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Long-term measurements of radon, thoron and their airborne progeny in 25 schools in Republic of Srpska
Journal of Environmental Radioactivity 148 (2015) 163e169
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Long-term measurements of radon, thoron and their airborne progeny in 25 schools in Republic of Srpska a Z. Curguz , Z. Stojanovska b, Z.S. Zuni c c, P. Kolar z d, T. Ischikawa e, Y. Omori e, R. Mishra f, c g, P. Uji c c, *, P. Bossew h B.K. Sapra f, J. Vaupoti University of East Sarajevo, Faculty of Transport and Traffic Engineering, Vojvode Misica 52, 74000 Doboj, Bosnia and Herzegovina Goce Delcev University, Faculty of Medical Sciences, Stip, Republic of Macedonia c Institute of Nuclear Sciences “Vinca”, University of Belgrade, 11000 Belgrade, Serbia d University of Belgrade, Institute of Physics, Serbia e Fukushima Medical University, Department of Radiation Physics and Chemistry, Hikariga-oka 1, Fukushima, 960-1295, Japan f Bhabha Atomic Research Centre, Radiological Physics and Advisory Division, Mumbai, India g Institute Jozef Stefan, Radon Centre, Jamova 39, 1000 Ljubljana, Slovenia h €penicker Allee 120-130, 10318 Berlin, Germany German Federal Office for Radiation Protection, Ko a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2015 Received in revised form 17 June 2015 Accepted 28 June 2015 Available online xxx
This article reports results of the first investigations on indoor radon, thoron and their decay products concentration in 25 primary schools of Banja Luka, capital city of Republic Srpska. The measurements have been carried out in the period from May 2011 to April 2012 using 3 types of commercially available nuclear track detectors, named: long-term radon monitor (GAMMA 1)- for radon concentration measurements (CRn); radon-thoron discriminative monitor (RADUET) for thoron concentration measurements (CTn); while equilibrium equivalent radon concentration (EERC) and equilibrium equivalent thoron concentrations (EETC) measured by Direct Radon Progeny Sensors/Direct Thoron Progeny Sensors (DRPS/ DTPS) were exposed in the period November 2011 to April 2012. In each school the detectors were deployed at 10 cm distance from the wall. The obtained geometric mean concentrations were CRn ¼ 99 Bq m3 and CTn ¼ 51 Bq m3 for radon and thoron gases respectively. Those for equilibrium equivalent radon concentration (EERC) and equilibrium equivalent thoron concentrations (EETC) were 11.2 Bq m3 and 0.4 Bq m3, respectively. The correlation analyses showed weak relation only between CRn and CTn as well as between CTn and EETC. The influence of the school geographical locations and factors linked to buildings characteristic in relation to measured concentrations were tested. The geographical location and floor level significantly influence CRn while CTn depend only from building materials (ANOVA, p 0.05). The obtained geometric mean values of the equilibrium factors were 0.123 for radon and 0.008 for thoron. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Indoor air Radon Thoron Radon progenies Thoron progenies Equilibrium factor Primary schools Nuclear track detectors
1. Introduction Radon (222Rn), a decay product of 226Ra, and thoron (220Rn), a decay product of 224Ra, are naturally occurring radioactive gases. They can be found in rocks, soil, and water of the earth's crust, and they can accumulate to high concentrations in the indoor environment. The difference in half lives of radon (3.825 d) and thoron
* Corresponding author. Institute of Nuclear Sciences “Vinca”, University of Belgrade, 11000 Belgrade, Serbia. E-mail address: [email protected] (P. Uji c). http://dx.doi.org/10.1016/j.jenvrad.2015.06.026 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
(55.6 s) implies that they behave very differently in indoor air (Doi et al., 1994; Urosevic et al., 2008). The main source of indoor radon is 226Ra in soil. The radon gas can diffuse out of the underlying soil into indoor air. In some cases, building materials can make a significant contribution. The pathway for radon generation in rock and soil to its accumulation indoors is controlled by a number of geogenic and anthropogenic factors (Cosma et al., 2013 and Cosma et al., 2015). The source of indoor thoron is thought to be building material in most cases. Due to its short half-life it cannot be expected to migrate over long distances. Therefore a possible geogenic contribution, which is usually most relevant for radon, is normally thought to be of minor importance, for thoron. Another consequence of the short half-life
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of thoron is that there is a distinctly decreasing concentration profile of thoron away from exhaling surface (Doi et al., 1994; Urosevic et al., 2008). Since radon is the main natural source of ionizing radiation, measures to control the public health risk from this gas within each country, must be determined using knowledge of indoor radon concentration distribution all over the country. This implies the need for radon surveys and radon mapping. Many national radon programmes have been started since 1970, mostly in developed countries, to obtain a representative distribution of radon concentration and set appropriate standards and actions (WHO, 2009). Worldwide data are published by the UNSCEAR, most recently in 2006 (UNSCEAR, 2006). On the other hand, no comparable number of survey data exists for thoron in the literature. Mainly, exposure to thoron and its decay products in indoor environment is usually much lower than that from radon and its decay products. But in recent years, exposure to thoron and its decay products and its possible health effects has gained increasing attention as indicated in the UNSCEAR 2006 Report. During the past few years, several studies of indoor thoron gas (Stojanovska et al., 2013) and its decay product concentrations have been published for the Balkan region (Gulan et al., 2012; Stojanovska et al., 2014; Mishra et al., 2014). The health risk associated to radon and thoron rises from the inhalation of the short-lived decay products, mainly reported as equilibrium equivalent concentration (EEC). The equilibrium equivalent concentration for radon EERC and equilibrium equivalent concentration for thoron EETC are the quantities directly related to the Potential Alpha Energy Concentration in air and hence to the inhalation dose. EERC is related to individual 218Po, 214 Pb and 214Bi activity concentrations, denoted C1, C2 and C3 respectively, through the relation (UNSCEAR annex B p. 103, 2000):
Fig. 1. Map of Republic of Srpska and location of Banja Luka city.
