128 Zdrav VaZr d2 r0a0v7 V; a4r6;2012048-; 14433 THE ROLE OF NANOSIZED AEROSOLS OF RADON DECAY PRODUCTS IN RADON DOSIMETRY VLOGA NANO AEROSOLOV RADONOVIH RAZPADNIH PRODUKTOV V DOZIMETRIJI RADONA Janja Vaupoti~1, Ivan Kobal1 Prispelo: 6. 7. 2007 – Sprejeto: 8. 11. 2007 Original scientific article UDC 546.296:614.87 Abstract Aim: In order to demonstrate the difference in dose conversion factors obtained based on epidemiological studies (DCFE) and by applying dosimetric models for mouth (DCFDJ and nasal (DCFDJ breathing the unattached fraction of nanosize radon short-lived decay products (fuJ was measured. Methods: Portable SARAD EQF3020 and EQF3020-2 devices vvere used to continuously measure activity levels of radon (Rn) and radon short-lived decay products (RnDP) , equilibrium factors betvveen Rn and RnDR and unattached fractions of RnDR Measurements were carried out in kindergartens, karst caves and vvineries. Conclusion: In kindergartens, f ranged from 0.03 to 0.24 with a geometric mean of 0.14. DCFn and DCFn are un Um D higher by a factor of 4.0 and 1.7, respectively than DCFE = 5 mSv WLM~1. At the lovvest point of the Postojna Cave, fun values ranged from 0.54 to 0.68 in summer and from 0.12 to 0.14 in vvinter. DCFDm is higher than DCFE by a factor of 11.5-14.2 in summer and 3.6-4.0 in vvinter, vvhile for DCFDn, these factors are 3.1-3.5 and 1.6-1.7, respectively In vvineries, fun values ranged from 0.08 to 0.20; DCFDm and DCFDn are higher than DCFE by factors of 2.8-5.1 and 1.5-1.9, respectively Key words: radon, radon short-lived decay products, unattached fraction of radon short-lived decay products, air, kindergartens, karst caves, vvineries Izvirni znanstveni ~lanek UDK 546.296:614.87 Izvle~ek Cilj: Meritve dele‘a prostih radonovih kratko‘ivih razpadnih produktov (fun) velikosti nanometrov z namenom, da bi prispevali k razlagi razlike med doznimi pretvorbenimi faktorji (DCF), ki jih dobimo na podlagi epidemiolo{kih izsledkov (DCFE) oziroma izra~unamo po dozimetri~nem modelu za dihanje skozi usta (DCFDm) in skozi nos (DCFDn). Metode: Za meritve smo uporabljali prenosna merilnika SARAD EQF3020 in EQF3020-2, s katerima smo kontinuirno merili koncentracije radona (Rn) in radonovih kratko‘ivih razpadnih produktov (RnDP), faktor radioaktivnega ravnote‘ja med Rn in RnDP (F) ter dele‘ prostih RnDP (fun). Meritve smo izvedli v otro{kih vrtcih, kra{kih jamah in vinskih kleteh. Zaklju~ki: V otro{kih vrtcih so bile vrednosti fun med 0,03 in 0,24 z geometri~no srednjo vrednostjo 0,14. DCFDm in DCFDn sta bila za faktor 4,0 oziroma 1,7 ve~ja od DCFE = 5 mSv WLM–1. V Postojnski jami je bil fun v obmo~ju 0,54– 0,68 poleti in v obmo~ju 0,12–0,14 pozimi na najni‘ji to~ki v jami. DCFDm je za faktor 11,5–14,2 poleti in faktor 3,6– Jo‘ef Stefan Institute, Jamova 39, PO Box 3000, 1001 Ljubljana, Slovenia Correcpondence to: e-mail: janja.vaupotic@ijs.si Vaupotič J., Kobal I. The role of nanosized aerosols of radon decay products in radon dosimetry 129 4,0 pozimi ve~ji od DCFE, medtem ko sta ta faktorja 3,1–3,5 oziroma 1,6–1,7 za DCFDn. V vinskih kleteh so bile vrednosti fun v obmo~ju 0,08–0,20; DCFDm je bil za faktor 2,8–5,1, DCFDn pa za faktor 1,5–1,9 ve~ji od DCFE. Klju~ne besede: radon, radonovi kratko‘ivi razpadni produkti, prosti radonovi razpadni produkti, zrak, vrtci, kra{ke jame, vinske kleti 1 Introduction Breathing air contaminated with radon and its short-lived decay products contributes about half the to-tal effective dose that a member of the general public receives on worldwide average from ali natural ra-dioactive sources (1). It is the second greatest cause of lung cancer, close to cigarette smoking (2). Radon dosimetry, in which nano sized radon decay products play a crucial role, is very complex and a number of questions stili remain to be re-solved. Radioactive decay of radon (222Rn: cc-decay, half-life, tV2 = 3.82 days) results in creating radon short-lived decay products (RnDP) (3): 218Po (cc-decay, tv 2 = 3.10 min), 214Pb ((3/8-decay, tV2 = 26.8 min), 214Bi (P/8-decay, tV2 = 19.9 min), and 214Po (cc-decay, tV2 = 164 us). Initially, RnDPs are positive free ionsvvhich, sooner or later, depending on experimental condi-tions, are partly neutralised by recombination and form nanosized clusters or so-called unattached RnDPs with sizes betvveen 0.5 and 3 nm (4). They further attach to aerosol particulates, forming at-tached decay products with an activity median aero-dynamic diameter (AMAD) of 200 nm (5). Because of plate-out of aerosols on the walls and floor of a room, as well as air movement and entry of fresh air, radioactive equilibrium betvveen decay products and radon is only partly achieved. It is expressed as a fraction betvveen 0 and 1, called the equilib-rium factor, F (3). Deposition of the short-lived de-cay products on the vvalls of the respiratory airways, the critical point in radon dosimetry, depends strongly on the particulate characteristics (6, 7). Therefore the unattached fraction (f ) of decav prod- lin J ucts is an important datum. In order to convert exposure to Rn and RnDP into dose, the so-called dose conversion factor, DCF is needed. In radon dosimetry, DCF is defined as the ratio of the vveighted equivalent dose to the lung (assuming a radiation vveighting factor for a par-ticles, wa, of 20, and a tissue vveighting factor for lung, w. , of 0.12) expressed in mSv, to the expo- sure to radon progeny expressed either in WLM or Bq m3 h. The old but stili widely used unit, 1 WLM (vvorking-level-month), is the exposure resulting from 170 hours breathing in air vvith an activity concen-trations of short-lived radon decay products of 1 WL (vvorking-level), vvhich vvas originally defined as the activity concentration of 218Po, 214Bi and 214Pb (214Po) vvhich are in radioactive equilibrium (F = 1) (3) vvith 100 pCi L-1 (3700 Bq m3) of 222Rn, resulting in an alpha energy concentration of 1.3x105 MeV L"1 (3). DCF values may be obtained either based on results of epidemiologic studies (hereafter denoted by DCFE) or by calculation applying dosimetric mod-els (hereafter denoted by DCFD). For DCFE, the ICRP Publication 65 (ICRP, 1994a) recommends 5 mSv/ WLM for vvorking, and 4 mSv/WLM for living envi-ronments. The dosimetric approach vvas elaborated by Birchall and James (8). DCFD is calculated based on a refined, recently proposed human respiratorv tract model (9). One of the parameters that affect DCFD most is fun (10, 11). DCFD values in the range 8-32 mSv/WLM vvere obtained under different conditions, vvith 15 mSv/WLM as the žbest estimate’ for the indoor air conditions in dvvellings. In addition, Porstendörfer (12) has shovvn that DCF = 101 x f + 6.7 x (1 - f ) Dm un un' for mouth breathing, and (1) DCFn = 23xf + 6.2 x (1 - f ) D un un' for nasal breathing (2) VVhich can be calculated separately based on the ex-perimental f values. un VVhile DCFE values are recommended by the ICRP-65 (ICRP, 1994a) methodology for use in general radon dosimetn/, DCFD (and so DCFDm, DCFDn) are suggested to be used for research purposes only (13). The lack of agreement betvveen DCFE and DCFD values has not been fully clarified yet, and the reason for DCFD > DCFE most probably originates from too high values being chosen for w (14, 15). Challenged bvthis 130 Zdrav Var 2007; 46 problem, our group has recentlv been studving f in a range of various environments in order to build up an experimental basis on which the discussion may con- tinue and lead to a better understand the disagreement and, eventually, to diminish the gap betvveen DCFD and DCFE. In this paper, results of Rn, RnDP, F and f monitoring un in air, with emphasis on f , are presented in three un environments in Slovenia: in kindergartens, karst caves (Postojna Cave) and vvineries. The influence of air temperature and relative humiditv, vvorking regime, and ventilation on f will be shown. DCFn values will be un D calculated and discussed, and compared with values OT L/Lsir-. 