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Radon exposure levels of the staff in the drinking water supply facilities in Bavaria, Germany
International Congress Series 1225 (2002) 81 – 86
Radon exposure levels of the staff in the drinking water supply facilities in Bavaria, Germany M. Trautmannsheimer *, W. Schindlmeier, K. Hu¨bel Bavarian Environmental Protection Agency, D-86177 Augsburg, Germany
Abstract Within the framework of a study covering the whole of Bavaria, water supply facilities were investigated as to radon concentrations in groundwater and indoor air and as to the radon exposure levels to which the staff working in these buildings were subjected. An inquiry sheet was sent to all drinking water supply facilities in Bavaria (Germany). Two thousand six hundred facilities were asked to provide detailed information about their facility and the duration of stay and about typical work processes. Over 500 water supply facilities were selected to take a 1-l ground water sample and to expose track-etch detectors in order to get the mean room concentration of the main working places. In addition, for a period of 2 months, the personnel had to wear a track-etch detector whilst they were in the supply facilities in order to get an estimate of their individual exposure level. In the east Bavarian crystalline region, indoor radon gas concentrations of up to 300 kBq/m3 were observed. About 10% of the processing plant workers of this region get an annual effective dose of more than 20 mSv. The application of effective means to reduce the radon exposure levels of these persons is in progress. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ground water; Working places; Track-etch detectors; Water purification; Effective dose
1. Introduction Since the publication of the results of several epidemiological studies on American and Canadian miners [1], there is no doubt that high exposure to radon and its progenies through inhalation can cause lung cancer. Besides mining, there are several other workplaces where increased radon exposure can be expected. Especially high radon concen-
* Corresponding author. Bayerisches Landesamt fu¨r Umweltschutz, D-86177 Augsburg, Germany. Tel.: +49821-9071-5294; fax: +49-821-9071-5554. E-mail address: [email protected] (M. Trautmannsheimer).
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 1 7 - 9
82
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Table 1 Average annual duration of stay of a processing plant worker in the buildings of the Bavarian water supply facilities Building type
Mean value [h/year]
Elevated reservoirs Water purification Collecting galleries All buildings
170 270 30 230
trations have been measured in visitor caves and mines, radon spas and water supply facilities. Therefore, in May 1996, the European Commission passed the Council Directive 96/29/Euratom [2], laying down the basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. The annual recommended upper limit for natural radiation for a worker is 20 mSv: the equivalent of the limit set for the effective dose of artificial radiation. The deadline for the implementation of legislation in the member states of this Council Directive was May 2000. In 1995, the Bavarian State Ministry for State Development and Environmental Affairs set up a research project to study the level of radon exposure to which the staff of the Bavarian water supply facilities were subjected. After various preliminary investigations, an inquiry sheet was sent to all 2600 Bavarian water supply facilities. Processing plant workers, supervisors and cleaning staff were asked to provide information about number, type, water extraction and ventilation of their supply facilities. They were also asked to
Fig. 1. Distribution of the duration of stay of a processing plant worker summarized over all visits in all of the elevated reservoirs, water purification units and collecting galleries. (The figure shows the distribution in two different scales. The left and lower axes belong to the black, the right and upper to the grey bars).
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
83
answer questions about the duration of stay and about their typical work processes in the elevated reservoirs and in the water purification buildings. About 50% of the water supply facilities returned the inquiry sheet containing data of 1000 employees and 9000 buildings. From this data, it was calculated that approximately 4500 persons work in about 20 000 buildings for the Bavarian water supply facilities. These persons stay in the supply buildings for regular inspections and cleaning of the elevated reservoirs, water purification units and collecting galleries. The average duration of stay in the water purification for back washing is about 2 h/week. In addition to this regular stay, the elevated reservoirs had to be cleaned thoroughly in intervals of approximately 1 year. The cleaning is often done by special staff or external companies. The average annual duration of stay of the processing plant workers extracted from the data in the inquiry sheets is given in Table 1. The distribution of the duration of stay summarized over all visits in all of the elevated reservoirs, water purification units and collecting galleries is shown in Fig. 1. The maximum of the distribution is around 30 h/ year, the median close to 100 h/year.