is to present analysis of long term radon, thoron and their decay products concentrations in schools. 2. Materials and methods 2.1. Detectors
EERC ¼ 0:105ðC1 Þ þ 0:516ðC2 Þ þ 0:380ðC3 Þ;
(1)
and, EETC is related to the individual 212Pb and 212Bi activity concentrations, denoted C1 and C2 respectively through the relation:
EETC ¼ 0:913ðC1 Þ þ 0:087ðC2 Þ:
(2)
For more precise dose estimation, accurate techniques to measure concentration of radon and thoron decay products are important. As in the cases of radon and thoron gases there are active and passive techniques. To measure radon and thoron progeny concentration in indoor environment, time integrating passive technique is more appropriate in assessment of human exposure than active techniques. For this purpose, few years ago, low cost time integrating passive detectors for EERC and EETC measurements have been developed (Zhou and Iida, 2000; Mishra and Mayya, 2008). A survey of radon, thoron and their decay product concentrations has been implemented in primary schools of Banja Luka city, largest and the most populated city in the Republic of Srpska (Fig. 1). The long term measurements were started as a research activity in 2011 and were conducted during one year. Primary schools in Banja Luka city were chosen for representative measurements due to their correlation with the number of residents c et al., 2010; Vaupoti si, 2010). A radon survey of (Zuni c and K ava primary schools may serve as a proxy to identify radon prone areas (Bossew et al., 2014). The main motivation for conducting research of radon concentrations in schools is children's health care, since they belong to the most sensitive categories of the human population, taking into account the time spent in that environment. Following the previous work (Curguz et al., 2013) in which active and passive measurements of radon concentrations in schools of Banja Luka city were investigated, the main objective of this study
The measurements were carried out with three different types of nuclear track detectors (SSNTD), explained in the following: a.) The CRn was measured by “Long term radon gas monitor” commercially named “Gamma 1”, consisted of a CR-39 detecting material placed on the bottom of a cylindrical diffusion chamber (dimensions ∅58 mm 20 mm), provided and analysed by Landauer company, Sweden (LANDAUER AB). The relative expanded combined uncertainty, given at 95% confidence level was in interval from 12% (for low CRn) to 28% (for high CRn). b.) The CTn was measured using the commercially named “Raduet”-radon-thoron discriminative monitor, originally developed by the National Institute of Radiological Science (NIRS), Chiba, Japan (Tokonami et al., 2005). It consists of two CR-39 detector chips fixed in the lower sections of two diffusion chambers (∅60 mm 30 mm). The detector's primary chamber is sensitive to radon, whereas the secondary chamber is sensitive to both radon and thoron. These detectors were provided and analysed by collaborators from Japan (NIRS) and used for measurements of thoron activity concentrations. The reported expanded combined uncertainty, at 95% confidence level for CTn in this survey was ranged from 5% (for high CTn and low CRn) up to 100% for low CTn and high CRn. c.) The EERC and EETC measurements were done by detector named Direct Radon Progeny Sensors/Direct Thoron Progeny Sensors (DRPS/DTPS) (Mishra and Mayya, 2008; Mishra et al., 2009). These monitors consist of two absorber mounted LR 115 type detecting material for measuring time-integrating decay products concentrations. DTPSs are absorber (aluminized mylar of 50 mm thickness) mounted LR-115 type
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169
nuclear track detectors which selectively detect only the alpha particles emitted from 212Po (8.78 MeV) atoms formed from the radioactive decay of 212Pb and 212Bi atoms deposited on the absorber surface. Similarly, DRPS has an absorber thickness of 37 mm to detect mainly the alpha particles emitted from 214Po (7.69 MeV) formed from the eventual decay of 218Po, 214Pb and 214Bi atoms deposited on it. The uncertainties associated with the measurements using DTPS/ DRPS are essentially due to change in unattached fraction in the environment, which may lead to alteration in deposition rate. For a typical air exchange rate of ~1 h1 in dwellings the deposition rate and hence the sensitivity factors tend to remain constant for time-integrated measurements (Mishra and Mayya, 2008; Mayya et al., 2012). However the sensitivity factor of DTPS/DRPS becomes increasingly affected by aerosol concentration and higher ventilation rates as observed in occupational environments (8e10 h1) (Mishra et al., 2009). The overall uncertainty of this deposition based technique is between 10 and 20% for indoor environments.
2.2. Detectors placement Long term measurements of CRn, CTn, EERC and EETC have been performed in 25 primary schools of Banja Luka. In one room, in every school, a set of three detectors was deployed. In 18 of 25 schools, CRn was measured in 2 rooms (17 schools) and in one school CRn was measured in three rooms. The school average radon concentration was estimated as an arithmetic mean of the measured values. In the case of single measurements, the individual result of concentrations was presented. The survey includes the following types of rooms: assembly halls, principal offices, classroom and hallways, situated on the ground or first floor of school buildings. Bricks with concrete foundations are used as building materials while some schools were made from concrete blocks and some from metal plates. Within the room, the detectors were deployed on the wall at the distance d z 10 cm, at a height of 1.5e2.0 m above the floor and at least 0.5 m far from corners. The Gamma 1 and Raduet detectors were exposed in these positions for 12 months starting in end of April 2011 and finished in the beginning of May 2012, while the DTPS/DRPS were deployed in the beginning of November 2011 and exposed for 6 months. In case of EERC and EETC measurements, we assumed that this period could be approximately representative for the annual exposure. After this period, they were collected and sent in laboratories for etching, counting of the tracks and subsequent statistical evaluation and analysis. 2.3. Results evaluation Evaluation of the results was made with commercially available statistical software's: XLSTAT Pro 7.5 and Minitab 16. Distribution fitting was tested applying KolmogoroveSmirnov (KS) test. The significance of the overall factors for measured concentration was tested, by means of analysis of variance (ANOVA) at a significance level of 95% (error probability p 0.05). If the main effect was significant, Fisher's LSD-test was applied to determine the differences between mean values. 3. Results and discussion Descriptive statistics of: radon concentrations CRn, thoron concentrations (CTn), equilibrium equivalent concentrations for radon (EERC) and for thoron (EETC) respectively, measured in the schools
165
of Banja Luka are given in Table 1. Frequency distributions of CRn, CTn, EERC and EETC are shown in Fig. 2. All concentrations were approximated as log-normal distribution except EERC which is normally distributed (KolmogoroveSmirnov at error probability p 0.05). In the 44 rooms measured the average CRn ranged from 36 to 549 Bq m3 with a geometric mean of 99 Bq m3. From Fig. 2 we can see that the highest frequencies of the CRn results are in intervals lower than 100 Bq m3 (24 of 44 rooms), and in 6 rooms the CRn were in range from 200 to 400 Bq m3. It should be mentioned that one of the examined school appeared to have annual CRn higher than 400 Bq m3, the action level proposed by the European Union (EU Council Directive, 1990), while in seven schools the CRn are above 300 Bq/m3, which is the highest allowable reference value in the new Euratom Basic safety standards (EU-BSS, 2014). In all 25 rooms, CTn was lower than 200 Bq m3, mainly with concentrations below 100 Bq m3 (23 of 25). Although, the geometric mean value of 51 Bq m3 for CTn was lower than the geometric mean of CRn, and in 6 rooms CTn were higher than CRn. The ratio between indoor CTn and indoor CRn was ranged from 0.10 to 1.55, approximately followed a log-normal distribution. The arithmetic mean of the CTn/CRn equals 0.68 (by SD ¼ 0.41) and geometric mean 0.56 (by GSD ¼ 1.97). As mentioned earlier, the CTn strongly depends on the distance from the wall, and such dependence arises for CTn/CRn as well. By comparison, in a survey of 18 Slovenian primary schools, where detectors were placed at 1 m distance from the wall, this value of CTn/CRn was lower, namely in the range of 0.02e0.82 (Vaupotic et al., 2012). The measured values of decay products ranged between 6.8 and 16.8 Bq m3 for EERC and from 0.09 to 1.16 Bq m3 for EETC. The ratio EETC/EERC was in the interval from 0.01 to 0.12 with geometric mean value of 0.04 (by GSD ¼ 2.22). The number of long term investigations and published papers are still limited especially for EETC. For example, in UNSCEAR 2006 the ICRP model is quoted (ICRP, 1987). By the ICRP model (air exchange rate of 0.7 h1), EERC and EETC are 15 Bq m3 (2e50 Bq/m3) and 0.5 Bq m3 (0.04e2 Bq m3), respectively. The corresponding value of the ratio EEТ C/ EERC ¼ 0.03. For instance, in Macedonia from measurements in 43 schools where detectors were positioned at distance >50 cm from the walls, the EEТ C/EERC values ranged between 0.004 and 0.360 with GM ¼ 0.028 (Stojanovska et al., 2014). 3.1. Correlation between concentrations The next step was investigation of correlation between concentrations measured in the same room. For this purpose a model of parametric linear regression (LR) was applied on ln-transformed data in order to reduce the influence of extremes on the tests. This model however does not suppose a physical dependency model of
Table 1 Descriptive statistics of all measured concentration obtained in schools of RS, where CRne radon concentration CTn e thoron concentration, EERC e equilibriumeequivalent radon concentration, EETC e equilibriumeequivalent thoron concentration.
