INDOOR RADON MAPPING IN FINLAND. Anne Voutilainen and Ilona Mäkeläinen

Transcription

1 Radon in the Living Environment, 058 INDOOR RADON MAPPING IN FINLAND Anne Voutilainen and Ilona Mäkeläinen STUK (Radiation and Nuclear Safety Authority) P.O. Box 14, FIN Finland Tel , fax anne.voutilainen@stuk.fi and ilona.makelainen@stuk.fi For over ten years STUK (The Radiation and Nuclear Safety Authority, Finland) has performed systematic indoor radon mapping with municipal health authorities. In the most radon-prone provinces (Uusimaa, Kymi and Häme) there are about 480,000 low-rise dwellings. It is expected that in 51,000 homes the action level of 400 Bq/m 3 is exceeded, of which about 6,000 have been detected. In the rest of Finland the numbers are: 820,000 low-rise dwellings, 17,000 expected cases of exceeding the limit, of which 800 were detected, respectively. In the three abovementioned provinces 20% of the houses investigated exceed the level of 400 Bq/m 3 and 40-50% exceed the level of 200 Bq/m 3. In the rest of Finland the figures are 4% and 13%, respectively. The most powerful search for radon should be focused on the provinces of Uusimaa, Kymi and Häme, where almost 80% of the houses exceeding the action level are situated. The search for high concentrations is worthwhile everywhere in Finland in areas where they have already been found. The search is easiest in esker areas and other sand and gravel deposits because they are easy to locate from geological maps. Key words: indoor radon, geology, mapping, esker, soil type INTRODUCTION In Finland, indoor radon concentrations are among the highest in the world. The reasons for high concentrations are the cold climate, the construction and air-tightness of buildings, elevated uranium concentration in the ground and permeable eskers. In some cases radon-rich water from drilled wells is a notable source. According to the resolution of the Ministry of Social Affairs and Health, in existing homes the indoor radon concentration should not exceed 400 Bq/m 3. New buildings should be planned and constructed in such a way that the radon concentration does not exceed 200 Bq/m 3. STUK has performed systematic radon mapping in cooperation with municipal health authorities since 1986 (National Board of Health 1986, Ministry of Social Affairs and Health 1997). STUK has made radon measurement plans and radon risk maps to identify radon-prone areas. The health authorities have distributed the dosemeters to homes. The results of the decade s work is published in two STUK s reports: Radon in dwellings in Finland (Voutilainen et al. 1997a) and Radon Atlas of Finland (Voutilainen et al. 1997b). The former summarizes the search for high radon concentrations with municipal health authorities. This article is a summary of that report. The latter contains detailed information on the radon situation in Finland. It offers radon maps of houses built on permeable or impermeable building sites by provinces, and statistics of radon concentrations by municipality. Some of the maps are also found on the internet The methodology of radon mapping in municipalities is also discussed elsewhere (Castrén et al. 1992). 513

