Zdrav Var 2007; 46; 117-127 117 EXPOSURE TO RADON AT UNDERGROUND WORKPLACES IZPOSTAVLJENOST RADONU NA PODZEMNIH DELOVNIH MESTIH Janja Vaupotič1, Ivan Kobal1 Prispelo: 6. 7. 2007 - Sprejeto: 8. 11. 2007 Original scientific article UDC 546.296:614.87 Abstract Aim: The main aim of this contribution is to revievv the radon (222Rn) surveys carried out over the last two decades at underground vvorkplaces in Slovenia in coal mineš, karstic caves, vvater supply plants, vvineries and hospitals. Methods: Alpha scintillation cells, etched track detectors, an AlphaGuard PQ2000 multiparameter radon monitor and EQF 3020 and EQF 3020-2 radon and radon progeny monitor systems were used to measure concentrations ot radon and radon short-lived decay products, equilibrium factor and unattached fraction of radon decay products in air. Conclusions: (1) Radon levels are low in coal mineš; (2) elevated radon levels can be present in karstic caves; prior to a longer stay in a karstic cave, the radon level should be checked and, if necessary stay in the cave limited; (3) although elevated radon levels are frequently found at vvater supply plants, attendance times at underground vvorkplaces are short and the effective doses lovv; čare is necessary for longer maintenance vvorks; (4) under normal vvorking regimes in a winery exposure to radon in underground facilities is lovv; (5) radon levels are lovv in the majority of basement rooms in hospitals, but precautions are necessary in old buildings vvhere the floor may not always be a sufficient barrier to Rn entry and indoor radon levels may be elevated. Key words: radon, radon short-lived decay products, underground vvorkplaces, effective doses Izvirni znanstveni članek UDK 546.296:614.87 Izvleček Cilj: Glavni namen je bil podati pregled preiskav radona (222Rn) na podzemnih delovnih mestih v zadnjih dveh desetletjih, in sicer v premogovnikih, kraških jamah, vodnih zajetjih, vinskih kleteh in bolnišnicah. Metode: Uporabljali smo komplementarno merilno opremo, s katero smo merili koncentracijo radona (Rn) in radonovih kratkoživih razpadnih produktov (RnDP), faktor radioaktivnega ravnotežja med Rn and RnDP ter delež prostih RnDP, in sicer: alfa scintilacijske celice, detektorje jedrskih sledi, radonski merilnik AlphaGuard PQ2000 ter kombinirana merilnika EQF 3020 in EQF 3020-2 za Rn in RnDP. Zaključki: (1) Koncentracije Rn v zraku rudnikov so bile nizke. (2) V kraških jamah lahko naletimo na zelo visoke koncentracije Rn v zraku, zato je potrebno pred daljšim zadrževanjem ali delom v jami predhodno izmeriti radon, delo v jami načrtovati in, če je potrebno, časovno omejiti; (3) v podzemnih prostorih vodnih zajetij lahko naletimo na povišane koncentracije Rn v zraku, ker pa je tu zadrževalni čas delavcev kratek, so dobljene efektivne doze nizke; vendar je potrebno daljša vzdrževalna dela načrtovati in, če je potrebno, omejiti delovni čas; (4) pri normalnem delovnem režimu so v vinskih kleteh koncentracije Rn v zraku nizke; previdnost je potrebna samo pri izvajanju del ob izključenem prezračevanju; (5) na večini delovnih mest v kletnih prostorih bolnišnic je izpostavljenost radonu zadovoljivo nizka - previdnost je potrebna le pri starejših zgradbah, v 1 Jožef Štefan Institute, Jamova 39, PO Box 3000, 1001 Ljubljana, Slovenia Correcpondence to: e-mail: janja.vaupotic@ijs.si 118 Zdrav Var 2007; 46 katerih morda tla niso bila kakovostno izvedena ali pa je pri{lo z leti do po{kodb talne plo{~e, s ~imer se je radonu olaj{al dostop v prostor. Klju~ne besede: radon, radonovi kratko‘ivi razpadni produkti, podzemna delovna mesta, efektivne doze 1 Introduction Recent epidemiological studies in homes indicate that in Europe (1) the radioactive noble gas radon (222Rn or Rn) together with its short-lived decay products (RnDP) accounts