TA6 – Radiation Protection of the Public and the Environment

Radon in houses and soil of

V. Radolić*, B. Vuković, D. Stanić, I. Miklavčić and J. Planinić

Department of Physics, University of Osijek, P.O. Box 125 31000 Osijek, Croatia

E-mail*: [email protected] fax: ++ 385 31 232 701

Abstract

Long-term indoor radon measurements in thousand Croatian homes, randomly selected, were performed by the LR-115 track etch detectors during a year 2003/2004. The obtained values of arithmetic means of radon concentrations in 20 Croatian counties were in range from 33 to 198 Bq/m3, while the arithmetic and geometric means for Croatia were 68 and 50 Bq/m3, respectively. Indoor radon concentrations follow log-normal distribution and the percentage of dwellings with concentrations above 400 Bq/m3 was 1.8 %. Radon concentrations in soil gas, at depth of 0.8 m, were measured by AlphaGUARD measuring system. Association between levels of indoor and soil radon was investigated.

Introduction

Radon is a naturally radioactive gas that is, by far, the main contributor to the total dose received by the general population from exposure to natural background radiation. It is also considered to be the main leading cause of lung cancer second to smoking [1]. Consequently, many countries have adopted a number of regulations and made large efforts to identify radon-prone areas. Because indoor radon levels can fluctuate largely over short scale, establishing radon risk maps can become very difficult. Nevertheless, most European countries have organized large sampling campaigns, mainly by performing indoor and soil-gas measurements [2]. Radon research group from the Department of Physics at the University of Osijek made a lot of measurements of indoor radon concentrations in dwellings, schools and kindergartens [3, 4] then in soil gas mainly in the area of Osijek town [5], as well as in Croatian spas systematically [6]. At the end of 2003 we started a year-long nationwide representative survey in order to estimate the distribution of the annual indoor radon concentrations in Croatia with the identification of the radon prone areas. The second objective was to determine the percentage of dwellings where radon concentration exceeded certain reference values, from which the future Croatian action levels could be chosen. Preliminary measurements of radon in soil gas on national level started at the end of 2005 and are still going on. Spatial distribution of sampling points was determined using integrated population, grid square and geological approaches (the geological structure of the Croatian territory comprises two main regions: the sedimentary rocks at the north and the karst structure at the south). Materials and methods

In order to perform a representative national survey on radon exposure of population from dwellings of Croatia (with 4.4 million inhabitants, 1.6 million dwellings), a random sample of thousand addresses was prepared as follows. In all twenty counties (Table 1), with administrative boundaries as presented in Figure 1, respective number of measurement locations was chosen proportionally to the number of inhabitants of the county (about 1 measurement location per 4000 inhabitants). The house owners were contacted by phone and if they had been agreed with performing the radon measurement in their houses, we were interviewing them about type of dwelling, measurement site inside the house (suggesting living room or bedroom at ground floor) and number of occupants especially number of children below 12 years; most of the interviewing owners agreed to participate in investigation. Then we sent the detectors with short instruction for regular placement of the detector vessel during exposure time in dwelling and with another envelope for the detector return to our laboratory. Small difficulties appeared by smashing detector vessels during transport (to measurement location as well as from it), then some shipments were returned because the address or house owner was unknown and some detectors were lost during exposure period. In total, about 20 % of sent detectors have never been returned; they lost for us (we received 782 detectors). Integrated measurements of indoor radon were performed by means of the passive track etching method with strippable LR-115 SSNTD film, type II (Kodak-Pathe, France). The cylindrical plastic vessel of detector, with the diameter and height of 11 and 7 cm, respectively, was covered with a filter paper of 0.078 kg/m2 surface density. Inside, at the bottom of the vessel, a LR-115 film of 2x3 cm was fixed and it presents the diffusion detector. The other film was fixed outside on the cylindrical shell of the vessel and presents the open detector. The diffusion detector registers only tracks of alpha particles emitted by radon because radon daughters do not pass through the filter paper. The open LR-115 detector registers the total number of alpha particles of radon and its short-lived progeny. The measurement method with two detectors (diffusion and open) enabled estimation of the equilibrium factor, F, for radon and its progeny in air, as well as a better assessment of [7] the radon dose . For the track densities D and D0 of the open and diffusion detectors, respectively, the equilibrium factor was calculated according the following relation:

F = a D/Do + b, (1) where the parameters were a = 0.50 and b = -0.53. Considering the definition of the equilibrium factor, the equilibrium equivalent concentration of the radon daughters corresponds to the product of the equilibrium factor value and the radon concentration in the air. The LR-115 detectors were exposed in the air for one year, and then etched in 10 % NaOH aqueous solution at 50 ºC for 180 min. After this chemical treatment, the detector tracks were automatically counted by spark counter AIST-2V (V.G. Khlopkin Radium Institute, St. Petersburg, Russia). The sensitivity coefficient of the diffusion detector was 44.5 Bq m-3/tr cm-2 d-1, but the background was near 45 tr cm-2. Radon concentration in air was determined as product of the sensitivity coefficient and track density of the diffusion detector. Radon concentrations in soil gas in the area of Osijek town were measured by passive track etched detectors LR-115 type II at depth of 0.5 m and exposure time of one week [5]. The radon measurements in soil gas at national level are performing by AlphaGUARD measuring system using a standard sampling procedure described and proposed by manufacturer [8]. At the first phase (screening) of this national survey, two locations are chosen in each county; usually, sites with minimum and maximum indoor radon levels. The samplings of soil gas and radon measurements are done near the houses (in gardens) at the depth of 0.8 m. Results and discussion