Fig. 2. The 10 main geological regions of Bavaria. Rock types of the regions: new red sandstone (1); shell limestone, Keuper (2); Franconian Keuper (3); Upper Jurassic, Dogger, Cretaceous (4); Granite, Gneiss (5, 10); Ejection material of the Ries meteorite (6); Sediment rocks, Molasses (7); young moraine (8); Trias, Jura, Tertiary (9).
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M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Bavaria, which has an area of 70 600 km2, can be partitioned into 10 main geological regions (see Fig. 2) according to the geological formations and the main aquifers. Taking into account the results of the radon soil gas measurements [3] and the information of the mean uranium and radium contents in the ground, the 10 regions can be classified according to their ‘‘radon potential’’. The highest potential is assigned to the east Bavarian region (No. 5) with its mainly granite substructure.
2. Radon measurements From each region, a total number of more than 500 water supply facilities proportional to the size of the region was selected for radon (Rn-222) measurements. The processing plant workers were asked to take a 1-l ground water sample in a radon tight PET bottle. For the sampling, a water hose was fixed at a tap of the raw water pipe and pushed down to the base of the bottle in order to fill the bottle very slowly and without turbulence. Preliminary experiments revealed that the loss of radon gas during this filling procedure is negligible. After filling, the sample was immediately sent to the gamma-spectroscopy lab at the Bavarian Environmental Protection Agency for analysis. Several track-etch detectors were sent by mail to the water supply facilities. They were exposed for a period of 2 weeks in order to get the mean room concentration of the main working places of the staff. In addition, the processing plant worker had to wear a personal track-etch detector for 2 months, which was fixed to his clothes during his stay in the supply facilities. From the recorded measurements, his individual effective dose was estimated. When not in use, the personal detector was stored near a reference detector at a place with low radon concentration. An instruction sheet was enclosed to aid the selection of a suitable storage place. Sites such as car garages, car boots or well ventilated areas such as rooms with constantly open windows, office rooms outside the supply facilities and roofed balconies were recommended. In spite of the fact that track-etch detectors cannot be switched off, the exposure resulting purely from the stay in the water supply facilities can be calculated by the exposure of the reference detector and the protocol of the duration of stay. From the results of these 2-month measurements, the annual dose of the processing plant worker was estimated. If the processing plant worker had to participate in the annual cleaning of the elevated reservoirs during the 2-month measuring period, he had to order an additional track-etch detector to record his exposure levels during the cleaning separately. The track-etch detectors, about 3000 in number, were obtained and evaluated by the GSF-National Research Center for Environment and Health, Neuherberg, Germany. These are CR39 type detectors in a diffusion chamber [4]. The system was also successfully tested for short time exposure measured in minutes. It is insensitive to Thoron. The measuring error (2-sigma standard deviation) is about 25% and almost unaffected by the recorded exposure. Much larger errors can occur by the calculation of the personal exposure of the processing plant workers. In order to keep the error level below 30%, the average radon concentration at the storage place must be below 200 Bq/m3. This concentration can be checked by the exposure of the reference detector. About 10% of the measurements with personal detectors had to be repeated because of poor measuring
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
85
Table 2 Results from the measurements of the raw water samples Region
1
2
3
4
5
6
7
8
9
10
Number of samples Maximum [Bq/l] Median [Bq/l]
25 46 12
31 68 11
99 266 10
51 38 6
181 1220 50
10 16 3
67 132 7
51 140 8
26 29 7
9 40 9
precision. In this case, the processing plant worker was advised by telephone how to find a better storage place. Using electronic systems, additional time-resolved measurements were taken in a few supply facilities. These continuous measurements were taken over a period of about 3 weeks. With these measurements, the influence of the different working processes on the radon concentration and the time variation of the equilibrium factor could be observed.