for about 9 % of deaths due to lung cancer and about 2 % of ali deaths due to cancer. It origi-nates in rocks and soils by radioactive decay of ra-dium (226Ra) in the uranium (238U) decay chain (2). Because uranium is widely distributed, though at low levels, ali over the earth’s crust, radon is present ev-erywhere in the environment, the highest activity con-centration being in soil gas, i. e., from several hun-dred kilo Bq m-3 to several mega Bq m-3 (activity of 1 Bq equals 1 disintegration per second). Radon ema-nates from its plače of origin in a mineral grain, into soil gas or water present in the void space. Then, dis-solved in either a carrier geogas or thermal-mineral vvater and driven by advection, it moves tovvards the surface and eventually reaches the atmosphere (3). There, it is rapidly diluted and its resulting concentra-tion in the outdoor air is low, in the range of 5-50 Bq m-3. On the other hand, it accumulates in closed un-derground places (karst caves, mineš, tunnels), as well as in basements and ground floors of the living and vvorking environment. In the indoor air of a build-ing, its concentration depends on the geology and pedology of the site, shape, size and quality (less often building material) of the structure, and living hab-its of residents and the vvorking regime (4). Because of its decay, Rn (cc-decay, half-life, tV2 = 3.82 days) is always accompanied by its short-lived decay products (RnDP): 218Po (cc-decay, tV2 = 3.05 min), 214Pb ((3/ y-decay, tV2 = 26.8 min), 214Bi (p/y-decay, tV2 = 19.7 min) and 214Po (cc-decay, tV2 = 164 us). RnDPs are present attached to aerosols or as unattached nanosize clusters. Although theoretically possible, radioactive equilibrium betvveen Rn and RnDP is never reached in the actual environment and activity con-centrations of RnDP are lovver than that of Rn, as described by the equilibrium factor F, vvhich has a value betvveen 0.40 and 0.60 in indoor air. VVhile extensive surveys of radon in indoor air in dvvellings and vvorkplaces have been carried out in most developed countries, much less attention has been paid to underground rooms, despite the fact that elevated Rn levels are expected here. Hovvever, underground places have not been ignored in Slovenia. In a nationvvide indoor radon programme, radon in air has been monitored in 730 kindergartens (5), 890 schools (6) and 1000 randomly selected homes (7) together vvith a large number of underground vvorkplaces, vvhere elevated radon levels can be ex-pected. In this paper, vve reported radon levels in vvorkplaces in Slovenian non-uranium underground mineš, karst caves, vvater supply plants, vvineries and hospitals. The effective doses estimated for the employees are discussed. 2 Experimental 2.1 Survey methods Different instruments have been used to measure radon concentration in air, the choice depending on the purpose of measurement. - To obtain instantaneous Rn concentration, different size alpha scintillation cells (8) vvere used. The cells vvere calibrated vvith a standard 226RaCI2 solution (National Institute of Standards and Technology (NIST Standard Reference Material 4953D), accord-ing to the Rushing procedure (9, 10). Using the same procedure, cells vvere checked monthly. Celi effi-ciencies lie around 1.4x10-3 s-1 Bq~1 m3, vvhich gives a lovver limit of detection of 10-30 Bq m-3 at a 1-2 min-1 background and 30 minutes counting tirne. Air vvas sampled directh/ into a celi, vvhich vvas trans-ported to the laboratory, vvhere gross alpha radia-tion vvas counted in PRM 145 counters (AMES, Slovenia) three hours after sampling, vvhen equilib-rium betvveen radon and its short-lived decay products vvas reached. - Average radon concentration vvas obtained by ex-posing etched-track detectors provided by vari-ous manufacturers: Forschungszentrum Karlsruhe (Germany), Radonlab (Oslo, Norway), and National Institute of Radiological Sciences (Chiba, Japan). After 1 to 3 months exposure, detectors vvere Vaupotič J., Kobal I. Exposure to radon at underground vvorkplaces 119 mailed back to the manufacturer for development and evaluation of the results. At these exposure times the lower limit of detection was 3 to 5 Bq m-3. The detectors were calibrated by the manu-facturers. - An AlphaGuard PQ2000 multiparameter radon monitor (Genitron, Germany) was used to mea-sure radon concentration, air temperature and barometric pressure continuously, with a fre-quency of one per hour over a period of 5-20 days in order to determine diurnal fluctuations of Rn concentration. The lower limit of detection was 50 to 100 Bq m-3. The instrument was calibrated in the manufacturer’s radon chamber before de-livery. - EQF 3020 and EQF 3020-2 radon and radon prog-eny monitor systems (Sarad, Germany) were used to measure continuously (with a frequency of once every two hours) concentrations of Rn and RnDP, equilibrium factor betvveen Rn and RnDP and unattached fraction (f ) of RnDP to- lin gether with air temperature and humidity, over a period of 5 to 20 days in order to determine diurnal fluctuations of these parameters. The lower limit of detection was 30 to 80 Bq m-3. The in-struments were calibrated before delivery and re-calibrated every two years in the manufacturer’s radon chamber. In order to comply with the quality assurance - quality control recommendations, the devices were checked regularly at the inter-comparison experiments organized annualh/ by the Slovenian Nuclear Safety Administra-tion (11), and at each site, radon concentration was also measured with alpha scintillation cells calibrated with a NIST standard as described above. Results ob-tained with the celi and the other devices agree vvithin experimental errors. 2.2 Measurement protocol The survey of radon at underground vvorkplaces vvas designed and performed vvith assistance from the Ra-diation Protection Administration at the Ministn/ of Health, except for the first measurements in underground mineš and karst caves. Places to be moni-tored vvere selected jointly, taking into account the elevated radon levels previoush/ found, the radon po-tential based on geology and our previous experience in indoor radon levels. Prior to our survey, the man-agement of each plače vvas provided vvith general in-formation about the radon problem at underground vvorkplaces, and the programme of our study vvas ex- plained. Underground rooms vvere selected jointly vvith the management, giving priority to those attended by larger numbers of persons for longer times. Air vvas sampled by alpha scintillation cells to obtain a quick and rough estimate of radon levels. Then, at the 2 or 3 points vvith the highest radon levels, etched track detectors vvere exposed for 1-3 months. In addition, at representative places, concentrations Rn (CRn) and RnDP (CRnDP), as vvell as the equilibrium factor (F) and unattached fraction of RnDP (f ), vvere recorded un continuoush/ for 5-20 days. 3 Results 3.1 Underground non-uranium mineš High radon levels have been found in air of uranium mineš and also of other underground vvorkings, espe-cially metal and coal mineš (12, 13). Rn concentrations in the air vvere measured betvveen 1978 and 1986 in the follovving Slovene mineš: Mežica lead-zinc mine, Idrija mercury mine and Velenje-Preloge, Trbovlje, Zagorje, Hrastnik, Laško and Senovo coal mineš (14). In total, about a hundred samples vvere taken vvith alpha scintillation cells. At several sites in the Mežica and Idrija mineš Rn concentrations exceeded the Slovene national limit of 1000 Bq m-3 (15). At the tirne of our survey, both mineš started to be shut dovvn and the investigation vvas therefore not continued. Rn concentrations in ali coal mineš hovvever vvere lovv, never exceeding 500 Bq m-3. 