THE RESULTS OF THE LITHUANIAN RADON SURVEY. Gendrutis Morkunas 1, Gustav Akerblom 2

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1 Radon in the Living Environment, 008 THE RESULTS OF THE LITHUANIAN RADON SURVEY Gendrutis Morkunas 1, Gustav Akerblom 2 1 Radiation Protection Centre, Kalvariju 153, LT-2042, Vilnius, Lithuania Tel , fax , genmo@takas.lt 2 Swedish Radiation Protection Institute, SE , Stockholm, Sweden Tel , fax , gustav.akerblom@ssi.se A national survey of indoor radon levels in Lithuania was performed between 1995 and The main objective of this survey was to evaluate the average of indoor radon concentrations in Lithuania and to determine whether there were significant variations with different areas. Measurements have been carried out in 400 randomly selected detached houses. The duration of one measurement was at least 3 weeks. The levels in two commonly used rooms on the lowest level were measured using passive electrets. Measurements were carried out during the cold weather season, 1 October and 30 April, i.e., when doors and windows generally are closed and the heating on. Information on house construction and layout, including the age of the house, the building materials and whether there was a basement, the type of water supply, as well as the ambient gamma dose rate, were also recorded. The results show that the arithmetic mean of indoor radon in the randomly selected detached houses is (55±4) Bq m -3 (confidence level 95%) and the geometric mean is 22 Bq m -3. A separate set of measurements was performed in Birzai karst region. The arithmetic and geometric mean values in detached houses in this region are (98±16) and 50 Bq m -3, respectively. Five regions (excluding the karst region ) where the indoor radon concentrations are two or more times higher than the average concentrations in the rest of Lithuania have been found. The source of indoor radon in Lithuania is the bedrock and the soils. The type and construction of house have significant influence on the indoor radon concentrations. The levels in houses built during the last 14 years are on average half those in houses built before The radon concentration in ground water is less than 30 Bq l -1. No statistically significant differences between winter and summer indoor radon concentrations have been found. The annual effective doses as a result of indoor radon have been estimated and are presented, the average value for detached houses being 0.97 msv. The national standards for restricting exposures to natural radiation sources have been adopted, action levels and intervention determined on the basis of the results obtained, intervention being different for dwellings and for workplaces. Key words: Lithuania, radon, concentration, surveys. INTRODUCTION Since indoor radon is one of the most important sources of human exposure to ionizing radiation the UNSCEAR statements "there is a justification to evaluate the natural background baseline in some detail" and "efforts should be made to continue to broaden the database used to determine the representative values and the extremes in exposures and to improve the dosimetric procedures" (UNSCEAR, 1999) are addressed to indoor radon, as well. Indoor radon surveys are being performed in many regions and countries. One of the aims of such surveys is the evaluation of the situation regarding indoor radon and the determination of locations where concentrations of indoor radon may be enhanced. 61

2 008 Radon in the Living Environment, The Lithuanian indoor radon survey started in 1990s. Recently data on indoor radon situation are available from this survey. They are presented in this paper. Lithuania is one of the Baltic States with a population of approximately 3.7 millions and territory of 65 thousand square kilometers. The bedrock in Lithuania consists of sedimentary consolidated and unconsolidated rock units from the Cambrian to Jurassic periods covering the Precambrian basement which consists of crystalline rocks. The basement rocks are not exposed anywhere. The thickness of sedimentary rock is meters. Uranium concentrations in the bedrock and the soils are low, usually less than 2.5 ppm (30 Bq kg -1 ). Since all space of Lithuania was covered by glacial ice during the last glacial period the soil mainly consists of moraine and glacial deposits of gravel, sand, silt and clay. Eskers have been formed along the ancient edge of the glacial ice. Another type of geological structure, which causes higher indoor radon concentrations, is karst in limestone areas. Such areas are situated in the northern part of Lithuania in the area around the city of Birzai. Measurements of concentrations of natural radionuclides in building materials show that building materials are not a source of radon indoors. Neither is domestic waters an important source of radon as no radon concentrations exceeding 30 Bq l -1 have been detected in the ground water. MATERIALS AND METHODS Selection of houses 400 detached houses were selected randomly (see Figure 1) using a specially designed computer code. Information on distribution of these houses (numbers of houses in all the administrative units - districts, cities and towns) was obtained from the Department of Statistics. Geographical co-ordinates of houses to be investigated have been generated by the code. When inhabitants, for some reason refused to cooperate or were absent the next closest house was investigated. One dwelling house in 1096 in rural areas and one house in 1120 in urban areas was investigated. Buildings containing a number of apartments were not included in this survey. Because higher concentrations of indoor radon would be expected to exist in the karst areas in the Birzai region, additional measurements were performed in houses of this region. Measurements of indoor radon concentration E-PERM TM electrets were used for measurements (Kotrappa et al. 1990, Rad Elec Inc. 1990). As part of the quality assurance program the measuring system has been tested through intercomparisons (Burian, 1997, Naismith et al. 1998). Indoor radon concentrations were measured in two permanently used rooms closest to the ground surface. In 98% of all cases these rooms were on the first floor. The rest of investigated rooms were on the second floor. Measurements were performed during the heating season, i.e., between November and March. The duration of one measurement was at least three weeks. Measurements of ambient gamma dose rate indoors were performed at the same time. The inhabitants were asked not to change their living habits during measurements. Additional information about each building was obtained. This information included geographical location of the building, its age, building material, presence or absence of a basement, type of water supply and number of floors. The 62