3. Results The results of about 550 measurements of raw water samples from the whole of Bavaria are shown Table 2. As expected, region 5 shows an increased average radon concentration in the raw water samples. Around 1100 measurements were taken in order to obtain information about the indoor concentrations in the water supply facilities (Table 3). As not every room in the buildings could be supplied with a track-etch detector, the processing plant workers were asked to place the detectors mainly in elevated reservoirs, water purification units and rooms with long duration of stay. In all regions, there are some rooms with concentrations of more than 10 kBq/m3. Taking into account that the normal annual working time is 2000 h/year, indoor concentrations of more than 3 kBq/m3 can lead to a dose above the limit of 20 mSv. Region 5 revealed the highest concentrations and, therefore, the median is significant higher than that of the other regions. The main purpose of this research project was to obtain information about the annual radon exposure levels of the staff in the Bavarian water supply facilities. About 500 measurements taken over a period of 2 months were performed. From this data, a linear estimate of the annual exposures was made (Table 4). Under ‘‘normal conditions’’ (equilibrium factor of about 0.4, unattached fraction of about 5– 10%), an exposure of 2 and 6 MBq h/m3 is equivalent to an effective dose of 6 and 20 mSv, respectively. In Table 3 Distribution of the radon indoor concentrations in the Bavarian supply facilities Region
1
2
3
4
5
6
7
8
9
10
Number of samples >3 kBq/m3 [%] >10 kBq/m3 [%] Maximum [kBq/m3] Median [kBq/m3]
48 6 4 30 0.5
60 18 7 130 0.7
205 15 4 18 0.7
95 11 3 35 0.7
360 52 31 271 3.3
21 10 10 16 0.5
141 26 3 16 1.1
106 17 9 27 0.7
53 17 8 29 0.7
18 39 28 33 1.2
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M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Table 4 Distribution of the annual radon exposure levels of the staff of the Bavarian water supply facilities Region
1
2
3
4
5
6
7
8
9
10
Number of samples >2 MBq h/m3 [%] >6 MBq h/m3 [%] Median [MBq h/m3]
24 8 0 0.3
25 8 4 0.3
94 14 4 0.3
43 2 0 0.3
149 18 11 0.4
8 12 0 0.1
54 8 4 0.2
43 12 0 0.1
24 0 0 0.1
8 0 0 0.4
accordance with the regulations of the Council Directive 96/29/Euratom, a continuous supervision of persons with annual exposure levels of between 2 and 6 MBq h/m3 is recommended. Since radon exposure levels are not only influenced by the radon concentration in the ground water, the special position of region 5 in Table 4 is not as clear as in Table 2.
4. Conclusions Prediction of the radon exposure levels of the staff without measurements is very difficult because exposure levels are influenced by (1) radon concentration in the untreated ground (raw) water, (2) characteristics of the buildings and work processes such as ventilation of the rooms, fraction of room volume to water surface of the basins, water flow through the basins, turbulent filling of the basins (cascades), enhanced radon production by special work processes (back-washing, filling times of the reservoirs), and (3) duration of stay in the water supply facilities. In all geological regions, exposure levels over 2 MBq h/m3/year can occur. In the granite region in east Bavaria, about 70 workers (10% of the staff of region 5), and in the rest of Bavaria about 50 (2% of the staff of all regions except 5) workers, are subjected to exposure levels of over 6 MBq h/m3/year. To reduce the exposure levels, it is very important to provide detailed information for the water supply facilities because there are usually simple ways in which to minimize the duration of stay in the buildings. Other effective means to reduce the exposure levels of the staff are optimization of the ventilation or air-conditioning systems, separation of frequently used working places from basins or water purification systems and the avoidance of turbulence whilst filling the basins.
References [1] J.H. Lubin, et al., Radon and lung cancer risk: a joint analysis of 11 underground miners studies. US National Institutes of Health. NIH publication No. 94-3644, 1994. [2] Council Directive 96/29/Euratom, basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. Official Journal No. 159: 1 – 114, 29/06/ 1996. [3] D.T. Bertlett, G.J. Gilvin, R. Still, D.W. Dixon, J.C.H. Miles, The NRPB radon personal dosimetry service, J. Radiol. Prot. 8 (1) (1988) 19 – 24. [4] J. Kemski, R. Klingel, A. Siehl, Classification and mapping of radon affected areas in Germany, Environ. Int. 22 (Suppl. 1) (1996) 789 – 798.
