Acta Chim. Slov. 2004, 51, 159-168. 159 Short Communication RADON DOSES BASED ON ALPHA SPECTROMETRY Janja Vaupotič,* Ivan Kobal Jožef Štefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 17-10-2003 Abstract At the raihvay station in Postojna Cave and at the lowest point of the cave, repeated short-term monitoring in summer and in winter of air concentrations of radon (C-g^) and radon decay products (Cpj^p), of the equilibrium factor (F) and unattached fraction of radon decay products (fun), of barometric pressure (P), relative air humidity in the cave (RH) and air temperature outside (Tout) and in the cave (Tm) has been carried out, with the emphasis on^. Dose conversion factors (DCF), calculated from the^n values (ranging from 0.09 to 0.65) exceed the ICRP-65 value of 5 mSvAVLM by a factor of 12-14 in summer and of 3.0-3.5 in winter. Key words: radon decay products, unattached fraction, dose conversion factors Introduction Postojna Cave is the largest of the 12 show caves in Slovenia, and also one of the largest in the world, visited by about half a million tourists a year. An electric train takes visitors from the entrance to the railway station in the cave, from where they start a walking route which, in about 1.5 hours, brings them back to the railway station. Visits are scheduled for every full hour from 9 a. m. to 6 p. m. in spring, summer and autumn, and every second full hour from 10 a. m. to 4 p. m. in winter. Because high air radon concentrations were found in the cave, " the Health Inspectorate at the Ministry of Health of Slovenia introduced regular radon monitoring in 1995. Radon is measured at the inner raihvay station and at the lowest point, somewhere in the middle of the walking route. " Etched track detectors are exposed for 3 months ali the year round, and radon exposure is calculated on the basis of the number of hours spent by a worker in the cave, as provided by the Cave management. Semi-annual and annual effective doses, obtained by using the ICRP-65 methodology, are reported to the Health Inspectorate, with the tirne to be spent by a person in the cave in the following half year limited accordingly. Since the preliminary measurements of the unattached fraction of short-lived radon decay products in the cave, carried out by the Porstendörfer's group, showed values J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry 160 Acta Chim. Slov. 2004, 51, 159-168. from 0.056 to 0.16, the doses obtained with the ICRP-65 methodology might be underestimated. In order to check this point, in the period 1998-2001 we performed systematic monitoring of the concentration and the unattached fraction of short-lived radon decay products in air at the railway station and the lowest point. In this contribution, results of the monitoring are discussed, with emphasis on the temporal variation of the equilibrium factor, the concentration of short-lived radon decay products and their unattached fraction, and the dependence of these parameters on the barometric pressure, outdoor air temperature, relative air humidity in the cave and working regime of the cave. Experimental Measuring sites The same measuring sites as for the regular radon monitoring programme were used, i.e., the railway station and the lowest point. Air temperature in the cave is almost constant ali the year round, at between 13 and 15 °C, and relative air humidity is practically 100%. There is no forced ventilation in the cave and air is exchanged only through numerous cracks, corridors and shafts connecting the cave with the outdoor atmosphere. Measuring techniaues Portable EQF3020 and EQF3020-2 devices (manufactured by SARAD, Dresden, Germany) have been used to measure concentrations of radon and radon short-lived decay products, equilibrium factor, unattached fraction of decay products, air temperature and relative air humidity. The sampling and analysing frequency is once in two hours. The two Po isotopes are not distinguished by their alpha energies, but can be analysed using a quasi spectroscopy based on measuring the total alpha activity at three appropriately chosen tirne intervals. The devices have been in operation for 10-15 days in summer and the same in winter from 1998 to 2002. The instruments were calibrated by the manufacturer on purchase, and have since then been regularly checked at the intercomparison experiments organized annually by the Nuclear Safety Administration at the Ministry of the Environment, Spatial Planning and Energy of Slovenia. The hourly average values of the barometric pressure and the outdoor air temperature at the Postojna J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry Acta Chim. Slov. 2004, 51, 159-168. 