3.2 Karst caves In 1984 and 1985, radon in air vvas measured vvith alpha scintillation cells at about five hundred sites in more than fifty Slovene caves (16), vvhere elevated Rn levels vvere expected (17). Measurable Rn concentrations ranged from 2 to 6 kBq m-3, although in many caves it vvas belovv the detection limit. At one site in the Postojna Cave off the tourist guided route, it vvas as high as 22 kBq m-3 (18). A further study in this cave, lasting for several years and in vvhich vari-ous complementan/ measuring devices vvere used to obtain CRn, F, fun, CRnDP, as vvell as meteorological parameters, shovved (19) that radon concentrations may reach 4-6 kBq m-3 in summer. The diurnal variation of the measured parameters in summer is shovvn in Fig. 1. Rn and RnDP levels vvere lovver in vvintertime by a factor of about 2. 120 Zdrav Var 2007; 46 8000 1.00 0.75 50 Et, 0.25 0.00 /un 1^^~\ f*l 3000- ^ ^VA^ * h r = *w^M A a h J o- \ \ \ / / «/ w H 2000- v \/oC \x( \ J \ i i/p *>s\* \ JL/w \ 2 o- i ^ . /^\ / / / ^Kl/A » J\ . 0 \ /^v« j^iJi k/ V/ / / V\V \J^\i ^ S »^ T^/^^VAr *'V/ / yv v >A^> o 1000- r i n . CrdDP 1 1.00 0.75 0.50 0.25 0.00 "Š q p q O « rt Date and tirne Figure 1. Radioactivity measurements at the lowest point in Postojna Cave, June 30 – July 8, 1999. Concentrations of Rn (CRn) and equilibrium factor (F); concentrations of RnDP (CRnDP) and unattached fraction of RnDP (fun); relative air humidity in the cave (RH) and barometric pressure (P); air temperature outdoor (Tout) and in the cave (Tin). Vaupotič J., Kobal I. Exposure to radon at underground vvorkplaces 121 3.3 Water supply plants Elevated Rn levels may also be found in air at vvorkplaces in vvater supply plants (20). In 2001, Rn, RnDP and F vvere monitored at vvorkplaces in 53 underground premises of vvater plants in Ljubljana, Grosuplje, Kočevje, Maribor, Koper, Ilirska Bistrica, Nova Gorica, Postojna, Sežana and Metlika (21). Both instantaneous and monthlv average radon concentrations vvere found to be relativelv lovv, exceeding 1000 Bq m-3 only at 4 places. 3.4 Wineries Although vvine production is an important national in-dustry in many countries reports on radon in air at vvorkplaces in their underground facilities are sparse (21). We focused our attention on this environment in 2002. Rn and RnDP concentrations, F and f vvere measured un in 22 underground rooms of the larger vvineries in the follovving cities/tovvns: Sežana, Koper, Vipava, Ptuj, Ormož, Ljutomer, Gornja Radgona and Maribor (23). VVhile 1-2 month average Rn concentrations obtained vvith etched track detectors vvere belovv 150 Bq m-3 in ali the rooms surveyed, alpha scintillation cells shovved an instantaneous value above 1000 Bq m-3 in one room of an old winery. The reason for this disagreement is evident from Fig. 2, vvhich shovvs the diurnal variation of Rn concentration. In the periods June 6-7 and 27-28, the ventilation system vvas shut dovvn, resulting in high Rn levels. The air sample vvas taken on June 6, vvhen Rn concentration vvas highest, vvhile etched track detector shovved the average, lovver, value. Time runs of the parameters monitored under the normal vvorking regime of a nevv winery are shovvn in Fig. 3. 