2 058 Radon in the Living Environment, RADON MEASUREMENTS STUK began investigating indoor radon concentrations in the 1970s. Measurements have been carried out on a larger scale since 1986 and they have been taken mostly in low-rise dwellings and first-floor flats. The radon dosemeter used by STUK for measurements gives a long-term mean radon concentration (Mäkeläinen 1986). Measurements were taken during the heating period (from early November to late April), because radon concentration is higher during the cold season. The measurement period was two months. In some STUK projects radon dosemeters were placed in homes for a whole year. To obtain comparable annual averages from two-month measurements, a model that adjusts to the effects of outdoor temperature and wind speed was applied (Arvela 1995). Radon dosemeters were sent to customers and returned to STUK by post. The health authorities usually distributed the dosemeters belonging to municipal radon-mapping programmes to the dwellings. They marked the positions of the houses under study on maps and mailed the maps to STUK. STUK determined the coordinates of the houses and the soil types of the building sites from the questionnaires filled out by the home owners and from geological maps. The information from the questionnaires concerning whether a house was situated on bedrock or not was considered to be reliable. The soil types for other building sites were determined from soil maps and maps of sand and gravel deposits. We used a TOPOS mapping system in digitizing the coordinates and in determining the soil types (Timo Pekkonen Inc., Helsinki, Finland). In the radon-mapping programme we usually used one dosemeter placed on the lowest residential floor, not in a cellar but always in a dwelling unit. If we used two dosemeters, one was placed on the lowest residential floor and the other in the bedroom or living-room, regardless of the floor. Data on radon concentration, the type of construction and the geology of the building site were recorded in STUK s indoor radon database, which today includes data on 52,000 houses. The exact coordinates of 38,000 dwellings are known. During the municipalities have used 3.4 million FIM (approx. 630,000 USD) by ordering 33,000 dosemeters for radon measurements. Private persons have ordered 24,000 dosemeters and STUK has used 34,000 dosemeters for its own investigations. RADON MEASUREMENT PLANS AND PROGNOSIS MAPS The aim of the radon measurement plan is to locate houses with high indoor radon concentration. The purpose of the radon prognosis map is to prevent the building of new houses with high radon concentrations. Measurement plans describe the radon situation among the prevailing housing stock, whereas prognosis maps show the radon potential of the building ground. In our prognosis maps radon potential stands for indoor radon concentration of the potential worst case house type built without any countermeasures against radon. STUK started to make radon measurement plans in 1986, and in 1994 we completed the work of making first-stage radon measurement plans. Now they cover all of the 455 municipalities in Finland. In areas where high indoor radon concentrations are found, further measurements are carried out, and so STUK's work in identifying radon-prone areas continues. We consider a "radonprone" area as being an area in which it is possible to measure indoor radon concentrations exceeding 400 Bq/m 3. Now we draw up so called radon situation reports which include maps of the 514

3 Radon in the Living Environment, 058 radon situation in the municipality and recommendations for further measurements. These reports now cover about 183 municipalities out of the total of 455. Radon prognoses are regional radon risk maps. The purpose of the radon prognosis is to classify areas and soil types with different levels of radon risk. The radon prognosis gives the percentages of future homes expected to have indoor radon concentration exceeding the levels of 200 and 400 Bq/m 3, when no protection against the entry of radon is used in construction. We use indoor radon monitoring data and geological information adjusted according to an empirical statistical model (Voutilainen and Mäkeläinen 1992 and 1993). RADON CONCENTRATIONS IN FINLAND Representative radon survey STUK made a representative indoor radon survey to investigate the exposure of Finns to radon (Arvela et al. 1993, Castrén 1994). Approx. 2,000 low-rise dwellings and 1,000 block of flats were measured during Table 1 shows the mean radon concentrations in low-rise dwellings and flats and the estimated number of houses exceeding levels 200, 400 and 800 Bq/m 3, respectively. All indoor radon measurements STUK s indoor radon data base currently consists of information on 51,700 low-rise dwellings. The exact coordinates are known from about 38,000 homes. The measurement results are higher than in the random sample because the aim of most of these measurements has been to search for high concentrations. Table 2 shows statistics by province based on all 51,700 radon measurements taken by STUK. Figure 1 shows the location of provinces as they existed before 1 September Figure 2 shows the mean radon concentrations in municipalities. The map gives an illustration of the geographical distribution of radon, although the measurements are not representative. THE EFFECT OF THE SOIL TYPE ON THE INDOOR RADON CONCENTRATION The differences between the uranium concentration and the permeability of the ground effect the geographical distribution of indoor radon concentrations. The mean radon concentration and the number of high concentrations are usually higher in houses built on permeable soil types than on other soil types. The random sample of indoor radon measurements provides representative information on the distribution of low-rise dwellings on different building sites. Table 3 shows the distribution of building sites between permeable and low-permeable soil types. It also shows the percentages of homes exceeding 400 Bq/m 3 in the whole country, in the most radon-prone area (area 1) and in the rest of Finland. Less than 20% of houses are built on eskers and other permeable soil types. Although about 40% of cases of exceeding limits are found in areas consisting of sand and gravel. In the most radon-prone area the percentages are about 30 and 50, respectively. THE SUCCESS OF THE RADON MEASUREMENT PROGRAMMES According to the representative radon survey it can be estimated that in Finland there are about 66,000 houses in which the indoor radon concentration exceeds 400 Bq/m 3. These include 59,000 low-rise 515