Radon exposure levels of the staff in the drinking water supply facilities in Bavaria, Germany M. Trautmannsheimer *, W. Schindlmeier, K. Hu¨bel Bavarian Environmental Protection Agency, D-86177 Augsburg, Germany
Abstract Within the framework of a study covering the whole of Bavaria, water supply facilities were investigated as to radon concentrations in groundwater and indoor air and as to the radon exposure levels to which the staff working in these buildings were subjected. An inquiry sheet was sent to all drinking water supply facilities in Bavaria (Germany). Two thousand six hundred facilities were asked to provide detailed information about their facility and the duration of stay and about typical work processes. Over 500 water supply facilities were selected to take a 1-l ground water sample and to expose track-etch detectors in order to get the mean room concentration of the main working places. In addition, for a period of 2 months, the personnel had to wear a track-etch detector whilst they were in the supply facilities in order to get an estimate of their individual exposure level. In the east Bavarian crystalline region, indoor radon gas concentrations of up to 300 kBq/m3 were observed. About 10% of the processing plant workers of this region get an annual effective dose of more than 20 mSv. The application of effective means to reduce the radon exposure levels of these persons is in progress. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ground water; Working places; Track-etch detectors; Water purification; Effective dose
1. Introduction Since the publication of the results of several epidemiological studies on American and Canadian miners [1], there is no doubt that high exposure to radon and its progenies through inhalation can cause lung cancer. Besides mining, there are several other workplaces where increased radon exposure can be expected. Especially high radon concen-
* Corresponding author. Bayerisches Landesamt fu¨r Umweltschutz, D-86177 Augsburg, Germany. Tel.: +49821-9071-5294; fax: +49-821-9071-5554. E-mail address: [email protected] (M. Trautmannsheimer).
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 1 7 - 9
82
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Table 1 Average annual duration of stay of a processing plant worker in the buildings of the Bavarian water supply facilities Building type
Mean value [h/year]
Elevated reservoirs Water purification Collecting galleries All buildings
170 270 30 230
trations have been measured in visitor caves and mines, radon spas and water supply facilities. Therefore, in May 1996, the European Commission passed the Council Directive 96/29/Euratom [2], laying down the basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. The annual recommended upper limit for natural radiation for a worker is 20 mSv: the equivalent of the limit set for the effective dose of artificial radiation. The deadline for the implementation of legislation in the member states of this Council Directive was May 2000. In 1995, the Bavarian State Ministry for State Development and Environmental Affairs set up a research project to study the level of radon exposure to which the staff of the Bavarian water supply facilities were subjected. After various preliminary investigations, an inquiry sheet was sent to all 2600 Bavarian water supply facilities. Processing plant workers, supervisors and cleaning staff were asked to provide information about number, type, water extraction and ventilation of their supply facilities. They were also asked to
Fig. 1. Distribution of the duration of stay of a processing plant worker summarized over all visits in all of the elevated reservoirs, water purification units and collecting galleries. (The figure shows the distribution in two different scales. The left and lower axes belong to the black, the right and upper to the grey bars).
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
83
answer questions about the duration of stay and about their typical work processes in the elevated reservoirs and in the water purification buildings. About 50% of the water supply facilities returned the inquiry sheet containing data of 1000 employees and 9000 buildings. From this data, it was calculated that approximately 4500 persons work in about 20 000 buildings for the Bavarian water supply facilities. These persons stay in the supply buildings for regular inspections and cleaning of the elevated reservoirs, water purification units and collecting galleries. The average duration of stay in the water purification for back washing is about 2 h/week. In addition to this regular stay, the elevated reservoirs had to be cleaned thoroughly in intervals of approximately 1 year. The cleaning is often done by special staff or external companies. The average annual duration of stay of the processing plant workers extracted from the data in the inquiry sheets is given in Table 1. The distribution of the duration of stay summarized over all visits in all of the elevated reservoirs, water purification units and collecting galleries is shown in Fig. 1. The maximum of the distribution is around 30 h/ year, the median close to 100 h/year.