Number of measurements Minimum (Bq m3) Median (Bq m3) Maximum (Bq m3) Arithmetic mean (Bq m3) Standard deviation (Bq m3) Standard error (Bq m3) Geometric mean (Bq m3) Geometric standard deviation
CRn
CTn
EERC
EETC
44 36 82 549 128 111 17 99 1.94
25 6.6 57 198 63 40 8 51 2.07
25 6.8 11.5 16.8 11.4 2.47 0.49 11.2 1.26
25 0.09 0.38 1.16 0.52 0.34 0.07 0.40 2.20
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Fig. 2. Histograms of radon concentrations measured with all detectors fitted with log-normal function (left) and probability plot of ln transformed radon data fitted with normal function.
that kind. As a measure of this linear dependence, Person coefficient R2 of determination was used. The analysis parameters are presented in Table 2. A linear regression analysis confirmed the correlation between CRn and CTn and between CTn and EETC. Correlations between other two paired concentrations were insignificant. The results show that even correlation is statistically significant; it is weak in both cases. This is not surprising because different factors affect the concentrations of radon and thoron gases and their decay products. For example generation of radon and thoron gas depends on the concentration of radium 226Ra and radium 224Ra in the source (soil, building materials). The CRn is distributed about homogeneously in the rooms because of its sufficiently long half-life as opposed to CTn which clearly depends on the distance to the source (wall) and whose distribution is therefore inhomogeneous. The concentration of radon and thoron decay products depends on the CRn and CTn and they are additionally affected with the indoor concentration of aerosols and air exchange rate on which they are more sensitive than the gases themselves.
Different correlations were obtained in several recent studies, dedicated to time-integrating measurements of indoor radon, thoron gas and their decay products. Gulan et al. (2012) shows correlations between EERC and EETC (R2 ¼ 0.59), as well between EERC and CRn (R2 ¼ 0.58), measured in 48 houses of Kosovo and Metohija. In the same study correlation between CRn and CTn and between CTn and EETC was insignificant which is contrary to our results. Poor correlations between CTn and EETC were reported for dwellings in China (R2 ¼ 0.04), (Janik et al., 2013; Yamada et al., 2006), Ireland (R2 < 0.01) (McLaughlin et al., 2011) and Korea (R2 ¼ 0.03) (Tokonami et al., 2005). 3.2. Influence of the location on the measured concentrations In order to investigate the influence of location on measured concentrations, we grouped all data by the factors “individual schools”, “floors”, “type of rooms” and “building materials”. According to first categorization we found that only CRn was subject to variability within the investigated region (ANOVA,
Table 2 Parameters of linear model and correlations between radon concentration, thoron concentration, and their decay products (significant correlations are bolded). R2
Measured C with paired detectors
p*
Linear model: y ¼ ax þ b a
b
ln(CRn)- ln(CTn) ln(EERC)- ln(EETC) ln(EERC)- ln(CRn) ln(EETC)- ln(CTn)
p ¼ 0.008 p ¼ 0.605 p ¼ 0.352 p ¼ 0.020
0.457 ± 0.158
2.715 ± 0.63
2.883 ± 0.799
0.501 ± 0.201
*In the case of significant correlation, the error probability is p < 0.05.
0.266 0.012 0.038 0.213
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169
p ¼ 0.0002), which can be assumed to be the variability of the underlying geology. In addition to the differences of CRn between different schools, the CRn also varied within the same school. In Fig. 3, the measured concentrations in all 44 rooms of 25 schools, together with expanded uncertainty are given. In 10 out of 18 schools, CRn within the room varied in the uncertainty interval of the measured results (bars coloured light grey in Fig. 3). In another 8 (bars coloured dark grey in Fig. 3), the variations of the measured CRn were higher, starting from 37% in school 12 and reaching the 108% in the school coded with number 19. Taking into account that the higher variation was obtained from the results measured on the same floor, once again are confirmed radon spatial trend due to their dependence on geology. The influence of factors related to building characteristics on the measured concentrations was investigated. The following factors were taken into consideration: floor level, type of room, and building materials. The factors which able a differentiation into subgroups (ANOVA, p ¼ 0.05) were found to be significant only for radon and thoron concentrations. The dependence of the measured concentrations on floor level was significant only for CRn (LSD, p ¼ 0.046). The descriptive statistic is given in Table 3. The grouped data according to the floor (ground and first) showed that, the geometric mean of CRn measured in the ground floor as 108 Bq m3 (GSD ¼ 1.98), is higher than the geometric mean of CRn in the first floor, 65 Bq m3 (GSD ¼ 1.40). Furthermore, we tested the influence of the room type and building materials on measured concentrations. The descriptive statistic of measured data grouped according to room type is present in Table 4. Although different occupation patterns, this factor not affected significantly measured concentrations. On the other hand the effect of building materials was significant for CRn (LSD, p 0.05). The geometric means of CTn depending on construction materials are plotted in Fig. 4. It is obvious that the mean values of CRn are divided in to two groups, where the CRn measured in buildings constructed with concrete (GM ¼ 51 Bq m3 by GSD ¼ 1.95), bricks (GM ¼ 59 Bq m3 by GSD ¼ 1.65) and brick 1 (GM ¼ 64 Bq m3 by GSD ¼ 1.24) are higher than that where metal (GM ¼ 22 Bq m3 by GSD ¼ 2.94) was used as construction material.
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Table 3 Radon concentrations measured at different floors. CRn
Number of measurements Minimum (Bq m3) Maximum (Bq m3) Arithmetic mean (Bq m3) Standard deviation (Bq m3) Geometric mean (Bq m3) Geometric standard deviation
First floor
Ground floor
7 38 1062 68 21 65 1.40
37 36 549 139 117 108 1.98
3.3. Equilibrium factors As the exposure is mainly induced by radon and thoron decay products and not by the radon and thoron gas, in order to estimate the effective dose correctly it is necessary to estimate the decay products concentration. The direct measurement of the decay products is not always possible, thus the estimation of its concentration is based on the measurement of radon and thoron gas concentration using the equilibrium factors separately for radon and for thoron. In this survey, equilibrium factors F were estimated as: F ¼ EEC/C, where C is a radon or thoron concentration and EEC is whether EERC or EETC. The obtained results are present in Table 5. The obtained GM value of 0.123 for radon equilibrium factor (FRn) is significantly lower than the value of 0.4 recommended by ICRP (ICRP, 1994). Although well-defined, this factor strongly dependent of the environmental conditions and show regional variations as well variations within the region. For example: an FRn in Banja Luka schools has a values ranged from 0.013 to 0.320 but in Macedonian schools this range is from 0.1 to 0.84 (Stojanovska et al., 2014), while in the investigation performed in the Sokobanja dwellings, the results for FRn have showed variation in range from 0.06 to 0.95 (Mishra et al., 2014). While relatively large amounts of data are available for the FRn, this is not the case for FTn. The reason may be its not straight-forward definition and the question of what could be the use of FTn if its value is strongly dependent on the location in a room. The short half-life of thoron which results in its non-uniform distribution in the room also leads to non-uniform FTn in the room. As seen in the present case, a correlation between the CTn and EETC was observed because the
Fig. 3. Radon concentrations with expanded uncertainty for 44 rooms of 25 schools; The white bars present single measurements, grey colour present schools where more than one measurements were performed.
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169
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Table 4 Radon, thoron and its decay products measured in different rooms.
CRn (Bq m3)
CTn (Bq m3) EERC (Bq m3) EETC (Bq m3)
Room
N
Min
Max
AM
SD
GM
GSD
Assembly halls Hallway Principal offices Assembly halls Hallway Assembly halls Hallway Assembly halls Hallway
10 18 15 10 15 10 15 10 15
38 48 36 17 7 6.79 6.86 0.14 0.09
549 314 374 198 124 14.76 16.84 0.92 1.16
1256 94 137 72 57 11.60 11.31 0.53 0.51
163 63 107 50 33 2.68 2.41 0.29 0.39
109 82 108 60 45 11.27 11.07 0.45 0.37
2.27 1.63 2.00 1.90 2.19 1.30 1.24 1.99 2.38
Fig. 4. Geometric mean of CTn measured in buildings made of different construction materials. Bars are one standard error of the mean.