161 meteorological station were obtained from the Office of Meteorology of the Environmental Agency of the Republic of Slovenia. Results and discussion Temporal variations of the parameters under consideration Results obtained at the lowest point are shown in Figure 1 and Figure 2 for summer and winter, respectively, of 1998. Average values of ali parameters, i.e., concentrations of radon (Crh) and radon decay products (Crudp), equilibrium factor (F), unattached fraction of radon decay products (fm), barometric pressure (P), relative air humidity in the cave (RH) and air temperature in the cave (Tin) and outdoors (Tout), during the whole period of measurement (denoted by t and called total average, e.g., Crd*, Crudp*, F^.fan, etc.) and also during working hours only (denoted by w, and called working average, e.g., CrhW, CRnopw, Fw, funw, etc.) were calculated and are shown on the graphs. Standard deviations of the above averages, based on the experimental errors, are betvveen 5 and 10%. The patterns of temporal variation of parameters in summer differ substantially from those in winter. It is also clear that C^n and C^dp values are lower in winter (CsJ = 1466 Bqm" and Crudp* = 736 Bqm" in the period December 14-22, 1998, Figure 2) than in summer (C^J = 4089 Bqm" and C^dp* = 1367 Bqm" in the period August 10-18, 1998, Figure 1). This is because of the so called chimney effect: in winter, the temperature outdoor is lower than in the cave, thus enhancing a natural draught of air from the cave through vertical channels into the outdoor atmosphere. In summer, the situation is reversed and the draught is minimal, if any. The opposite is true with F which is lower in summer (F1 = 0.34 in the period August 10-18, 1998) and higher in winter (F1 = 0.56 in the period December 14-22, 1998). The number of visitors is much higher in summer than in winter, thus causing a higher plate-out of RnDP, and thus, at the same Tm reducing F. In summer (Figure 1), daily decreases of Crb coincide with decreases in P. When the outdoor pressure, measured outdoors, starts to decrease, the cave system becomes overpressurized with respect to outdoors and starts to release radon-rich air, in accordance with the general effect of pressure fluctuations on radon exhalation. " In winter, this effect appears to be assisted by the chimney effect. J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry 162 Acta Chim. Slov. 2004, 51, 159-168. 8000 6000 4000 2000 *- Rn r\ ^ ^ A^V [V-.A vA-v N... **' "v .--, v\ i •••¦; '*i * • "¦¦"¦ '*""'- ¦ ¦. ?. F »^ »i i' Rnvv/C Rnt! 3934 Bqm /4089 Bqm Fw/Ft: 0.32/0.34 1.00 0.75 0.50 0.25 0.00 8000 6000 4000 2000 v> -•„ "7 * Aa 1 <¦¦ 1 r ^RnDP V \s v w\ C /C /C RnDP RnDP : 1255 J3qm /1367 J3qm un un 980 970 960 950 940 930 PW/nt l l IP : 953 nPa/953 nPa 1.00 0.75 0.50 0.25 0.00 - 100 - 90 - 80 - 70 - 60 - 50 w1 r> t t w / n -r -r i rt/ KH IKH : 98.8 %/99.4 "/o 35 30 25 20 15 10 5 0 Date and tirne Figure 1. Time fluctuation of the measured parameters at the lowest point, August 1998 (see the text for definitions of parameters). o o J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry Acta Chim. Slov. 2004, 51, 159-168. 163 8000 4000 Rn /%— Rn : 1365 J3qm /1466 J3qm Wt Rn '^ t—'VV/7—rt t1 /t1 : 0.57/0.56 1.00 0.50 0.00 8000 6000 4000 2000 0.75 0.50 0.25 ^ RnDP /^ RnDP : 6/O Bqm /736 Bqm un (/un: 0.09/0. lO 980 970 960 950 940 930 100 90 80 70 60 50 Pw/Pt: 954 hPa/954 hPa RHw/RHt: 98.5 %/99.4 % 30 7 25 20 -15 f 10 5 0 -5 -10 Date and tirne Figure 2. Time fluctuation of the measured parameters at the lowest point, December 1998 (see the text for definitions of parameters). 6000 0.75 2000 0.25 O 1.oo o o.oo J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry 164 Acta Chim. Slov. 2004, 51, 159-168. 