3.5 Hospitals In many hospitals there are laboratories, shops, kitch-ens and other facilities in basements vvhere elevated Rn levels may be expected (24, 25). In 2002, a Rn survey vvas carried out in hospitals in the follovving cities/tovvns: Ankaran, Begunje, Brežice, Celje, Golnik, Idrija, Izola, Jesenice, Kranj, Ljubljana, Maribor, Murska Sobota, Nova Gorica, Novo mesto, Ormož, Postojna, Ptuj, Sežana, Slovenj Gradec, Šentvid pri Stični, Topolšica, Trbovlje and Vojnik (26). In each hospital, tvvo to four etched track detectors vvere exposed. In total, 207 air samples vvere taken from 186 rooms, and 215 etched track detectors vvere exposed in 198 rooms. In addition, in 12 rooms of 9 hospitals, concentrations of Rn and RnDP as vvell as F and f vvere recorded un continuously for periods of 5 to 11 days. Monthly average Rn concentrations obtained vvith etched track detectors vvere belovv 100 Bq m-3 in more than 70 % of places (Table 1) and only in 7 rooms vvere they above 400 Bq m-3 (Table 2). Diurnal variations of Rn and RnDP concentrations in tvvo shops vvith elevated levels are presented in Fig. 4. Variations vvere very small in the first (Fig. 4a), fluctu-ating by about ±50 % around the mean value, vvhile they vvere much more pronounced in the second (Fig. 4b), radon concentrations ranging from several hun-dred Bq m-3 in the morning up to several thousand Bq m~3 during the night. Surprisingly, both shops are on the ground floor and not in a basement. VVhile a constantly high concentration is seen for the vveekend of October 18-21 (Fig. 4b) this is not so clear for the vveekend of September 27-30 (Fig. 4a). Permanent ventilation in the first shop keeps radon concentrations almost con- 4000 Date and tirne Figure 2. Continuous measurement of concentrations of Rn (CRJ and RnDP (CRnDP) in an old winery, using the Sarad EQF3020-2 instrument betvveen June 6 and July 3, 2002. 122 Zdrav Var 2007; 46 stant, while the second shop is only ventilated natu-rally by opening doors and windows, and hence a typi-cal diurnal variation of radon concentration occurs (27). 4 Discussion To relate concentrations of Rn or RnDP to health, the dose conversion factor (DCF) is needed, defined as the ratio of the vveighted equivalent dose to the lung (ex-pressed in mSv) to the exposure to RnDP (expressed either in WLM, if RnDP activity concentration in air is known, or Bq m-3 h, if Rn activity concentration in air is known). The old but stili widely used unit, 1 WLM (work-ing-level-month), is the exposure resulting from 170 hours breathing air with an activity concentration of short-lived radon decay products of 1 WL (working-level). 1 WL was originally defined as the activity concentrations of RnDP vvhich are in radioactive equilibrium (F = 1) with 100 pCi L-1 (3700 Bq m3) of 222Rn, resulting in a potential alpha energy concentration of 1.3x105 MeV L-1 (2). DCF val-ues have been obtained from epidemiological studies on uranium miners. At present, the International Com-mission for Radiological Protection (ICRP) in Publica-tion 65 (28) recommends 5 mSv WLM~1 for vvorking and 4 mSv WLM"1 for living environments. 1.00 0.80 0.60 b) 1.00 0.60 400 4000 g 3000 o- h 2000 i 1000 CN O O CN CN O O CN S o CN s o CN O CN CN O O CN CN O o CN CN O o CN CN O O CN S o CN o CN CN O o CN CN O O CN CN o o CN CN O o CN s o CN o 0\ o 0\ O O 3\ O o o o o O O o 0\ o O O O CN CN in CN CN CN C~ CN C~ CN OG CN 00 CN CN CN o o m o o CN O Date and tirne 4000 ooooooooooooooooo ooooooooooooooooo ooooooooooooooooo ^¦ininsdt^i-^odoNONOi-^i-irieof^i^-i/"! Date and tirne Figure 4. Continuous measurement of Rn and RnDP concentrations using the Sarad EOF3020-2 instrument: a) in the pharmacy shop of hospital 01-ID-PB between September 24 and October 2, 2002; b) in the pharmacy shop of hospital 07-NM-SB between October 14 and 25, 2002. 124 Zdrav Var 2007; 46 Table 2. Hospitals (coded names) with indoor air radon concentrations higher than 400 Bq rrr3. Shovvn are: number of persons vvorking in the rooms surveyed and their annual exposure times, monthly average radon concentrations (CRn) obtained with etched track detectors in the periods indicated (in 2002), and monthly effective doses (E) (mo stands for month), received by the personnel in the period from 23 September to 22 October. Hospital Period of Om No. Exposure tirne E Code measurement Bq m3 persons h per year pSv mo"1 01-LJ-PK 17.9.