Fig. 2. The 10 main geological regions of Bavaria. Rock types of the regions: new red sandstone (1); shell limestone, Keuper (2); Franconian Keuper (3); Upper Jurassic, Dogger, Cretaceous (4); Granite, Gneiss (5, 10); Ejection material of the Ries meteorite (6); Sediment rocks, Molasses (7); young moraine (8); Trias, Jura, Tertiary (9).
84
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Bavaria, which has an area of 70 600 km2, can be partitioned into 10 main geological regions (see Fig. 2) according to the geological formations and the main aquifers. Taking into account the results of the radon soil gas measurements [3] and the information of the mean uranium and radium contents in the ground, the 10 regions can be classified according to their ‘‘radon potential’’. The highest potential is assigned to the east Bavarian region (No. 5) with its mainly granite substructure.
2. Radon measurements From each region, a total number of more than 500 water supply facilities proportional to the size of the region was selected for radon (Rn-222) measurements. The processing plant workers were asked to take a 1-l ground water sample in a radon tight PET bottle. For the sampling, a water hose was fixed at a tap of the raw water pipe and pushed down to the base of the bottle in order to fill the bottle very slowly and without turbulence. Preliminary experiments revealed that the loss of radon gas during this filling procedure is negligible. After filling, the sample was immediately sent to the gamma-spectroscopy lab at the Bavarian Environmental Protection Agency for analysis. Several track-etch detectors were sent by mail to the water supply facilities. They were exposed for a period of 2 weeks in order to get the mean room concentration of the main working places of the staff. In addition, the processing plant worker had to wear a personal track-etch detector for 2 months, which was fixed to his clothes during his stay in the supply facilities. From the recorded measurements, his individual effective dose was estimated. When not in use, the personal detector was stored near a reference detector at a place with low radon concentration. An instruction sheet was enclosed to aid the selection of a suitable storage place. Sites such as car garages, car boots or well ventilated areas such as rooms with constantly open windows, office rooms outside the supply facilities and roofed balconies were recommended. In spite of the fact that track-etch detectors cannot be switched off, the exposure resulting purely from the stay in the water supply facilities can be calculated by the exposure of the reference detector and the protocol of the duration of stay. From the results of these 2-month measurements, the annual dose of the processing plant worker was estimated. If the processing plant worker had to participate in the annual cleaning of the elevated reservoirs during the 2-month measuring period, he had to order an additional track-etch detector to record his exposure levels during the cleaning separately. The track-etch detectors, about 3000 in number, were obtained and evaluated by the GSF-National Research Center for Environment and Health, Neuherberg, Germany. These are CR39 type detectors in a diffusion chamber [4]. The system was also successfully tested for short time exposure measured in minutes. It is insensitive to Thoron. The measuring error (2-sigma standard deviation) is about 25% and almost unaffected by the recorded exposure. Much larger errors can occur by the calculation of the personal exposure of the processing plant workers. In order to keep the error level below 30%, the average radon concentration at the storage place must be below 200 Bq/m3. This concentration can be checked by the exposure of the reference detector. About 10% of the measurements with personal detectors had to be repeated because of poor measuring
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
85
Table 2 Results from the measurements of the raw water samples Region
1
2
3
4
5
6
7
8
9
10
Number of samples Maximum [Bq/l] Median [Bq/l]
25 46 12
31 68 11
99 266 10
51 38 6
181 1220 50
10 16 3
67 132 7
51 140 8
26 29 7
9 40 9
precision. In this case, the processing plant worker was advised by telephone how to find a better storage place. Using electronic systems, additional time-resolved measurements were taken in a few supply facilities. These continuous measurements were taken over a period of about 3 weeks. With these measurements, the influence of the different working processes on the radon concentration and the time variation of the equilibrium factor could be observed.