Table 5 Descriptive statistics of the equilibrium factor for radon and for thoron in schools, of Banja Luka.
No. of values used Minimum Median Maximum Arithmetic mean Standard deviation Geometric mean GSD
FRn
FTn
25 0.013 0.150 0.320 0.149 0.078 0.123 2.065
25 0.003 0.009 0.048 0.011 0.010 0.008 2.201
detectors were placed relatively close to the walls, but because of uniform mixing of the thoron decay products throughout the room, we did not obtain very high EETC near the walls. On the other hand, high CTn were recorded very close to the walls. Consequently, a low FTn was obtained. The obtained GM of FTn ¼ 0.008 in this study is very similar with GM value: FTn ¼ 0.006 reported for Sokobanja dwellings, where detectors were deployed on the wall (Mishra et al., 2014). As well, similar with GM of FTn ¼ 0.0024 obtained from the measurements at 10 cm distance from the wall in dwellings of Kosovo (Gulan et al., 2012). On the other hand from measurements at distance more than 50 cm from the wall, the GM value of FTn ¼ 0.067 was obtained for Macedonian schools (Stojanovska et al., 2014). 4. Conclusion Indoor radon, thoron and their decay products concentrations in 25 primary schools of Banja Luka were measured. For that purpose, three types of nuclear track detectors were used. The obtained
geometric mean values of concentrations are 99 Bq m3 for radon and 51 Bq m3 for thoron as well as for EERC and EETC are 11.2 and 0.40 Bq m3, respectively. Weak correlations were found only between CRn and CTn as well as between CTn and EETC. This evaluation indicated that radon, thoron and their decay products were practically independent quantities, so that it would be inappropriate to estimate the concentration of one from those of the others. Furthermore, geographical positions of the buildings as well anthropogenic factors such as floor level of a room within the building have significant impact only on CRn. The building factor which affected CTn was construction material. The measurements of CRn, CTn and their decay products allow determination of long-term mean equilibrium factors. The obtained geometric mean values of the equilibrium factors are 0.123 for radon and 0.008 for thoron. While the value for radon is lower than what is generally assumed as typical (0.4), the one for thoron is in line with literature data for FTn close to the walls. Acknowledgements The authors acknowledge the support given by the Ministry of Science and Technology of Republic of Srpska within the project 19/ 06-020/961-132/11 and the support of the Ministry of Education, Science and Technological Development of the Republic of Serbia within the projects P 171020, III 45003, P 41028 and P 171018. References Bossew, P., Zuni c, Z.S., Stojanovska, Z., Tollefsen, T., Carpentieri, C., Veselinovi c, N., Komatina, S., Vaupoti c, J., Simovi c, R.D., Antignani, S., Bochicchio, F., 2014. Geographical distribution of the annual mean radon concentrations in primary schools of Southern Serbia e application of geostatistical methods. J. Environ. Radioact. 127, 141e148. Cosma, Constantin, Cucos¸-Dinu, Alexandra, Papp, Botond, Begy, Robert, Sainz, Carlos, 2013. Soil and building material as main sources of indoor radon in B ait¸a-S¸tei radon prone area (Romania). J. Environ. Radioact. 116, 174e179. Cosma, C., Papp, B., Cucos¸ Dinu, A., Sainz, C., 2015. Testing radon mitigation techniques in a pilot house from B ait¸a-S¸tei radon prone area (Romania). J. Environ. Radioact. 140, 141e147. Curguz, Z., Zuni c, Z.S., Tollefsen, T., Jovanovi c, P., Nikezi c, D., Kolar z, P., 2013. Active and passive radon concentration measurements and first-step mapping in schools of Banja Luka, Republic of Srpska. Rom. J. Phys. 58 (S), 90e98, 1221146X. Doi, M., Fujimoto, K., Kobayashi, S., Yonehara, H., 1994. Spatial distribution of thoron and radon concentrations in the indoor air of a traditional Japanese wooden house. Health Phys. 66 (1), 43e49. EU-BSS, 2014. Council Directive 2013/59/Euratom of 5 December 2013 Laying Down Basic Safety Standards for Protection against the Dangers Arising from Exposure to Ionising Radiation, and Repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom. http://eur-lex. europa.eu/JOHtml.do?uri¼OJL:2014:013:SOM:EN:HTML. Gulan, L., Mili c, G., Bossew, P., Omori, Y., Ishikawa, T., Mishra, R., Mayya, Y.S., Stojanovska, Z., Vu ckovi c, B., Zuni c, Z.S., 2012. Field experience on indoor radon. thoron and their progenies with solid state detectors in a survey of Kosovo and Metohija. Radiat. Prot. Dosim. 152 (1e3), 189e197. ICRP (International Council on Radiological Protection), 1994. Human Respiratory Tract Model for Radiological Protection. Publication 66. Elsevier Science, Oxford.
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169 ICRP (International Council on Radiological Protection), 1987. Publication 50. Pergamon Press, Oxford. Janik, M., Tokonami, S., Kranrod, C., Sorimachi, A., Ishikawa, T., Hosoda, M., Mclaughlin, J., Chang, B.U., Kim, Y.J., 2013. Comparative analysis of radon. thoron and thoron progeny concentration measurements. J. Radiat. Res. 54 (4), 597e610. LANDAUER AB, Box 6522, SE-751 38 Uppsala e SWEDEN, www.landauer-se.com. Mishra, R., Mayya, Y.S., 2008. Study of a deposition based Direct Thoron Progeny Sensor (DTPS) technique for estimating Equilibrium Equivalent Thoron Concentration (EETC) in indoor environment. Radiat. Meas. 43, 1408e1416. Mishra, R., Mayya, Y.S., Kushwaha, H.S., 2009. Measurement of 220Rn/222Rn progeny deposition velocities on surfaces and their comparison with theoretical models. J. Aerosol Sci. 40, 1e15. Mishra, R., Zunic, Z.S., Venoso, G., Bochicchio, F., Stojanovska, Z., Carpentieri, C., Prajith, R., Sapra, B.K., Mayya, Y.S., Ishikawa, T., Omori, Y., Veselinovic, N., Tollefsen, T., Uji c, P., Bossew, P., 2014. An evaluation of thoron (and radon) equilibrium factor close to walls based on long-term measurements in dwellings. Radiat. Prot. Dosim. 160 (1e3), 164e168. Mayya, Y.S., Mishra, R., Prajith, R., Gole, A.C., Sapra, B.K., Chougaonkar, M.P., Nair, R.R.K., Ramola, R.C., Karunakara, N., Koya, P.K.M., 2012. Deposition based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies. Radiat. Prot. Dosim. 152 (1e3), 18e24. McLaughlin, J., Murray, M., Currivan, L., Pollard, D., Smith, V., Tokonami, S., Sorimachi, A., Janik, M., 2011. Long-term measurements of thoron. its airborne progeny and radon in 205 dwellings in Ireland. Radiat. Prot. Dosim. 145 (2e3), 189e193. Stojanovska, Z., Bossew, P., Tokonami, S., Zuni c, Z.S., Bochicchio, F., Boev, B., Ristova, M., Januseski, J., 2013. Indoor thoron concentration in FYR of Macedonia. Radiat. Meas. 49, 57e66. Stojanovska, Z., Zunic, Z.S., Bossew, P., Bochicchio, F., Carpentieri, C., Venoso, G., Rosaline Mishra, R., Rout, R.P., Sapra, B.K., Burghele, B.D., Cucos¸-Dinu, A., Boev, B., Cosma, C., 2014. Results from the time integrated measurements of
