsa of indoor radon and thoron concentrations. Campaign Winter Winter–summer Summer Radon AM [Bq m−3] 56 83 127 SD [Bq m−3] 32 37 34 Median [Bq m−3] 52 94 114 Range [Bq m−3] 10–202 22–129 81–183 GM [Bq m−3] 49 73 123 GSD 1.69 1.81 1.31 No. of samples (above LLD) 66 8 13 Thoron AM [Bq m−3] 17 20 19 SD [Bq m−3] 5 — 9 Median [Bq m−3] 16 — 16 Range [Bq m−3] 10 (LLD)-31 — 10 (LLD)-35 GM [Bq m−3] 17 — 18 GSD 1.29 — 1.57 No. of samples (above LLD) 66 (14) 8 (1) 13 (5) Campaign Winter Winter–summer Summer Radon AM [Bq m−3] 56 83 127 SD [Bq m−3] 32 37 34 Median [Bq m−3] 52 94 114 Range [Bq m−3] 10–202 22–129 81–183 GM [Bq m−3] 49 73 123 GSD 1.69 1.81 1.31 No. of samples (above LLD) 66 8 13 Thoron AM [Bq m−3] 17 20 19 SD [Bq m−3] 5 — 9 Median [Bq m−3] 16 — 16 Range [Bq m−3] 10 (LLD)-31 — 10 (LLD)-35 GM [Bq m−3] 17 — 18 GSD 1.29 — 1.57 No. of samples (above LLD) 66 (14) 8 (1) 13 (5) aAM, arithmetic mean; SD, standard deviation; GM, geometric mean; GSD, geometric standard deviation; LLD, lower limit of detection. Table 1. Descriptive statisticsa of indoor radon and thoron concentrations. Campaign Winter Winter–summer Summer Radon AM [Bq m−3] 56 83 127 SD [Bq m−3] 32 37 34 Median [Bq m−3] 52 94 114 Range [Bq m−3] 10–202 22–129 81–183 GM [Bq m−3] 49 73 123 GSD 1.69 1.81 1.31 No. of samples (above LLD) 66 8 13 Thoron AM [Bq m−3] 17 20 19 SD [Bq m−3] 5 — 9 Median [Bq m−3] 16 — 16 Range [Bq m−3] 10 (LLD)-31 — 10 (LLD)-35 GM [Bq m−3] 17 — 18 GSD 1.29 — 1.57 No. of samples (above LLD) 66 (14) 8 (1) 13 (5) Campaign Winter Winter–summer Summer Radon AM [Bq m−3] 56 83 127 SD [Bq m−3] 32 37 34 Median [Bq m−3] 52 94 114 Range [Bq m−3] 10–202 22–129 81–183 GM [Bq m−3] 49 73 123 GSD 1.69 1.81 1.31 No. of samples (above LLD) 66 8 13 Thoron AM [Bq m−3] 17 20 19 SD [Bq m−3] 5 — 9 Median [Bq m−3] 16 — 16 Range [Bq m−3] 10 (LLD)-31 — 10 (LLD)-35 GM [Bq m−3] 17 — 18 GSD 1.29 — 1.57 No. of samples (above LLD) 66 (14) 8 (1) 13 (5) aAM, arithmetic mean; SD, standard deviation; GM, geometric mean; GSD, geometric standard deviation; LLD, lower limit of detection. The radon and thoron average values for each district are presented in Figure 1. In all the 65 surveyed dwellings, the minimum concentration of radon was 10 Bq m−3 and the maximum value was quite high at 202 Bq m−3. The arithmetic means of radon and thoron concentrations for winter were 56 and 17 Bq m−3, respectively. On the other hand, thoron concentration levels were found to be low. The minimum value was below of the LLD as 10 Bq m−3 while the maximum value was 31 Bq m−3. Figure 1. View largeDownload slide Map of Kuwait with municipality average radon (A) and thoron (B) concentrations results. Figure 1. View largeDownload slide Map of Kuwait with municipality average radon (A) and thoron (B) concentrations results. It should be noted that earlier radon measurements in Kuwait using charcoal PicoRad dosimeters in 300 dwellings (with more than 600 locations) yielded a mean value of 33 Bq m−3 and maximum value of 241 Bq m−3.(3) Because the exposure time for the charcoal PicoRad dosimeters was only 2 days, the statistical dispersion is much higher than for the etch-track detectors with measurements during a 3-month exposure time. The concentration levels for radon and thoron measured during the winter campaign were approximately log-normal distributions as presented in Figure 2; an alternative hypothesis (distributions are different) was rejected at p = 0.05 by Lilliefors (Kolmogorov–Smirnov), Cramer–von Mises and Anderson–Darling tests. Figure 2. View largeDownload slide Radon (A) and thoron (B) Q–Q plots and histograms for the winter campaign. Figure 2. View largeDownload slide Radon (A) and thoron (B) Q–Q plots and histograms for the winter campaign. Figure 3 shows the variation of radon and thoron concentrations as measured in the three campaigns. The median values, the lower and upper quartiles and the minimum and maximum values are presented. The highest radon concentration level was in summer and it was lower in winter–summer and winter whereas the thoron concentration results show no difference in the three campaigns. Figure 3. View largeDownload slide Radon (A) and thoron (B) results during the three campaigns. Figure 3. View largeDownload slide Radon (A) and thoron (B) results during the three campaigns. The non-parametric Kruskal–Wallis and Wilcox tests for radon data accepted an alternative hypothesis (data are statistically different) at p = 0.05 and indicated that differences in radon concentration in the different campaigns were statistically significant. These tests were performed if two assumptions were fulfilled: (1) the number of measured data for each campaign was not lower than four and (2) the Lavene test criterion