3. Results The results of about 550 measurements of raw water samples from the whole of Bavaria are shown Table 2. As expected, region 5 shows an increased average radon concentration in the raw water samples. Around 1100 measurements were taken in order to obtain information about the indoor concentrations in the water supply facilities (Table 3). As not every room in the buildings could be supplied with a track-etch detector, the processing plant workers were asked to place the detectors mainly in elevated reservoirs, water purification units and rooms with long duration of stay. In all regions, there are some rooms with concentrations of more than 10 kBq/m3. Taking into account that the normal annual working time is 2000 h/year, indoor concentrations of more than 3 kBq/m3 can lead to a dose above the limit of 20 mSv. Region 5 revealed the highest concentrations and, therefore, the median is significant higher than that of the other regions. The main purpose of this research project was to obtain information about the annual radon exposure levels of the staff in the Bavarian water supply facilities. About 500 measurements taken over a period of 2 months were performed. From this data, a linear estimate of the annual exposures was made (Table 4). Under ‘‘normal conditions’’ (equilibrium factor of about 0.4, unattached fraction of about 5– 10%), an exposure of 2 and 6 MBq h/m3 is equivalent to an effective dose of 6 and 20 mSv, respectively. In Table 3 Distribution of the radon indoor concentrations in the Bavarian supply facilities Region
1
2
3
4
5
6
7
8
9
10
Number of samples >3 kBq/m3 [%] >10 kBq/m3 [%] Maximum [kBq/m3] Median [kBq/m3]
48 6 4 30 0.5
60 18 7 130 0.7
205 15 4 18 0.7
95 11 3 35 0.7
360 52 31 271 3.3
21 10 10 16 0.5
141 26 3 16 1.1
106 17 9 27 0.7
53 17 8 29 0.7
18 39 28 33 1.2
86
M. Trautmannsheimer et al. / International Congress Series 1225 (2002) 81–86
Table 4 Distribution of the annual radon exposure levels of the staff of the Bavarian water supply facilities Region
1
2
3
4
5
6
7
8
9
10
Number of samples >2 MBq h/m3 [%] >6 MBq h/m3 [%] Median [MBq h/m3]
24 8 0 0.3
25 8 4 0.3
94 14 4 0.3
43 2 0 0.3
149 18 11 0.4
8 12 0 0.1
54 8 4 0.2
43 12 0 0.1
24 0 0 0.1
8 0 0 0.4
accordance with the regulations of the Council Directive 96/29/Euratom, a continuous supervision of persons with annual exposure levels of between 2 and 6 MBq h/m3 is recommended. Since radon exposure levels are not only influenced by the radon concentration in the ground water, the special position of region 5 in Table 4 is not as clear as in Table 2.
4. Conclusions Prediction of the radon exposure levels of the staff without measurements is very difficult because exposure levels are influenced by (1) radon concentration in the untreated ground (raw) water, (2) characteristics of the buildings and work processes such as ventilation of the rooms, fraction of room volume to water surface of the basins, water flow through the basins, turbulent filling of the basins (cascades), enhanced radon production by special work processes (back-washing, filling times of the reservoirs), and (3) duration of stay in the water supply facilities. In all geological regions, exposure levels over 2 MBq h/m3/year can occur. In the granite region in east Bavaria, about 70 workers (10% of the staff of region 5), and in the rest of Bavaria about 50 (2% of the staff of all regions except 5) workers, are subjected to exposure levels of over 6 MBq h/m3/year. To reduce the exposure levels, it is very important to provide detailed information for the water supply facilities because there are usually simple ways in which to minimize the duration of stay in the buildings. Other effective means to reduce the exposure levels of the staff are optimization of the ventilation or air-conditioning systems, separation of frequently used working places from basins or water purification systems and the avoidance of turbulence whilst filling the basins.
References [1] J.H. Lubin, et al., Radon and lung cancer risk: a joint analysis of 11 underground miners studies. US National Institutes of Health. NIH publication No. 94-3644, 1994. [2] Council Directive 96/29/Euratom, basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. Official Journal No. 159: 1 – 114, 29/06/ 1996. [3] D.T. Bertlett, G.J. Gilvin, R. Still, D.W. Dixon, J.C.H. Miles, The NRPB radon personal dosimetry service, J. Radiol. Prot. 8 (1) (1988) 19 – 24. [4] J. Kemski, R. Klingel, A. Siehl, Classification and mapping of radon affected areas in Germany, Environ. Int. 22 (Suppl. 1) (1996) 789 – 798.