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indoor radon, thoron and their decay product concentrations in schools in the Republic of Macedonia. Radiat. Prot. Dosim. 162 (1e2), 152e156. Tokonami, S., Takahashi, H., Kobayashi, Y., Zhuo, W., 2005. Up-to-date radon-thoron discriminative detector for a large scale survey. Rev. Sci. Instrum. 76, 113505e113509. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) Anex B, 2000. Sources and Effects of Ionizing Radiation. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation to the General Assembly. United Nations, New York, USA. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation), 2006. Effects of Ionizing Radiation. Report to the General Assembly with Scientific Annexes (Annex E) (United Nations, New York, USA). Urosevic, V., Nikezic, D., Vulovic, S., 2008. A theoretical approach to indoor radon and thoron distribution. J. Environ. Radioact. 99 (12), 1829e1833. Vaupoti c, J., K av asi, N., 2010. Preliminary study of thoron and radon levels in various indoor environments in Slovenia. Radiat. Prot. Dosim. 141 (4), 383e385. va si, N., Ishikawa, T., Yonehara, H., Tokonami, S., 2012. Vaupoti c, J., Bezek, M., Ka Radon and thoron doses in kindergartens and elementary schools. Radiat. Prot. Dosim. 152 (1e3), 242e252. WHO (World Health Organization), 2009. WHO Handbook on Indoor Radon: a Public Health Perspective, ISBN 978 92 4 154767 3. www.who.int/ionizing_ radiation/env/radon/en/index1.html. Yamada, Y., Sun, Q., Tokonami, S., Akiba, S., Zhuo, W., Hou, C., Zhang, S., Ishikawa, T., Furukawa, M., Fukutsu, K., Yonehara, H., 2006. Radon-thoron discriminative measurements in Gansu Province. China and their implication for dose estimates. J. Toxicol. Environ. Health A 69 (7), 723e734. Zhou, W., Iida, T., 2000. Estimation of thoron progeny concentrations in dwellings with their deposition rate measurements. Health Phys. 35, 365e370. Zuni c, Z.S., Veselinovi c, N., Bochicchio, F., Carelli, V., Vaupoti c, J., Cukni c, O., Simovi c, R., Vojinovic, Z., Kisic, D., Tollefsen, T., 2010. The indoor radon survey in Serbian schools: can it reflect also the general population exposure? Nukleonika 55 (4), 419e427.
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Long-term measurements of radon, thoron and their airborne progeny in 25 schools in Republic of Srpska a Z. Curguz , Z. Stojanovska b, Z.S. Zuni c c, P. Kolar z d, T. Ischikawa e, Y. Omori e, R. Mishra f, c g, P. Uji c c, *, P. Bossew h B.K. Sapra f, J. Vaupoti University of East Sarajevo, Faculty of Transport and Traffic Engineering, Vojvode Misica 52, 74000 Doboj, Bosnia and Herzegovina Goce Delcev University, Faculty of Medical Sciences, Stip, Republic of Macedonia c Institute of Nuclear Sciences “Vinca”, University of Belgrade, 11000 Belgrade, Serbia d University of Belgrade, Institute of Physics, Serbia e Fukushima Medical University, Department of Radiation Physics and Chemistry, Hikariga-oka 1, Fukushima, 960-1295, Japan f Bhabha Atomic Research Centre, Radiological Physics and Advisory Division, Mumbai, India g Institute Jozef Stefan, Radon Centre, Jamova 39, 1000 Ljubljana, Slovenia h €penicker Allee 120-130, 10318 Berlin, Germany German Federal Office for Radiation Protection, Ko a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2015 Received in revised form 17 June 2015 Accepted 28 June 2015 Available online xxx
This article reports results of the first investigations on indoor radon, thoron and their decay products concentration in 25 primary schools of Banja Luka, capital city of Republic Srpska. The measurements have been carried out in the period from May 2011 to April 2012 using 3 types of commercially available nuclear track detectors, named: long-term radon monitor (GAMMA 1)- for radon concentration measurements (CRn); radon-thoron discriminative monitor (RADUET) for thoron concentration measurements (CTn); while equilibrium equivalent radon concentration (EERC) and equilibrium equivalent thoron concentrations (EETC) measured by Direct Radon Progeny Sensors/Direct Thoron Progeny Sensors (DRPS/ DTPS) were exposed in the period November 2011 to April 2012. In each school the detectors were deployed at 10 cm distance from the wall. The obtained geometric mean concentrations were CRn ¼ 99 Bq m3 and CTn ¼ 51 Bq m3 for radon and thoron gases respectively. Those for equilibrium equivalent radon concentration (EERC) and equilibrium equivalent thoron concentrations (EETC) were 11.2 Bq m3 and 0.4 Bq m3, respectively. The correlation analyses showed weak relation only between CRn and CTn as well as between CTn and EETC. The influence of the school geographical locations and factors linked to buildings characteristic in relation to measured concentrations were tested. The geographical location and floor level significantly influence CRn while CTn depend only from building materials (ANOVA, p 0.05). The obtained geometric mean values of the equilibrium factors were 0.123 for radon and 0.008 for thoron. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Indoor air Radon Thoron Radon progenies Thoron progenies Equilibrium factor Primary schools Nuclear track detectors
1. Introduction Radon (222Rn), a decay product of 226Ra, and thoron (220Rn), a decay product of 224Ra, are naturally occurring radioactive gases. They can be found in rocks, soil, and water of the earth's crust, and they can accumulate to high concentrations in the indoor environment. The difference in half lives of radon (3.825 d) and thoron
* Corresponding author. Institute of Nuclear Sciences “Vinca”, University of Belgrade, 11000 Belgrade, Serbia. E-mail address: [email protected] (P. Uji c). http://dx.doi.org/10.1016/j.jenvrad.2015.06.026 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
(55.6 s) implies that they behave very differently in indoor air (Doi et al., 1994; Urosevic et al., 2008). The main source of indoor radon is 226Ra in soil. The radon gas can diffuse out of the underlying soil into indoor air. In some cases, building materials can make a significant contribution. The pathway for radon generation in rock and soil to its accumulation indoors is controlled by a number of geogenic and anthropogenic factors (Cosma et al., 2013 and Cosma et al., 2015). The source of indoor thoron is thought to be building material in most cases. Due to its short half-life it cannot be expected to migrate over long distances. Therefore a possible geogenic contribution, which is usually most relevant for radon, is normally thought to be of minor importance, for thoron. Another consequence of the short half-life
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of thoron is that there is a distinctly decreasing concentration profile of thoron away from exhaling surface (Doi et al., 1994; Urosevic et al., 2008). Since radon is the main natural source of ionizing radiation, measures to control the public health risk from this gas within each country, must be determined using knowledge of indoor radon concentration distribution all over the country. This implies the need for radon surveys and radon mapping. Many national radon programmes have been started since 1970, mostly in developed countries, to obtain a representative distribution of radon concentration and set appropriate standards and actions (WHO, 2009). Worldwide data are published by the UNSCEAR, most recently in 2006 (UNSCEAR, 2006). On the other hand, no comparable number of survey data exists for thoron in the literature. Mainly, exposure to thoron and its decay products in indoor environment is usually much lower than that from radon and its decay products. But in recent years, exposure to thoron and its decay products and its possible health effects has gained increasing attention as indicated in the UNSCEAR 2006 Report. During the past few years, several studies of indoor thoron gas (Stojanovska et al., 2013) and its decay product concentrations have been published for the Balkan region (Gulan et al., 2012; Stojanovska et al., 2014; Mishra et al., 2014). The health risk associated to radon and thoron rises from the inhalation of the short-lived decay products, mainly reported as equilibrium equivalent concentration (EEC). The equilibrium equivalent concentration for radon EERC and equilibrium equivalent concentration for thoron EETC are the quantities directly related to the Potential Alpha Energy Concentration in air and hence to the inhalation dose. EERC is related to individual 218Po, 214 Pb and 214Bi activity concentrations, denoted C1, C2 and C3 respectively, through the relation (UNSCEAR annex B p. 103, 2000):
Fig. 1. Map of Republic of Srpska and location of Banja Luka city.