The annual indoor radon concentrations in Croatia were in the range from 4 to 751 Bq m-3, with the arithmetic mean and standard deviation of 68 and 85 Bq m-3, respectively; the geometric mean and its standard deviation were 50 and 2.3 Bq m-3, respectively. The distribution of the annual indoor radon concentrations on the counties, with the respective number of measurements (n), arithmetic mean (c), standard deviation (σc), geometric mean (cg) and the equilibrium factor (F), is given in Table 1.

Table 1. – Total number of inhabitants on twenty Croatian counties according the census of population of March 2001 (total number of inhabitants in the Republic of Croatia was 4,437,460), with the number of measurements (n), arithmetic mean (c / Bqm-3) and -3 respective standard deviation (σc), as well as geometric mean (cg / Bqm ) of the annual indoor radon concentrations, and the equilibrium factor (F).

Total number County Symbol of n c σc cg F inhabitants I 1,088,841 190 47 55 36 0.59 Krapina - Zagorje II 142,432 18 71 112 46 0.64 Sisak - III 185,387 28 58 35 48 0.65 Karlovac IV 141,787 28 104 120 69 0.55 Varaždin V 184,769 32 79 110 52 0.60 - Križevci VI 124,467 21 46 17 43 0.64 - VII 133,084 22 62 45 49 0.55 Primorje – VIII 305,505 52 141 181 71 0.50 - Senj IX 53,677 8 198 268 94 0.40 Virovitica - X 93,389 15 33 20 27 0.74 Požega - XI 85,831 17 66 34 59 0.60 Slavonski Brod - XII 176,765 36 94 74 75 0.53 P i Zadar XIII 162,045 30 40 42 29 0.55 Osijek - Baranja XIV 330,506 67 74 51 61 0.56 Šibenik - Knin XV 112,891 22 39 30 32 0.61 Vukovar - Srijem XVI 204,768 41 92 69 73 0.52 Split - XVII 463,676 84 40 39 29 0.56 XVIII 206,344 27 76 81 49 0.50 Dubrovnik - Neretva XIX 122,870 21 71 55 51 0.60 Međimurje XX 118,426 23 75 56 59 0.66 CROATIA - total 4,437,460 782 68 85 50 0.57

Figure 1 also presents the arithmetic means of the annual indoor radon concentrations on the counties with gradations between 33 (county X) and 198 (country IX) Bq m-3 as shown in the legend according Table 1. Exception was made only for islands: they were divided into two classes (northern and southern) and the border between them was chosen as the border-line between XV and XVII counties. The common average concentration of the northern and southern islands were calculated and presented in Figure 1. Of course, the radon concentration means of counties had different statistical weights (number of measurements, n), because the n was chosen proportionally to county population size (in reality, the number of sent detectors was proportional to the county population, while the percentage of returned detectors on counties was from 60% to 90% nearly).

XX V VI II

I VII X XIV VIII III XI IV XVIII XII XVI

IX 50 Bq/m3

100 Bq/m3 XIII

3 XV 150 Bq/m

3 XVII 200 Bq/m

XIX

Figure 1. - The arithmetic means of the annual indoor radon concentrations on the Croatian counties with gradations between 33 (county X) and 198 (county IX) Bq m-3.

The further investigation was related to the distribution of the empirical frequencies (number of dwellings) on the classes of radon concentration. As presented in Fig. 2, the modal class of the empirical histogram, with the class width of log c = 0.2, had the frequency and the class mean of 175 and log c = 1.5 (c = 31.6 Bq/m3), respectively. The theoretical frequencies of the log-normal distribution were fitted to the empirical frequencies (log c), distributed on the class means, and the theoretical curve was obtained, as presented in Fig. 2. The statistical χ 2 – test, applied on the empirical and theoretical frequencies, did not show that the empirical frequency distribution for the radon in dwellings of Croatia belong to the log-normal distribution (calculated parameter, χ 2 = 52.04, was 2 higher than the theoretical one, χ 0.05 = 19.67, for 11 degrees of freedom and significance level of 0.05). Although, it was found, that the parameter χ 2 depended on choice of class width and in case of class width of 25 Bqm-3, the respective distribution of empirical frequencies was close to obey log-normal distribution.

180

160

140

120

100

80

Frequencies, f Frequencies, 60

40

20

0 0.5 1.0 1.5 2.0 2.5 3.0 log c

Figure 2. – Histogram of the number of dwellings (empirical frequencies, f) on the classes of the radon concentration (log c / Bq m-3) and fitted theoretical curve of the log- normal distribution.