8000 6000 4000 2000 Rn 'L Rn : 3427 Bqm /3333 Bqm 1.00 0.75 0.50 0.25 0.00 8000 6000 4000 2000 0 980 970 960 950 940 930 -*¦ 1.00 0.75 /C ^-" RnDP f *-" RnDP /un 'j un : 0.17/0.15 : 1932 13qm /2080 13qm 0.50 0.25 0.00 100 90 80 70 60 50 /P : 950 nPa/949 hPa /t/i /RH : 91.9 %/92.1 % 35 30 25 20 10 5 0 Date and tirne Figure 3. Time fluctuation of the measured parameters at the railway station, July 2001 (see the text for definitions of parameters). o It : 0.58/0.63 J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry Acta Chim. Slov. 2004, 51, 159-168. 165 While in summer, daily Crh increases/decreases are generally accompanied by F decreases/increases, the Crb - F relationship in winter appears to be more complicated. Although higher F values are expected to be accompanied by lower/,n values, " this was observed in winter but not always in summer, when a minimum F value was often followed by a delayed maximum mfun. Due to the chimney effect the air draught from the cave to the outdoor atmosphere is stronger in winter than in summer, thus the cave air is more stagnant and the/m value lower in summer than in winter. The opposite was found, with much higher fun values in summer (fUn = 0.58 in the period August 10-18, 1998) than in winter (fur? = 0.10 in the period December 14-22, 1998). The chimney effect appears to be obscured by the air mixing produced by the visitors' moving through narrow corridors. A low/m value in stagnate air during the night was rapidly increased on starting the visits in the morning, and started to decrease again in the afternoon. Fluctuations of/,n are much more pronounced in summer than in winter, most probably because of the much larger numbers of visitors in summer. There may, however, be more complex factors at work. Figure 1 and Figure 3 compare the summer situation for the two measuring sites, the lowest point in the period August 10-18, 1998 and the raihvay station in the period July 3-18, 2001. Crb values are higher and F values lower at the lowest point than at the railway station, the Crudp values being similar at the two sites. fun values are significantly lower at the raihvay station. This marked difference cannot be explained by a small difference in RH (92% at the raihvay station and 99% at the lowest point) at the same Tm, ' but is rather caused by higher aerosol concentartion ' at the raihvay station, produced by train traffic and presence of a number of groups of tourist waiting for the train. While diurnal variation of/U is pronounced at the lowest point, it is almost constant at the raihvay station. The railway station is situated in a large hali in which the microclimatic conditions affecting/m are more constant than in the narrow corridors at the lowest point. Dose conversion factors The ICRP-65 methodology for estimating doses due to radon and short-lived radon decay products is based on the results of epidemiological studies and recommends as dose conversion convention 4 mSv/WLM at home and 5 mSv/WLM at the workplace. J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry 166 Acta Chim. Slov. 2004, 51, 159-168. A refined dosimetric approach, based on a new lung model, has recently been proposed. The activity median aerodynamic diameter (AMAD) of the unattached short-lived radon decay products is taken as 0.8 nm, while that of the attached fraction, as 200 nm. Dose conversion factor (DCF in mSv/WLM) is expressed by: DCFm = 101x/in + 6.7x(l- fun) for mouth breathing, and DCFn = 23 x/,„ + 6.2x (1 - fun) for nasal breathing. Table 1. Dose conversion factors (DCF in mSvAVLM) for mouth (m) and nasal (n) breathing calculated from the working average of the unattached fraction of radon short-lived decay products (4nW), measured in summer and winter at the lowest point in the Postojna Cave. Also DCF 15 values are given. season, year r w 7un DCFm mSvAVLM DCFm 1 5 DCFn mSvAVLM DCF J 5 winter, 1998 0.12 18.0 3.6 8.2 1.6 summer, 1998 0.54 57.6 11.5 15.3 3.1 winter, 1999 0.14 19.9 4.0 8.6 1.7 summer, 1999 0.61 64.2 12.8 16.5 3.3 summer, 2000 0.56 59.5 11.9 15.6 3.1 summer, 2001 0.68 70.8 14.2 17.6 3.5 summer, 2002 0.67 69.9 14.0 17.5 3.5 Using these equations, dose conversion factors were calculated from funw for the periods under investigation, and are collected in Table 1 for the lowest point. They are also divided by 5 (ICRP-65) and the ratio is also given in the table. For mouth breathing DCF value at the lowest point is higher than the value recommended