is to present analysis of long term radon, thoron and their decay products concentrations in schools. 2. Materials and methods 2.1. Detectors
EERC ¼ 0:105ðC1 Þ þ 0:516ðC2 Þ þ 0:380ðC3 Þ;
(1)
and, EETC is related to the individual 212Pb and 212Bi activity concentrations, denoted C1 and C2 respectively through the relation:
EETC ¼ 0:913ðC1 Þ þ 0:087ðC2 Þ:
(2)
For more precise dose estimation, accurate techniques to measure concentration of radon and thoron decay products are important. As in the cases of radon and thoron gases there are active and passive techniques. To measure radon and thoron progeny concentration in indoor environment, time integrating passive technique is more appropriate in assessment of human exposure than active techniques. For this purpose, few years ago, low cost time integrating passive detectors for EERC and EETC measurements have been developed (Zhou and Iida, 2000; Mishra and Mayya, 2008). A survey of radon, thoron and their decay product concentrations has been implemented in primary schools of Banja Luka city, largest and the most populated city in the Republic of Srpska (Fig. 1). The long term measurements were started as a research activity in 2011 and were conducted during one year. Primary schools in Banja Luka city were chosen for representative measurements due to their correlation with the number of residents c et al., 2010; Vaupoti si, 2010). A radon survey of (Zuni c and K ava primary schools may serve as a proxy to identify radon prone areas (Bossew et al., 2014). The main motivation for conducting research of radon concentrations in schools is children's health care, since they belong to the most sensitive categories of the human population, taking into account the time spent in that environment. Following the previous work (Curguz et al., 2013) in which active and passive measurements of radon concentrations in schools of Banja Luka city were investigated, the main objective of this study
The measurements were carried out with three different types of nuclear track detectors (SSNTD), explained in the following: a.) The CRn was measured by “Long term radon gas monitor” commercially named “Gamma 1”, consisted of a CR-39 detecting material placed on the bottom of a cylindrical diffusion chamber (dimensions ∅58 mm 20 mm), provided and analysed by Landauer company, Sweden (LANDAUER AB). The relative expanded combined uncertainty, given at 95% confidence level was in interval from 12% (for low CRn) to 28% (for high CRn). b.) The CTn was measured using the commercially named “Raduet”-radon-thoron discriminative monitor, originally developed by the National Institute of Radiological Science (NIRS), Chiba, Japan (Tokonami et al., 2005). It consists of two CR-39 detector chips fixed in the lower sections of two diffusion chambers (∅60 mm 30 mm). The detector's primary chamber is sensitive to radon, whereas the secondary chamber is sensitive to both radon and thoron. These detectors were provided and analysed by collaborators from Japan (NIRS) and used for measurements of thoron activity concentrations. The reported expanded combined uncertainty, at 95% confidence level for CTn in this survey was ranged from 5% (for high CTn and low CRn) up to 100% for low CTn and high CRn. c.) The EERC and EETC measurements were done by detector named Direct Radon Progeny Sensors/Direct Thoron Progeny Sensors (DRPS/DTPS) (Mishra and Mayya, 2008; Mishra et al., 2009). These monitors consist of two absorber mounted LR 115 type detecting material for measuring time-integrating decay products concentrations. DTPSs are absorber (aluminized mylar of 50 mm thickness) mounted LR-115 type
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169
nuclear track detectors which selectively detect only the alpha particles emitted from 212Po (8.78 MeV) atoms formed from the radioactive decay of 212Pb and 212Bi atoms deposited on the absorber surface. Similarly, DRPS has an absorber thickness of 37 mm to detect mainly the alpha particles emitted from 214Po (7.69 MeV) formed from the eventual decay of 218Po, 214Pb and 214Bi atoms deposited on it. The uncertainties associated with the measurements using DTPS/ DRPS are essentially due to change in unattached fraction in the environment, which may lead to alteration in deposition rate. For a typical air exchange rate of ~1 h1 in dwellings the deposition rate and hence the sensitivity factors tend to remain constant for time-integrated measurements (Mishra and Mayya, 2008; Mayya et al., 2012). However the sensitivity factor of DTPS/DRPS becomes increasingly affected by aerosol concentration and higher ventilation rates as observed in occupational environments (8e10 h1) (Mishra et al., 2009). The overall uncertainty of this deposition based technique is between 10 and 20% for indoor environments.
2.2. Detectors placement Long term measurements of CRn, CTn, EERC and EETC have been performed in 25 primary schools of Banja Luka. In one room, in every school, a set of three detectors was deployed. In 18 of 25 schools, CRn was measured in 2 rooms (17 schools) and in one school CRn was measured in three rooms. The school average radon concentration was estimated as an arithmetic mean of the measured values. In the case of single measurements, the individual result of concentrations was presented. The survey includes the following types of rooms: assembly halls, principal offices, classroom and hallways, situated on the ground or first floor of school buildings. Bricks with concrete foundations are used as building materials while some schools were made from concrete blocks and some from metal plates. Within the room, the detectors were deployed on the wall at the distance d z 10 cm, at a height of 1.5e2.0 m above the floor and at least 0.5 m far from corners. The Gamma 1 and Raduet detectors were exposed in these positions for 12 months starting in end of April 2011 and finished in the beginning of May 2012, while the DTPS/DRPS were deployed in the beginning of November 2011 and exposed for 6 months. In case of EERC and EETC measurements, we assumed that this period could be approximately representative for the annual exposure. After this period, they were collected and sent in laboratories for etching, counting of the tracks and subsequent statistical evaluation and analysis. 2.3. Results evaluation Evaluation of the results was made with commercially available statistical software's: XLSTAT Pro 7.5 and Minitab 16. Distribution fitting was tested applying KolmogoroveSmirnov (KS) test. The significance of the overall factors for measured concentration was tested, by means of analysis of variance (ANOVA) at a significance level of 95% (error probability p 0.05). If the main effect was significant, Fisher's LSD-test was applied to determine the differences between mean values. 3. Results and discussion Descriptive statistics of: radon concentrations CRn, thoron concentrations (CTn), equilibrium equivalent concentrations for radon (EERC) and for thoron (EETC) respectively, measured in the schools
165
of Banja Luka are given in Table 1. Frequency distributions of CRn, CTn, EERC and EETC are shown in Fig. 2. All concentrations were approximated as log-normal distribution except EERC which is normally distributed (KolmogoroveSmirnov at error probability p 0.05). In the 44 rooms measured the average CRn ranged from 36 to 549 Bq m3 with a geometric mean of 99 Bq m3. From Fig. 2 we can see that the highest frequencies of the CRn results are in intervals lower than 100 Bq m3 (24 of 44 rooms), and in 6 rooms the CRn were in range from 200 to 400 Bq m3. It should be mentioned that one of the examined school appeared to have annual CRn higher than 400 Bq m3, the action level proposed by the European Union (EU Council Directive, 1990), while in seven schools the CRn are above 300 Bq/m3, which is the highest allowable reference value in the new Euratom Basic safety standards (EU-BSS, 2014). In all 25 rooms, CTn was lower than 200 Bq m3, mainly with concentrations below 100 Bq m3 (23 of 25). Although, the geometric mean value of 51 Bq m3 for CTn was lower than the geometric mean of CRn, and in 6 rooms CTn were higher than CRn. The ratio between indoor CTn and indoor CRn was ranged from 0.10 to 1.55, approximately followed a log-normal distribution. The arithmetic mean of the CTn/CRn equals 0.68 (by SD ¼ 0.41) and geometric mean 0.56 (by GSD ¼ 1.97). As mentioned earlier, the CTn strongly depends on the distance from the wall, and such dependence arises for CTn/CRn as well. By comparison, in a survey of 18 Slovenian primary schools, where detectors were placed at 1 m distance from the wall, this value of CTn/CRn was lower, namely in the range of 0.02e0.82 (Vaupotic et al., 2012). The measured values of decay products ranged between 6.8 and 16.8 Bq m3 for EERC and from 0.09 to 1.16 Bq m3 for EETC. The ratio EETC/EERC was in the interval from 0.01 to 0.12 with geometric mean value of 0.04 (by GSD ¼ 2.22). The number of long term investigations and published papers are still limited especially for EETC. For example, in UNSCEAR 2006 the ICRP model is quoted (ICRP, 1987). By the ICRP model (air exchange rate of 0.7 h1), EERC and EETC are 15 Bq m3 (2e50 Bq/m3) and 0.5 Bq m3 (0.04e2 Bq m3), respectively. The corresponding value of the ratio EEТ C/ EERC ¼ 0.03. For instance, in Macedonia from measurements in 43 schools where detectors were positioned at distance >50 cm from the walls, the EEТ C/EERC values ranged between 0.004 and 0.360 with GM ¼ 0.028 (Stojanovska et al., 2014). 3.1. Correlation between concentrations The next step was investigation of correlation between concentrations measured in the same room. For this purpose a model of parametric linear regression (LR) was applied on ln-transformed data in order to reduce the influence of extremes on the tests. This model however does not suppose a physical dependency model of
Table 1 Descriptive statistics of all measured concentration obtained in schools of RS, where CRne radon concentration CTn e thoron concentration, EERC e equilibriumeequivalent radon concentration, EETC e equilibriumeequivalent thoron concentration.