3 Radon in the Living Environment, 008 occupants have provided some of these data. RESULTS The average value of indoor radon concentration in detached houses was found to be (55±5) Bq m -3. The maximum detected concentration was 1860 Bq m -3, the geometric mean of values obtained in different rooms Bq m -3, in houses Bq m -3. Indoor radon concentrations in the Birzai karst region exceed the national average. The measurements were performed twice - in August and February. The results of these measurements are presented in Table 1. Application of the t-test indicates that there are no statistically significant differences between average values in winter and in summer. On the other hand, a statistically significant difference between concentrations in houses in the karst region and in randomly selected houses was found (p<0.01). There are 5 regions in Lithuania (except for the above mentioned karst region) that have higher than average indoor radon concentrations. The mean concentrations in these regions are presented in Table 2. These mean values differ from the mean values of indoor radon in the rest of Lithuania (p<0.01). This indicates that geographical factors have an influence on indoor radon concentrations. This variation is connected with geological conditions. For example, permeable hills of gravel and coarse sand which facilitates the transport of radon containing soil air from the soil into the buildings are more frequent in the eastern and southern regions. Reasons of higher indoor radon concentrations in other regions are not so clear. The distribution of indoor radon concentrations in separate rooms is presented in Fig 2. The distribution of indoor radon concentrations in houses obeys the same lognormal shape. The second maximum is observed in the distribution of indoor radon concentrations in houses of the Birzai karst region. This may be due to the fact that only some of the houses investigated in this region are affected by karst phenomena. The results for rural and urban houses are presented in Table 3. There are no statistically significant differences between these two groups. Indoor radon concentrations received in houses of different building materials are presented in Table 4. In this case only the major building material has been taken into account and no materials covering internal surfaces of walls, ceilings and floors have been considered. The t-test shows differences between houses made of: - concrete and all the other materials (except for clay), - wood with brick and brick, - clay and cinder. The relationship between year of construction, materials and radon concentrations, together with the intervals of confidence (2 sigmas) are shown in Table 4. Because of changes in construction 63

4 008 Radon in the Living Environment, technology, the building material used depends largely on the year of construction. The age dependence of the radon concentrations in houses is most likely due to differences in foundations and constructions of basements bottom slabs and structure of floors above the ground. There is only one exception - differences between wooden - brick houses and brick houses. The first category consists of wooden houses with the layer of bricks covering wooden walls. The space between the wooden wall and layer of bricks may change direction of flow of the air, which carries radon from soil so that a substantial part of this soil air may escape into open atmosphere. The dependence of indoor radon concentrations on building material is shown in Table 5. Houses of 15 years old and less (especially of 5 years and less) have lower values. This is most likely connected with more airtight construction barriers (e.g. basement slabs, basement walls and floors) in new houses. However, these barriers are not connected with presence of basements, as shown in Table 6. There is no statistically significant difference between the values in houses with a basement and houses without one. Also as shown in Table 7 the number of floors has no significant influence on indoor radon concentrations. However, indoor radon concentrations do vary significantly from floor level to floor level as shown in Table 8 (p<0.01). In respect of water supply Lithuanian houses may be divided into two categories - houses with municipal or individual water supply (with water supply) and houses where water is carried in buckets from wells (houses without water supply). It should be emphasized that houses with water supply have lower indoor radon concentrations (Table 9). This difference is due to differences in the ages of houses with water supply and without it. This variation with ages is statistically significant (p<0.01). No relationship between intensity of gamma radiation indoors and indoor radon concentrations has been observed (coefficient of correlation 0.07). The results from the radon measurements were used for calculation of the doses received from exposure to indoor radon in detached houses, using the ICRP conversion factors (ICRP 1993). In Lithuanian detached houses the average annual effective dose was (0.97±0.7) msv assuming 7000 hours stay indoors and an equilibrium factor of 0.4. Approximately, about 50 percent of Lithuanian population (1,8 million) live in detached houses. DISCUSSION The results show that the type of ground strongly influences indoor radon concentrations. The soil and the bedrock are the main sources of radon indoors. The results also show that the selection of houses to be investigated is a very important part of an indoor radon survey and give a good indication as to which types of houses and ground conditions are likely to cause high indoor radon concentrations and where remedial actions ought to be taken. This point is illustrated by the fact that the Birzai karst region with increased indoor radon concentrations was not detected during measurements in randomly selected houses, although the area of this region is a few thousands of square kilometers. 64