was satisfied, i.e. variances of both sets were equal. In general, the data for thoron were not tested because the number of data was insufficient for the calculation. The difference between measured radon concentration levels in winter and summer can be related to the climate conditions and behaviours of the residents. As mentioned earlier, the windows are opened during the day time in the winter until early summer for fresh-air ventilation; therefore, radon that is exhaled from the building materials does not accumulate in the house when the windows are open. This tendency is opposite to results presented by other authors for measurements in European countries where higher concentrations were observed in winter compared to summer.(29–32) This is explained as follows: during winter months, in countries with cold weather the buildings are heated, which generates a higher pressure difference between the soil surrounding the buildings and the indoor environment. This creates the so-called ‘chimney effect’ and radon can easily penetrate cracks in the basement and finally accumulate in the house. Additionally, inhabitants keep the windows closed to save energy in such cold weather; therefore, the air exchange rate is low. The presence of thoron can be related to exhalation from building materials. The results of the Rn–Tn measurements in various locations (floor levels) and for different air conditioning systems are presented in Figures 4 and 5, respectively. Radon gas is heavier than air and it can be accumulated in the lowest level of buildings. Therefore, it seems useful to investigate its concentration on upper floors. In the current study, the number of results was limited, so the data from the upper floors (the third and higher floors) are presented as one average value. Figure 4. View largeDownload slide Radon (A) and thoron (B) concentrations during the three campaigns for different building floor levels. Figure 4. View largeDownload slide Radon (A) and thoron (B) concentrations during the three campaigns for different building floor levels. Figure 5. View largeDownload slide Radon (A) and thoron (B) concentrations during the three campaigns for different air conditioning systems. Figure 5. View largeDownload slide Radon (A) and thoron (B) concentrations during the three campaigns for different air conditioning systems. Figure 4 shows the averages of the radon and thoron concentrations during the three campaigns for different building floor levels. As expected, the highest concentration of radon was registered in the basement for all campaigns. However, differences were not statistically significant as confirmed by the Kruskal–Wallis and Wilcox tests, except for the relationship between ground floor and upper floors for winter–summer. The tests for radon in winter–summer and summer campaigns and all campaigns for thoron were not performed due to the insufficient number of data. Figure 4 shows that thoron concentration (if present) was independent of location and campaign (i.e. the season). Furthermore, results for the relationship between radon and thoron concentrations of three different types of air conditioning systems presented in Figure 5. The radon concentration was not related to the type of air conditioning system within the same campaign. The Kruskal–Wallis and Wilcox tests for each type system within the winter campaign confirmed this statement and rejected the alternative hypothesis (mean values are different). As previously, in other campaigns statistical tests were not performed due to the insufficient number of data. The effect of different seasons was investigated in ten selected houses where the monitors were exposed during winter and summer at the same location within each house. The box plot in Figure 6 presents results for median, average (open square), 25th and 75th quantiles as well as minimum and maximum values. Radon and thoron concentrations in the individual houses are separately presented in Figure 7. It is evident from the graph that in all the houses, the radon concentrations in summer were higher than those in winter. This fact was confirmed by the statistical Kruskal–Wallis and Wilcox tests. The results in Table 2 show that the average radon concentration in winter varied in the range from 28 to 75 Bq m−3 with an arithmetic mean value of 49 ± 17 Bq m−3 and from 89 to 183 Bq m−3 with an arithmetic mean value of 132 ± 33 Bq m−3 in summer. The summer/winter ratio ranged from 1.93 to 3.91 with a median value of 2.65. This ratio was opposite to that obtained from a survey in Punjab, India, where the winter concentrations were higher than the summer values and the summer/winter ratio varied from 0.52 to 1.20 with the average of 0.68.