Number of measurements Minimum (Bq m3) Median (Bq m3) Maximum (Bq m3) Arithmetic mean (Bq m3) Standard deviation (Bq m3) Standard error (Bq m3) Geometric mean (Bq m3) Geometric standard deviation
CRn
CTn
EERC
EETC
44 36 82 549 128 111 17 99 1.94
25 6.6 57 198 63 40 8 51 2.07
25 6.8 11.5 16.8 11.4 2.47 0.49 11.2 1.26
25 0.09 0.38 1.16 0.52 0.34 0.07 0.40 2.20
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Fig. 2. Histograms of radon concentrations measured with all detectors fitted with log-normal function (left) and probability plot of ln transformed radon data fitted with normal function.
that kind. As a measure of this linear dependence, Person coefficient R2 of determination was used. The analysis parameters are presented in Table 2. A linear regression analysis confirmed the correlation between CRn and CTn and between CTn and EETC. Correlations between other two paired concentrations were insignificant. The results show that even correlation is statistically significant; it is weak in both cases. This is not surprising because different factors affect the concentrations of radon and thoron gases and their decay products. For example generation of radon and thoron gas depends on the concentration of radium 226Ra and radium 224Ra in the source (soil, building materials). The CRn is distributed about homogeneously in the rooms because of its sufficiently long half-life as opposed to CTn which clearly depends on the distance to the source (wall) and whose distribution is therefore inhomogeneous. The concentration of radon and thoron decay products depends on the CRn and CTn and they are additionally affected with the indoor concentration of aerosols and air exchange rate on which they are more sensitive than the gases themselves.
Different correlations were obtained in several recent studies, dedicated to time-integrating measurements of indoor radon, thoron gas and their decay products. Gulan et al. (2012) shows correlations between EERC and EETC (R2 ¼ 0.59), as well between EERC and CRn (R2 ¼ 0.58), measured in 48 houses of Kosovo and Metohija. In the same study correlation between CRn and CTn and between CTn and EETC was insignificant which is contrary to our results. Poor correlations between CTn and EETC were reported for dwellings in China (R2 ¼ 0.04), (Janik et al., 2013; Yamada et al., 2006), Ireland (R2 < 0.01) (McLaughlin et al., 2011) and Korea (R2 ¼ 0.03) (Tokonami et al., 2005). 3.2. Influence of the location on the measured concentrations In order to investigate the influence of location on measured concentrations, we grouped all data by the factors “individual schools”, “floors”, “type of rooms” and “building materials”. According to first categorization we found that only CRn was subject to variability within the investigated region (ANOVA,
Table 2 Parameters of linear model and correlations between radon concentration, thoron concentration, and their decay products (significant correlations are bolded). R2
Measured C with paired detectors
p*
Linear model: y ¼ ax þ b a
b
ln(CRn)- ln(CTn) ln(EERC)- ln(EETC) ln(EERC)- ln(CRn) ln(EETC)- ln(CTn)
p ¼ 0.008 p ¼ 0.605 p ¼ 0.352 p ¼ 0.020
0.457 ± 0.158
2.715 ± 0.63
2.883 ± 0.799
0.501 ± 0.201
*In the case of significant correlation, the error probability is p < 0.05.
0.266 0.012 0.038 0.213
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p ¼ 0.0002), which can be assumed to be the variability of the underlying geology. In addition to the differences of CRn between different schools, the CRn also varied within the same school. In Fig. 3, the measured concentrations in all 44 rooms of 25 schools, together with expanded uncertainty are given. In 10 out of 18 schools, CRn within the room varied in the uncertainty interval of the measured results (bars coloured light grey in Fig. 3). In another 8 (bars coloured dark grey in Fig. 3), the variations of the measured CRn were higher, starting from 37% in school 12 and reaching the 108% in the school coded with number 19. Taking into account that the higher variation was obtained from the results measured on the same floor, once again are confirmed radon spatial trend due to their dependence on geology. The influence of factors related to building characteristics on the measured concentrations was investigated. The following factors were taken into consideration: floor level, type of room, and building materials. The factors which able a differentiation into subgroups (ANOVA, p ¼ 0.05) were found to be significant only for radon and thoron concentrations. The dependence of the measured concentrations on floor level was significant only for CRn (LSD, p ¼ 0.046). The descriptive statistic is given in Table 3. The grouped data according to the floor (ground and first) showed that, the geometric mean of CRn measured in the ground floor as 108 Bq m3 (GSD ¼ 1.98), is higher than the geometric mean of CRn in the first floor, 65 Bq m3 (GSD ¼ 1.40). Furthermore, we tested the influence of the room type and building materials on measured concentrations. The descriptive statistic of measured data grouped according to room type is present in Table 4. Although different occupation patterns, this factor not affected significantly measured concentrations. On the other hand the effect of building materials was significant for CRn (LSD, p 0.05). The geometric means of CTn depending on construction materials are plotted in Fig. 4. It is obvious that the mean values of CRn are divided in to two groups, where the CRn measured in buildings constructed with concrete (GM ¼ 51 Bq m3 by GSD ¼ 1.95), bricks (GM ¼ 59 Bq m3 by GSD ¼ 1.65) and brick 1 (GM ¼ 64 Bq m3 by GSD ¼ 1.24) are higher than that where metal (GM ¼ 22 Bq m3 by GSD ¼ 2.94) was used as construction material.