The percentage of dwellings with a radon concentration above 200 and 400 Bq/m3 [9] was 5.4 and 1.8 %, respectively. The indoor radon concentrations in Croatia were near the radon levels of the surrounding countries of the Central or Southern Europe [2]; for instance, our arithmetic and geometric means (68 and 50 Bq/m3, respectively) were almost equal to those in Italy (70 and 52 Bq/m3), somewhat lower than in Slovenia (121 and 81 Bq/m3) and somewhat higher than those for Greece (55 and 44 Bq/m3). The assessment of the effective dose for indoor radon and its progeny (H) was performed by using the following equation [4, 10]:

H = (k1 + k2 F)cT, (2) 3 -1 -1 where the conversion factors were k1 = 0.17 nSv (Bq/m ) h and k2 = 9.0 nSv (Bq/m3)-1 h-1, but the F was the equilibrium factor, c was radon concentration and T (hours per year) dwelling time. Using method of two (diffusion and open) SSNT detectors [7], our measurements gave the average equilibrium factor F = 0.56; taking into account an occupancy factor of 0.60 (T = 0.6·365.25·24 h = 5,260 h) and the (above) average indoor radon concentration, c = 68 Bq/m3, we get for the Croatian population the average annual effective radon dose: H = 1.9 mSv. In the same way, around 243 thousand people in Croatia receive the annual radon dose higher than 5.5 mSv (> 200 Bq/m3), and 81 thousand people receive the annual radon dose higher than 11.0 mSv (> 400 Bq/m3). Indoor radon concentration versus radon in soil gas near the houses in northern parts of Croatia is presented in Figure 3; only results for houses without cellars are shown.

500

400

300 -3

200 c / Bqc / m

100

0 10 20 30 40 50 c / kBq m-3 s

-3 -3 Figure 3. – The indoor radon (c / Bq m ) versus radon in soil gas (cs / kBq m ) in northern part of Croatia; open circles (○) represent the empirical values but the line follow the regression equation (3).

The sedimentary types of rocks dominate at the northern parts of Croatia and the soil is usually of very low permeability. At some sites, even it was not possible to pump out soil gas into ionization chamber of AlphaGUARD detector during sampling time of 5 minutes. So, it could be said the diffusion is the dominant transport mechanism for radon in such soil types. This lack of vertical fluency of soil gas reduces the possibility of entry and accumulation of radon in houses so the amount of potential radon entry pathways (floor cracks and joints at concrete sub floors, sumps and drains …) become the key factor for elevated radon level in houses. Beside that, it is evident that the higher soil gas radon implies the higher indoor radon. For the total of 31 pair values (cs, c) from Figure 3, the correlation coefficient was r = 0.61; positive and statistically significant

r n − 2 which is showed by the test with Kendall’s variable ( t = = 4.146 > t0 = 1− r 2 2.045; this is limiting theoretical value of the Student’s variable for the significance level of 0.05 and (n-2) = 29 degrees of freedom [11]. So, by fitting a linear regression to the experimental data, the following equation of the regression between the indoor radon (c / Bq m-3) and -3 radon concentration in soil gas (cs / kBq m ):

c = a cs + b (3) where the parameters were a = 6.559 and b = -75.825. The radon measurements in soil of Croatia and the investigation of the influence of radon in soil gas on indoor radon are still going on especially on better determination of spatial distribution of sampling points using integrated population, grid square and geological approaches.

Conclusion

Integrated measurements of indoor radon with the LR-115 detectors in Croatia during one year gave radon concentrations in the range of 4 - 751 Bq/m3, with the arithmetic and geometric means of 68 and 50 Bq/m3, respectively. The arithmetic means of radon concentrations on 20 counties were between 33 and 198 Bq/m3. The statistical χ 2–test, applied on the empirical and theoretical frequencies, did not show that the empirical frequency distribution for the radon in dwellings of Croatia belonged to the log-normal distribution 2 2 (calculated parameter, χ = 52.04, was higher than the theoretical one, χ 0.05 = 19.67, for 11 degrees of freedom and significance level of 0.05). Although, it was found, that the parameterχ 2 depended on class choice and in case of class width of 25 Bqm-3, the respective distribution of empirical frequencies was close to obey log-normal distribution. The percentage of dwellings with radon concentrations above 200 and 400 Bq/m3 was 5.4 and 1.8 %, respectively. The indoor radon concentrations of Croatia were near the radon levels of the surrounding countries. The assessment of the annual effective dose for the indoor radon and its progeny, for the average equilibrium factor of 0.56 and occupancy factor of 0.6, gave the average effective dose of 1.9 mSv/y. In the same way, around 243 thousand people in Croatia receive the annual radon dose higher than 5.5 mSv (c ≥ 200 Bq/m3), and 81 thousand people receive the annual radon dose higher than 11.0 mSv (c ≥ 400 Bq/m3). Association between levels of indoor and soil radon was investigated and the statistically positive correlation was found.

Acknowledgements

This work was supported by the Croatian Ministry of Science. The authors are also grateful to Gabriela Poslon, Tanja Jačinović, Ana-Marija Elter and Darko Barešić for cooperation and technical assistance in this work.

References

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