by ICRP-65 by a factor 11.5-14.2 in summer and 3.6-4.0 in winter. Although not shown in the table, this factor is 4.5 in summer at the railway station. On the other hand, for nasal breathing the DCF values presently in use are underestimated by a factor of 3.1-3.5 in summer and 1.6-1.7 in winter at the lowest point. For a selected “working profile” DCF value is a proper combination of the DCFm and DCFn values. Conclusions The fraction of unattached radon short-lived decay products in air of the Postojna Cave ranged from 0.09 to 0.68. It was higher at the lowest point of the tourist guided route than at the raihvay station and, at the lowest point, higher in summer than in winter. The calculated dose conversion factors are higher in summer than the value of J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry Acta Chim. Slov. 2004, 51, 159-168. 167 5 mSvAVLM presently in use by a factor of 11.5-14.2 for mouth breathing and 3.1-3.5 for nasal breathing. Systematic measurements covering longer periods are underway, with the aim of re-evaluating the annual effective doses of the cave employees. Elevated DCF values, and thus resulting elevated effective doses of the employees, signify their increased health risk toward lung cancer. This should be the further step of the study. Acknowledgements The authors thank Ms. Petra Dujmovič for her measurements and analyses in the cave and in the laboratory. The cooperation of the Cave management and personnel is appreciated. References 1. I. Kobal, M. Ančik, M. Škofljanec, Radiat. Prot. Dosim. 1988, 25, 207-211. 2. I. Kobal, B. Smodiš, J. Burger, M. Škofljanec, Health Phys. 1987, 52, 473-479. 3. I. Kobal, M. Škofljanec, D. Zavrtanik, Naše jame 1978, 20, 41-47. 4. J. Vaupotič, P. Dujmovič, I. Kobal, Acta Carsol. 1998, 27, 395-406. 5. J. Vaupotič, I. Csige, V. Radolič, I. Hunyadi, J. Planinič, I. Kobal, Health Phys. 2001, 80, 142-147. 6. J. Vaupotič, P. Dujmovič, I. Kobal, Doses due to radon and progeny received by workers in the Postojna Cave (from January 1, 2001 to December 31, 2001); Jožef Štefan Institute Report IJS-DP-8537, 2002, pp 1-17. 7. J. Vaupotič, P. Dujmovič, I. Kobal, Doses due to radon and progeny received by workers in the Postojna Cave (from January 1, 2002 to December 31, 2002); Jožef Štefan Institute Report IJS-DP-8739, 2003, pp 1-24. 8. M. Urban, J. Schmitz, In: Fifth International Symposium on the Natural Radiation Environment, Tutorial Sessions; Report EUR 14411 EN 1993,151-183. 9. G. Buttenveck, J. Porstendörfer, A. Reineking, J. Kesten, Radiat. Prot. Dosim. 1992, 45, 167-170. 10. T. Streil, G. Holfeld, V. Oeser, C. Feddersen, K. Schönefeld, In: IRPA (International Radiological Protection Association) Regional Congress on Radiation Protection in Neighbouring Countries in Central Europe, Portorož, 1996, pp 334-337. 11. J. W. Thomas, Health Phys. 1970, 19, 691. 12. M. 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Isot. 1996, 47, 515-523. J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry 168 Acta Chim. Slov. 2004, 51, 159-168. 23. International Commission on Radiological Protection (ICRP), Protection against Radon-222 at Home and at Work; ICRP Publication 65, Pergamon Press, Oxford, 1994, pp 1–262. 24. J. Porstendörfer, Int. Congr. Ser. 2002, 1225, 149–160. 25. International Commission on Radiological Protection (ICRP), Human Respiratory Tract Model for Radiological Protection; ICRP Publication 66, Pergamon Press, Oxford, 1994, pp 1–482. Povzetek V letih od 1998 do 2001 smo v zraku Postojnske jame na najnižji točki in na železniški postaji merili konentracijo radona (CRn) in radonovih kratkoživih razpadnih produktov (CRnDP), ravnotežni faktor (F), delež nevezanih radonovih razpadnih produktov (fun), zračni tlak (P), relativno vlažnost zraka v jami (RH) in temperaturo zraka v jami (Tin) ter zunaj (Tout). Poseben poudarek je bil na fun in na njegovi odvisnosti od vremenskih razmer. Vrednosti fun so bile v širokem razponu, od 0,09 do 0,68. Z uporabo novega dozimetrijskega modela smo na osnovi izmerjenih vrednosti fun izračunali dozne pretvorbene faktorje in ugotovili, da so bili poleti za faktor 11,5-14,2 pozimi pa za faktor 3,1-3,5 višji od 5 mSv/WLM, to je vrednosti, ki jo priporoča metodologija ICRP-65. J. Vaupotič, I. Kobal: Radon Doses Based on Alpha Spectrometry