4 058 Radon in the Living Environment, dwellings and 7,000 flats (Arvela and Castrén 1994). Adjusted to 1995 housing statistics, the number of low-rise dwellings exceeding 400 Bq/m 3 is larger, about 68,000. In STUK s radon data base there are 6,621 low-rise dwellings in which the radon concentration exceeds 400 Bq/m 3. We have measured 4% of all low-rise dwellings in Finland and found only 10% of all estimated cases of exceeding limits. Table 4 shows the search for and the locating of houses exceeding 400 Bq/m 3. The estimates of the number of houses exceeding 400 Bq/m 3 in other provinces than Uusimaa, Kymi and Häme are not exact because in the random sample there were only 1-5 houses exceeding 400 Bq/m 3. In the most radon-prone provinces (Uusimaa, Kymi and Häme) there are about 480,000 low-rise dwellings. It is expected that in 51,000 homes the action level of 400 Bq/m 3 is exceeded, of which about 6,000 have been detected. In the rest of Finland the numbers are: 820,000 low-rise dwellings, 17,000 expected cases of exceeding limits of which 800 have been detected, respectively. Table 5 shows the number of houses built on different building sites exceeding the action level of 400 Bq/m 3. The most radon-prone area consists of the province of Häme and the eastern part of Uusimaa and the western part of Kymi (as in table 3). The numbers in table 5 are smaller than in table 4 because the soil type of the building site is not determined by all measurements. 13% of all houses measured exceed 400 Bq/m 3. The percentage is not higher because of the nature of systematic radon mapping. The purpose of the first stage of the radon measurement plan is to gain an overview of the radon situation in the municipality. Therefore some measurements are always directed to areas expected to have low radon concentrations. These low results are also useful. They tell about the radon situation in the area when building new houses. However, most of the measurements are always directed to areas expected to have high radon concentrations. When the municipalities later carry out the second or third stage of the measurement plan the results of locating houses exceeding the action level are improved. Seeking high radon concentrations has been only one purpose of STUK s radon measurement programmes. We have measured indoor radon concentrations when investigating the effects of radon on health. The representative survey gave information on the radon exposure of Finns. Private persons have ordered measurements all over Finland. Therefore it is evident that most of the measurements in our data base are below the action level of 400 Bq/m 3. CONCLUSIONS There are two ways to reduce radon risks: 1) Houses with radon levels in excess of the action level of 400 Bq/m 3 must be located and mitigated and 2) new radon problems should be prevented in new buildings. The aim of planning and building is to achieve as low indoor radon concentrations as possible, in all cases below 200 Bq/m 3. Detailed information about radon risks in building areas already exists in many municipalities. Since it is not available in all municipalities, STUK has published the Radon Atlas of Finland (Voutilainen et al. 1997b). With its maps and tables it is a useful tool for those who plan and decide what kind of radon mitigation measures are needed in municipalities. Many Finnish municipalities have provided a great deal of radon information for our data base. However, there are also many municipalities which have taken only a few radon measurements or which have not taken measurements following any measurement plan. Most of these municipalities are situated in low-radon provinces. It may be sufficient to measure radon concentration in