(33) Figure 6. View largeDownload slide Radon (A) and thoron (B) results versus measurement season for 10 selected houses (average values are presented as the open squares). Figure 6. View largeDownload slide Radon (A) and thoron (B) results versus measurement season for 10 selected houses (average values are presented as the open squares). Figure 7. View largeDownload slide Comparison of radon (A) and thoron (B) concentrations in 10 selected houses during winter and summer. Figure 7. View largeDownload slide Comparison of radon (A) and thoron (B) concentrations in 10 selected houses during winter and summer. Table 2. Descriptive statistics for 10 selected houses with repeated measurements. Season Winter Summer Radon AM [Bq m−3] 49 132 SD [Bq m−3] 17 33 Median [Bq m−3] 47 120 Range [Bq m−3] 28–75 89–183 GM [Bq m−3] 46 129 GSD 1.44 1.29 No. of samples (above LLD) 10 10 Thoron AM [Bq m−3] 19 24 SD [Bq m−3] 5 10 Median [Bq m−3] 16 21 Range [Bq m−3] 15–25 16–35 GM [Bq m−3] 18 22 GSD 1.31 1.47 No. of samples (above LLD) 10 (3) 10 (3) Season Winter Summer Radon AM [Bq m−3] 49 132 SD [Bq m−3] 17 33 Median [Bq m−3] 47 120 Range [Bq m−3] 28–75 89–183 GM [Bq m−3] 46 129 GSD 1.44 1.29 No. of samples (above LLD) 10 10 Thoron AM [Bq m−3] 19 24 SD [Bq m−3] 5 10 Median [Bq m−3] 16 21 Range [Bq m−3] 15–25 16–35 GM [Bq m−3] 18 22 GSD 1.31 1.47 No. of samples (above LLD) 10 (3) 10 (3) View Large Table 2. Descriptive statistics for 10 selected houses with repeated measurements. Season Winter Summer Radon AM [Bq m−3] 49 132 SD [Bq m−3] 17 33 Median [Bq m−3] 47 120 Range [Bq m−3] 28–75 89–183 GM [Bq m−3] 46 129 GSD 1.44 1.29 No. of samples (above LLD) 10 10 Thoron AM [Bq m−3] 19 24 SD [Bq m−3] 5 10 Median [Bq m−3] 16 21 Range [Bq m−3] 15–25 16–35 GM [Bq m−3] 18 22 GSD 1.31 1.47 No. of samples (above LLD) 10 (3) 10 (3) Season Winter Summer Radon AM [Bq m−3] 49 132 SD [Bq m−3] 17 33 Median [Bq m−3] 47 120 Range [Bq m−3] 28–75 89–183 GM [Bq m−3] 46 129 GSD 1.44 1.29 No. of samples (above LLD) 10 10 Thoron AM [Bq m−3] 19 24 SD [Bq m−3] 5 10 Median [Bq m−3] 16 21 Range [Bq m−3] 15–25 16–35 GM [Bq m−3] 18 22 GSD 1.31 1.47 No. of samples (above LLD) 10 (3) 10 (3) View Large In contrast to the present study, the previous investigation for Kuwait dwellings reported no significant difference between indoor radon concentrations on a seasonal basis.(3) Investigation based on 600 short-term measurement using Pico-Rad detectors showed that in during the two main seasons (short winter and long summer) concentration is nearly uniform, except for the basements where the concentration values were ~30% (mean value) higher in the winter season. In should be mention that the summer/winter ratio for measurement in basement to be 0.75 and was opposite to the current study. One of the possible explanation of the difference in radon concentration and seasonal variation between past and present study is the different measurement technique—2 days measurement by PicoRad detectors and at least 3 months measurement using RADUET monitors. In addition, in the present study, the average thoron concentrations were 19 ± 5 and 24 ± 10 Bq m−3 for winter and summer, with a higher median value for summer (21 Bq m−3) than for winter (16 Bq m−3). The maximum value of 35 Bq m−3 was registered in summer and the minimum of 10 Bq m−3 (as LLD) was registered in summer as well as in winter. Due to the limited number of thoron results (only three results above the LLD for each season) with high uncertainty, it cannot be stated that thoron concentration was subject to any seasonal variation. CONCLUSION The first measurement study of thoron concentration in Kuwait dwellings was carried out in parallel with a radon study using passive radon/thoron discriminative monitors that allow the simultaneous measurements of both gases. The thoron concentration levels showed low values without dependency on the floor levels within the dwellings or on the seasons (summer and winter). On the other hand, the radon concentration levels showed some seasonal variations between summer and winter. Although the number of reported data is limited in the study area, the average radon concentrations in the present study were found to be higher compared with previous research. It can be concluded from this study that 20% of the studied Kuwait dwellings have radon levels that exceeded the WHO reference level of 100 Bq m−3, but they are not above the 300 Bq m−3 level recommended by the European Union. REFERENCES 1 Chen, J., Rahman, N. M. and Atiya, I. A. 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Radiation Protection Dosimetry – Oxford University Press
Published: Feb 9, 2018
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