5 Radon in the Living Environment, 008 Now, then the regions with increased indoor radon concentrations have been identified, the findings should be taken into account when planning a strategy for making further measurements. One of results of Lithuanian indoor radon survey was the work out of a new national standard on natural exposure, which includes exposure to indoor radon, as well as other sources. The first standard was adopted in Since no results of measurements of indoor radon concentrations were available before 1995 the requirements of this standard concerning indoor radon levels were too strict and not at all essential issues were regulated by it (see Table 10). The new standard was adopted in the end of 1998 and is more realistic because it is based on results received from this survey of indoor radon. CONCLUSIONS 1. The main source of radon in Lithuanian houses is radon from the ground. 2. Indoor radon concentrations depend on the location of the house, its age and the floor level of the room where the measurement is undertaken. 3. There are regions with higher indoor radon concentrations. One of such regions is the Birzai karst region. 4. Special attention should be paid to the fact that not all the areas with higher indoor radon concentrations may be detected during measurements in randomly selected houses. ACKNOWLEDGMENTS The authors would like to extend their thanks to the Swedish Radiation Protection Institute for their support for this project as well as to colleagues at the Radiation Protection Centre A. Mastauskas, K. Gasiunas and J. Ziliukas and Dr. Ch. Hone from the Radiological Protection Institute of Ireland for their help and assistance. REFERENCES [1] Burian I. Intercomparisons of radon and its decay products measurement. Pribram, Czech Republic, 1997, 13 pp. [2] ICRP publication 65. Protection against radon-222 at home and at work. Pergamon Press, 1993; 23, 45 pp. [3] Kotrappa P, Dempsey JC, Ramsey RW, Stieff LR, A practical E-PERM TM (Electret Passive Environmental Radon Monitor) system for indoor 222 Rn measurement. Health Phys 1990; 58: [4] Naismith SP, Howarth CB, Miles JCH. Results of the 1997 European Commission intercomparison of passive radon detectors. National Radiological Protection Board, Chilton, United Kingdom, 1998, 111 pp. [5] Rad Elec Inc. E-PERM system manual. Rad Elec Inc. Frederick, USA, 1990, 39 pp. [6] United Nations Scientific Committee on the Effects of Atomic Radiation. Exposures from natural Sources. UNSCEAR, Vienna, 1999, 89 pp. 65

6 008 Radon in the Living Environment, Table 1. Indoor radon concentrations (with intervals of confidence) in detached houses of the Birzai karst region situated in a limestone area. Survey Average Geometric mean of concentrations, Bq m -3 concentration, Bq m -3 in rooms in houses February 95± August 101± Total 98± Table 2. Indoor radon concentrations (with intervals of confidence) in detached houses in 5 defined regions. Region Average concentration, Bq m -3 Eastern 77±10 Southern 82±22 Kretinga (north-western) 101±45 South-eastern 71±19 Northern 76±21 Table 3. Indoor radon concentrations (with intervals of confidence) in rural and urban detached houses. Houses Number of houses investigated Concentration, Bq m -3 Rural ±5 Urban 99 53±10 Table 4. Indoor radon concentrations as a function of the average of year of construction (with intervals of 95% confidence) in detached houses made of different building materials. Material Number of houses Concentration, Bq m -3 Year of construction investigated Concrete 5 9.6± ±3 Wood ±5 1944±2 Wood and concrete 6 76± ±9 Wood and bricks 13 42± ±8 Clay 2 34±22 Bricks ± ±3 Cinder 4 122± ±8 66

7 Radon in the Living Environment, 008 Table 5. Indoor radon concentrations (with intervals of confidence) in detached houses of different age. Age of house, in years Number of houses investigated Concentration, Bq m -3 less than ± ± ± ± ± ± ± ±14 75 and more 32 62±34 unknown 64 64±11 Table 6. Indoor radon concentrations (with intervals of confidence) in detached houses with basement and without it. Basement Number of houses investigated Concentration, Bq m -3 Present ±9 Absent ±5 Table 7. Indoor radon concentrations (with intervals of confidence) in detached houses with different number of floors. Number of floors Number of houses Concentration Bq m -3 investigated ± ±17 Table 8. Indoor radon concentrations on different floor levels of detached houses. Floor level Number of rooms investigated Concentration, Bq m ± ±11 67

8 008 Radon in the Living Environment, Table 9. Indoor radon concentrations in detached houses with water supply and without it. Concentrations and year of construction are presented with intervals of confidence. Water from wells is carried in buckets into the houses without a water supply. Water supply Number of houses Concentration, Bq m -3 Year of construction investigated Present ±8 1960±4 Absent ±5 1950±2 Table 10. Differences between Lithuanian indoor radon standards of 1993 and Item Standard of 1993 of 1998 Maximum permissible concentrations in existing houses, Bq m Maximum permissible concentrations in new houses, Bq m -3 Concept of radon-prone areas No Yes Maximum permissible concentrations in No 1000, depend on underground workplaces, Bq m -3 duration of work Maximum permissible concentrations in No 100 water, Bq l -1 68

9 Radon in the Living Environment, 008 Figure 1. Lithuania. Distribution of randomly selected detached houses where indoor radon measurements have been performed 69

10 008 Radon in the Living Environment, 0,4 0,3 N 0,2 0,1 0, C, Bq m -3 Figure 2. Distribution of indoor radon concentrations C in separate rooms 70