5 Radon in the Living Environment, 058 homes to gain a picture of the radon situation in the municipality. There are also municipalities which have made sufficient measurements and finished their radon measurement plans. The most concentrated search for radon should be focused on the provinces of Uusimaa, Kymi and Häme, where almost 80% of the houses exceeding the action level are situated. The search for high concentrations is worthwhile everywhere in Finland in areas where they have already been found. The search is easiest in esker areas and other sand and gravel deposits because these permeable formations are easy to locate from geological maps. If the indoor radon concentration exceeds 400 Bq/m 3 we recommend that the inhabitants take mitigation measures. Sub-slab suction and radon well have been proven to be the most efficient methods. It is cheaper and easier to decrease radon concentrations when building new houses than when renovating existing houses. Radon-safe construction is almost always cheaper than investigating radon levels on building sites. This is why it is reasonable to build radon-safe dwellings in all esker areas throughout the country and on other building sites in most parts of Finland following the recommendations of the guide published by the Ministry of the Environment (1994). It may be necessary to use more efficient methods in the areas with the highest radon levels. In the provinces of Oulu, Vaasa and Kuopio, especially in areas characterised by impermeable soil types, radon levels rarely exceed 200 Bq/m 3. In these areas, decisions about the need for radon-safe construction should be taken by individual municipalities. Building radon-safe is a new, demanding challenge which can be met only by cooperation between builders, designers, building authorities and other authorities. REFERENCES [1] Arvela H. Residential Radon in Finland: Sources, Variation, Modelling and Dose Comparisons. STUK-A124. Oy Edita Ab, Helsinki, 1995, 92 pp. [2] Arvela H ja Castrén O. Costs of radon mitigation in Finnish dwellings. STUK-A114. Oy Edita Ab, Helsinki, 1994, 39 pp. (abstract in English) [3] Arvela H, Mäkeläinen I and Castrén O. Residential Radon Survey in Finland. STUK-A108. Oy Edita Ab, Helsinki, 1993, 49 pp. (abstract in English) [4] Castrén O. Radon reduction potential of Finnish dwellings. Radiation Protection Dosimetry 1994; 56, 1-4: [5] Castrén O, Arvela H, Mäkeläinen I and Voutilainen A. Indoor radon survey in Finland: methodology and applications. Radiation Protection Dosimetry 1992; 45, 1/4: [6] Ministry of the Environment. Radon safe buildings, foundation construction. Guide 2/1993. Painatuskeskus Oy, Helsinki, 1994, 32 pp. (in Finnish) [7] Ministry of Social Affairs and Health. Indoor air guide 1997:1.Oy Edita Ab, Helsinki, 1997, 72 pp. (in Finnish) [8] Mäkeläinen I. Experiences with track etch detectors for radon measurements. 13th international conference on solid state nuclear track detectors, , Rome, Italy. Nuclear Tracks 1986; 12, 1-6: [9] National Board of Health. Directive 2/86, Painatuskeskus Oy, Helsinki,1986 (in Finnish) 517

6 058 Radon in the Living Environment, [10] Voutilainen A and Mäkeläinen I. The use of indoor radon measurements and geological data in making radon prognosis map of Tampere. Eastern European workshop on geological aspects of radon risk mapping, Prague, Czech and Slovak Federative Republic. Extended Abstracts, 1992: [11] Voutilainen A and Mäkeläinen I. Radon risk mapping using indoor monitoring data a case study of the Lahti area, Finland. Indoor Air 1993; 3: [12] Voutilainen A, Mäkeläinen I, Pennanen M, Reisbacka H and Castrén O. Radon Atlas of Finland. STUK-A148. Oy Edita Ab, Helsinki, 1997, 125 pp. (b). [13] Voutilainen A, Mäkeläinen I, Reisbacka H and Castrén O. Radon in dwellings in Finland. STUK- A146. Oy Edita Ab, Helsinki,1997, 34 pp. (abstract in English) (a) 518