-24.10. 600± 35 1098 191 17.9.-24.10. 2800 ±140 816 664 01-ID-PI 16.9.-24.10. 1400± 85 1590 646 24.9.-24.10. 1000± 50 1590 464 02-MB-SB 17.9.-24.10. 565± 35 1984 327 05-SE-BS 18.9.-22.10. 3000 ±150 4 348 300 07-NM-SB 17.9.-25.10. 1100± 55 7 1984 618 Based on radon concentrations measured at under-ground vvorkplaces, effective doses (L) received by a vvorker at these places were calculated applving the general formula (2): E = (CRn x F) 13700 x {ti 170) x DCF. (4) Here, CRn is the radon concentration (in Bq m-3) and t is the tirne (in hours) spent by vvorkers at this vvorkplace. According to ICRP-65 (28), F = 0.40 and DCF = 5 mSv WLM"1. 4.1 Underground non-uranium mineš Rn levels in coal mineš are low, as the result of the effective ventilation needed to prevent methane explo-sions. Therefore no precautionarv measures are needed from the radiation protection point of view. Effective doses have not been published. 4.2 Karst caves Radon concentrations in karst caves are higher in sum-mer than in vvinter due to the so-called “chimnev ef-fect”. For example, air temperature in the Postojna Cave is betvveen 8.0 and 9.0 °C and is practicallv constant ali the year round. In vvinter, the air temperature in the cave is higher than outdoors and the cave vvorks as a huge fire plače in vvhich the draught drives air from the cave rooms into the atmosphere. Because of the high radon levels in the Postojna Cave, in 1995 regular and permanent Rn monitoring vvas re-quired by the Radiation Protection Administration, and has been carried out ever since. The Cave manage- ment reports the effective doses for their vvorkers in the cave for the first half of every year and for the vvhole year. If the effective dose for a vvorker for the first half of the year exceeds 2.0 mSv, that vvorker vvould spend a reduced tirne in the cave in the second half of the year. Based on the annual effective doses, the tirne to be spent the follovving year by vvorkers in the cave is planned. The effective dose received by a tourist dur-ing one visit is negligible. 4.3 Water supply plants The attendance tirne of vvorkers in underground pre-mises of the vvater plants is short and hence despite the elevated Rn levels, the resulting annual effective doses vvere acceptably lovv: they never exceeded 3 mSv (Fig. 5). Therefore under the present vvorking regime no mitigation measures are needed. Nonetheless, the managements vvere recommended to prepare a tirne plan prior to every maintenance vvork underground, vvhich should be accompanied by Rn monitoring and, if necessary, tirne limited for a given vvorker. 4.4 Wineries Based on Rn concentrations obtained vvith etched track detectors, annual effective doses for the vvorkers in underground rooms of vvineries vvere estimated to range from 0.13 to 1.75 mSv. The latter value slightly ex-ceeds 1.25 mSv, the effective dose a member of the general public receives from Rn and RnDP in a year on a vvorldvvide average (29). Under the present operational regime in Slovene vvineries no precautions from the radiation protection point of vievv are necessan/ for Vaupotič J., Kobal I. Exposure to radon at underground vvorkplaces 125 > in a PlantID Figure 5. Annual effective doses of vvorkers in underground facilities in Slovene vvater supply plants in 2001 (Pl - plant identity number). vvorkers and no mitigation measures are foreseen. We have only dravvn the attention of the managements to the crucial role of ventilation in keeping lovv Rn levels, and not to allovv vvorks, or at least to limit vvorking times, at underground vvorkplaces during periods vvhen fans are shutdovvn. 