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Table 3 Radon concentrations measured at different floors. CRn
Number of measurements Minimum (Bq m3) Maximum (Bq m3) Arithmetic mean (Bq m3) Standard deviation (Bq m3) Geometric mean (Bq m3) Geometric standard deviation
First floor
Ground floor
7 38 1062 68 21 65 1.40
37 36 549 139 117 108 1.98
3.3. Equilibrium factors As the exposure is mainly induced by radon and thoron decay products and not by the radon and thoron gas, in order to estimate the effective dose correctly it is necessary to estimate the decay products concentration. The direct measurement of the decay products is not always possible, thus the estimation of its concentration is based on the measurement of radon and thoron gas concentration using the equilibrium factors separately for radon and for thoron. In this survey, equilibrium factors F were estimated as: F ¼ EEC/C, where C is a radon or thoron concentration and EEC is whether EERC or EETC. The obtained results are present in Table 5. The obtained GM value of 0.123 for radon equilibrium factor (FRn) is significantly lower than the value of 0.4 recommended by ICRP (ICRP, 1994). Although well-defined, this factor strongly dependent of the environmental conditions and show regional variations as well variations within the region. For example: an FRn in Banja Luka schools has a values ranged from 0.013 to 0.320 but in Macedonian schools this range is from 0.1 to 0.84 (Stojanovska et al., 2014), while in the investigation performed in the Sokobanja dwellings, the results for FRn have showed variation in range from 0.06 to 0.95 (Mishra et al., 2014). While relatively large amounts of data are available for the FRn, this is not the case for FTn. The reason may be its not straight-forward definition and the question of what could be the use of FTn if its value is strongly dependent on the location in a room. The short half-life of thoron which results in its non-uniform distribution in the room also leads to non-uniform FTn in the room. As seen in the present case, a correlation between the CTn and EETC was observed because the
Fig. 3. Radon concentrations with expanded uncertainty for 44 rooms of 25 schools; The white bars present single measurements, grey colour present schools where more than one measurements were performed.
Z. Curguz et al. / Journal of Environmental Radioactivity 148 (2015) 163e169
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Table 4 Radon, thoron and its decay products measured in different rooms.
CRn (Bq m3)
CTn (Bq m3) EERC (Bq m3) EETC (Bq m3)
Room
N
Min
Max
AM
SD
GM
GSD
Assembly halls Hallway Principal offices Assembly halls Hallway Assembly halls Hallway Assembly halls Hallway
10 18 15 10 15 10 15 10 15
38 48 36 17 7 6.79 6.86 0.14 0.09
549 314 374 198 124 14.76 16.84 0.92 1.16
1256 94 137 72 57 11.60 11.31 0.53 0.51
163 63 107 50 33 2.68 2.41 0.29 0.39
109 82 108 60 45 11.27 11.07 0.45 0.37
2.27 1.63 2.00 1.90 2.19 1.30 1.24 1.99 2.38
Fig. 4. Geometric mean of CTn measured in buildings made of different construction materials. Bars are one standard error of the mean.
Table 5 Descriptive statistics of the equilibrium factor for radon and for thoron in schools, of Banja Luka.
No. of values used Minimum Median Maximum Arithmetic mean Standard deviation Geometric mean GSD
FRn
FTn
25 0.013 0.150 0.320 0.149 0.078 0.123 2.065
25 0.003 0.009 0.048 0.011 0.010 0.008 2.201
detectors were placed relatively close to the walls, but because of uniform mixing of the thoron decay products throughout the room, we did not obtain very high EETC near the walls. On the other hand, high CTn were recorded very close to the walls. Consequently, a low FTn was obtained. The obtained GM of FTn ¼ 0.008 in this study is very similar with GM value: FTn ¼ 0.006 reported for Sokobanja dwellings, where detectors were deployed on the wall (Mishra et al., 2014). As well, similar with GM of FTn ¼ 0.0024 obtained from the measurements at 10 cm distance from the wall in dwellings of Kosovo (Gulan et al., 2012). On the other hand from measurements at distance more than 50 cm from the wall, the GM value of FTn ¼ 0.067 was obtained for Macedonian schools (Stojanovska et al., 2014). 4. Conclusion Indoor radon, thoron and their decay products concentrations in 25 primary schools of Banja Luka were measured. For that purpose, three types of nuclear track detectors were used. The obtained
geometric mean values of concentrations are 99 Bq m3 for radon and 51 Bq m3 for thoron as well as for EERC and EETC are 11.2 and 0.40 Bq m3, respectively. Weak correlations were found only between CRn and CTn as well as between CTn and EETC. This evaluation indicated that radon, thoron and their decay products were practically independent quantities, so that it would be inappropriate to estimate the concentration of one from those of the others. Furthermore, geographical positions of the buildings as well anthropogenic factors such as floor level of a room within the building have significant impact only on CRn. The building factor which affected CTn was construction material. The measurements of CRn, CTn and their decay products allow determination of long-term mean equilibrium factors. The obtained geometric mean values of the equilibrium factors are 0.123 for radon and 0.008 for thoron. While the value for radon is lower than what is generally assumed as typical (0.4), the one for thoron is in line with literature data for FTn close to the walls. Acknowledgements The authors acknowledge the support given by the Ministry of Science and Technology of Republic of Srpska within the project 19/ 06-020/961-132/11 and the support of the Ministry of Education, Science and Technological Development of the Republic of Serbia within the projects P 171020, III 45003, P 41028 and P 171018. References Bossew, P., Zuni c, Z.S., Stojanovska, Z., Tollefsen, T., Carpentieri, C., Veselinovi c, N., Komatina, S., Vaupoti c, J., Simovi c, R.D., Antignani, S., Bochicchio, F., 2014. Geographical distribution of the annual mean radon concentrations in primary schools of Southern Serbia e application of geostatistical methods. J. Environ. Radioact. 127, 141e148. Cosma, Constantin, Cucos¸-Dinu, Alexandra, Papp, Botond, Begy, Robert, Sainz, Carlos, 2013. Soil and building material as main sources of indoor radon in B ait¸a-S¸tei radon prone area (Romania). J. Environ. Radioact. 116, 174e179. Cosma, C., Papp, B., Cucos¸ Dinu, A., Sainz, C., 2015. Testing radon mitigation techniques in a pilot house from B ait¸a-S¸tei radon prone area (Romania). J. Environ. Radioact. 140, 141e147. Curguz, Z., Zuni c, Z.S., Tollefsen, T., Jovanovi c, P., Nikezi c, D., Kolar z, P., 2013. Active and passive radon concentration measurements and first-step mapping in schools of Banja Luka, Republic of Srpska. Rom. J. Phys. 58 (S), 90e98, 1221146X. Doi, M., Fujimoto, K., Kobayashi, S., Yonehara, H., 1994. Spatial distribution of thoron and radon concentrations in the indoor air of a traditional Japanese wooden house. Health Phys. 66 (1), 43e49. EU-BSS, 2014. Council Directive 2013/59/Euratom of 5 December 2013 Laying Down Basic Safety Standards for Protection against the Dangers Arising from Exposure to Ionising Radiation, and Repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom. http://eur-lex. europa.eu/JOHtml.do?uri¼OJL:2014:013:SOM:EN:HTML. Gulan, L., Mili c, G., Bossew, P., Omori, Y., Ishikawa, T., Mishra, R., Mayya, Y.S., Stojanovska, Z., Vu ckovi c, B., Zuni c, Z.S., 2012. Field experience on indoor radon. thoron and their progenies with solid state detectors in a survey of Kosovo and Metohija. Radiat. Prot. Dosim. 152 (1e3), 189e197. ICRP (International Council on Radiological Protection), 1994. Human Respiratory Tract Model for Radiological Protection. Publication 66. Elsevier Science, Oxford.
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