7 Radon in the Living Environment, 058 Table 1: STUK s representative radon survey (Arvela et al. 1993). Mean radon concentrations, and percentages and estimated number of houses exceeding 200, 400 and 800 Bq/m 3. Arithmetical mean Bq/m 3 >200 Bq/m 3 % (number) >400 Bq/m 3 % (number) >800 Bq/m 3 % (number) Low-rise dwellings Flats (209,000) 1.6 (16,000) 5.0 (59,000) 0.8 (7,000) 1.4 (16,000) 0.3 (3,000) All dwellings (225,000) 3.6 (66,000) 1.0 (19,000) Table 2: Statistics based on radon measurements from STUK s indoor radon database. Data include single-family houses, semi-detached houses, row houses and first-floor flats. Four columns include the percentage of houses investigated exceeding 200, 400, 800 and 2000 Bq/m 3. provinces houses investigated Arithm. mean Bq/m 3 >200 Bq/m 3 % >400 Bq/m 3 % >800 Bq/m 3 % >2000 Bq/m 3 % max Bq/m 3 whole country 51, ,700 Åland Häme 12, ,700 Central Finland 2, ,600 Kuopio 1, ,100 Kymi 7, ,200 Lapland 1, ,700 Mikkeli 1, ,900 Oulu 2, ,800 North Karelia 1, ,200 Turku and Pori 4, ,200 Uusimaa 12, ,000 Vaasa 2, ,

8 058 Radon in the Living Environment, Table 3: The percentages of houses built on different building sites of all houses and the percentages of houses exceeding 400 Bq/m 3 of all cases of exceeding limits in the area according to the representative survey. Building site Whole country Area 1* Rest of Finland All dwellings > 400 Bq/m 3 All dwellings > 400 q/m 3 All dwellings > 400 Bq/m 3 Permeable 18% 42% 28% 48% 15% 35% Impermeable 82% 58% 72% 52% 85% 65% * Häme province, eastern part of Uusimaa and western part of Kymi provinces Permeable = eskers and other sand and gravel formations Impermeable = bedrock, till, clay and silt Table 4: The number of low-rise dwellings, the number of homes exceeding 400 Bq/m 3 estimated according to the representative radon survey and the 1995 housing statistics, and the number of detected houses exceeding 400 Bq/m 3. Province Low-rise dwellings built in the area Houses estimated to exceed 400 Bq/m 3 Houses detected to exceed 400 Bq/m 3 Whole country 1,300, ,000 6,621 Åland 7, Häme 170,000 23,000 2,614 Central Finland 71,000 3, Kuopio 69,000 2, Kymi, whole West part East part 93,000 54,000 39,000 12,000 8,600 3,600 1,693 1, Lapland 61,000 1, Mikkeli 61,000 1, Oulu 130, North Karelia 56,000 1, Turku and Pori 190,000 1, Uusimaa, whole West part East part 220, ,000 38,000 16,000 11,000 4,800 1, ,054 Vaasa 140,000 1,

9 Radon in the Living Environment, 058 Table 5: Low-rise dwellings built on different building sites, the number of homes exceeding 400 Bq/m 3 estimated according to the representative radon survey and the 1995 housing statistics, and the number of detected houses exceeding 400 Bq/m 3. Area Soil type of building site Low-rise dwellings built in the area Houses estimated to exceed 400 Bq/m 3 Houses detected to exceed 400 Bq/m 3 Häme province Permeable 50,000 14,000 1,531 Impermeable 120,000 9, Western part of Kymi province Eastern part of Uusimaa province Permeable Impermeable Permeable Impermeable 14,000 40,000 7,800 30,000 1,700 6,100 1,700 2,800 Rest of Finland Permeable 142,000 9, Impermeable 850,000 20, Permeable = Eskers and other sand and gravel formations Impermeable = Bedrock, till, clay and silt 521

10 058 Radon in the Living Environment, Figure 1: Provinces as they existed before 1 September

11 Radon in the Living Environment, 058 Figure 2: Arithmetical mean radon concentrations in municipalities. Only two municipalities are missing measurements. 523

12 058 Radon in the Living Environment, 524