4.5 Hospitals The reason for elevated indoor Rn levels in hospitals vvas always that the floor not properlv made: either the concrete slab vvas of bad quality, vvith cracks appear-ing vvith age, or vvooden boards vvere fixed on vvooden beams laid directly on a gravel ground. Annual effective doses vvere calculated for 1025 per-sons vvorking in the rooms surveyed. 996 persons (94.2 %) received less than 1 mSv in a year and the remain-ing 59, betvveen 1.1 and 7.3 mSv For 16 of the latter, vvorking in rooms vvith radon concentration more than 400 Bq m-3, the monthly effective doses are shovvn in Table 2. Ali the rooms vvith annual effective doses more than 2 mSv vvere further investigated in order to obtain reliable data on vvhich to decide vvhether mitigation measures should be undertaken. Some rooms have been already successfully mitigated and the others are in the process of mitigation. 5 Conclusions This revievv of the Rn survey carried out at underground vvorkplaces in Slovenia over the last tvvo decades, high- lights the follovving main points: (i) because of effective ventilation, no concern is necessan/ for miners’ exposure to radon in coal mineš; (ii) prior to a longer stay in a karst cave, Rn level should be checked and, if necessary, the stay in the cave should be limited -as an example: based on the semi-annual and annual effective doses of the vvorkers in the Postojna Cave, their stay in the cave is planned ahead and vvorking tirne in the cave is limited; (iii) elevated Rn levels are frequently found in underground premises of vvater sup-ply plants but the effective doses received by the vvorkers are lovv because of short attendance times; hovv-ever, maintenance vvork underground should be accom-panied by Rn monitoring, and the management should plan the tirne needed for each vvorker, in order to avoid too high exposure; (iv) under normal vvorking regime in a winery, exposure of vvorkers to Rn in underground facilities is lovv; čare is necessan/ only vvhen vvork is to be carried out during ventilation shut dovvn; (v) in the majority of basement rooms in hospitals the exposure of personnel to Rn is acceptably lovv, nevertheless, the occupation safety officer should pay attention to rooms in old buildings vvhere the floor is not always a sufficient barrier to Rn entry, leading to the possibility of elevated indoor Rn levels. VVhile radon levels cannot be reduced in karst caves because the natural environment should be preserved, at ali other places Rn problem can be successfulh/ mitigated by undertaking appropriate technical measures. 126 Zdrav Var 2007; 46 Acknovvledgements The authors appreciate fruitful discussions with Dr. Janez Kristan, Dr. Milko Križman and Dr. Tomaž Šutej. The cooperation of managements and personnel of the coal mineš, the Postojna Cave, water supply plants, vvineries and hospitals is appreciated. The authors also thank Prof. Dr. Roger Pain for his lin-guistic corrections. Contributors: Ivan Kobal participated in field measurements and data evaluation for underground mineš and karst caves, vvhile Janja Vaupotič, as the head of the Radon Center at the Jožef Štefan Institute, designed the programme and, together with her co-workers, ran the measurements in the Postojna Cave, water supplv plants, vvineries and hospitals, evaluated the data obtained and prepared the paper jointlv vvith Ivan Kobal. Conflict of interest: none. Sources of support: This study vvas financed by the Slovene Research Agency and the Radiation Protection Administration at the Ministn/ of Health of Slovenia. List of abbreviations: ICRP - International Commission on Radiological Protection Rn - 222Rn isotope of radon RnDP - short-lived decay products of 222Rn: 218Po, 214Pb, 214 C.} ¦ rii^i-l 214 D/"\ References 1. 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