2 Survey methods Portable SARAD EQF3020 and EQF3020-2 devices (manufactured by SARAD, Dresden, Germany) have been used. Every two hours a pump sucks air over a silicon detector on which radon decay products are electro-deposited and counted. The data are stored and later transferred to a personal computer for evaluation. The instruments measure activity con-centrations (hereafter simply called concentrations) of Rn and RnDP (i. e., CRn and CRnDP), the equilib-rium factor F betvveen Rn and RnDP and unattached fraction f of RnDP, together with air temperature un and relative air humidity, the two parameters affect-ing f (16). The lower limit of detection for C and un Rn CRnDP is 50-60 Bq m3 and the experimental errors 10-20 % at low concentrations and less than 10 % for CRn and CRnDP higher than 200 Bq m3. The instruments were calibrated by the manufac-turer on purchase and are checked regularly at the inter-comparison experiments organized annually by the Slovene Nuclear Safety Administration (17). They are re-calibrated every two years in the manufacturer’s radon chamber. In addition, at each measurement site the instruments were regularly checked by taking samples and measuring CRn with alpha scintillation cells, calibrated annually by a standard 226RaCI2 solution (National Institute of Stan-dards and Technology (NIST Standard Reference Material 4953D) (18). The measurements were carried out at places where our previous survey had revealed elevated Rn lev-els (19). Thus, a device was operated for 10-15 days in one or more rooms of selected kindergartens (in total: 29 rooms in 13 kindergartens), at the lovvest point and in the railway station in the Postojna Cave and in underground premises of four vvineries. 3 Results In Fig. 1, diurnal variations of the measured parameters are shown for the LJ-S2-01b kindergarten. Simi-lar plots were obtained for ali kindergartens and aver-age values for each parameter for the whole period of measurement were calculated (20). The values for f un are given in Table 1, and the results of CRn, CRnDP and F obtained in ali kindergartens surveved are summarised in Table 2. The results of measurements at the lovvest point in the Postojna Cave (21) in summer are shovvn in Fig. 3. Similar plots vvere also obtained there in vvinter, and for both summer and vvinter at the railway station in the cave. CRn and CRnDP vvere lovver at the railway station than at the lovvest point and lovver in vvinter than in summer at both sites. In Table 3 only average values f for vvinter and summer tirne are shovvn for the un lovvest point. From ali measurements in vvineries (22), the results for VK-P-02 vvith lovv f values (Fig. 5) and VK-S-05 vvith un high fun values (Fig. 6) are shovvn and average values are listed in Table 4. 4 Discussion 4.1 Kindergartens Fig. 2 shovvs the dependence of f on F, relative air hu- un midity (HR) and air temperature (7) in the rooms sur-veyed. As expected (23-28), a negative correlation betvveen f and F vvas observed in aH kindergartens and, un for the measurement in the LJ-S2-01b kindergarten, may be approximated by a povver expression (29) fun = 0.056 po.875 (pjg 2a). During vvorking hours, the plate-out of aerosols is enhanced, resulting in reduced aerosol con-centration in the air (30) and thus lovvering F and in-creasing f . Also the f - RH correlation is expected to un un be negative (31): increasing RAVcauses an increase in size of aerosol particulates, thus increasing the attach-ment rate of RnDPs and decreasing f . This kind of f - un un RH relationship vvas observed in some kindergartens but not in others. Thus, for example, in the LJ-S2-01b kindergarten (Fig. 2b) even a positive, though ven/ vveak, correlation vvas found, vvhich cannot be explained based on the data obtained so far. The f - T relationship vvas un neither expected (26) nor observed, as shovvn for the LJ-S2-01b kindergarten in Fig. 2c. Vaupotič J., Kobal I. The role of nanosized aerosols of radon decay products in radon dosimetry 131 S 1000 500 2500 7 S -hOOOOOCN(N-h,-it-c^h,_cOOOO ooooooooooooooooo ooooooooooooooooo n r^ cs cs cs cs tN cs cs ts -< >"H >-< >"H >-< -H >-< -H ¦-< >-< -H >-< >-< "-1. ~^'~;^c>c?c!c!c!c!c!0. P *-! P P P (N02p3C ^ 0.00 0.00 0.40 0.30 § 0.20 0.25 0.50 F 0.75 1.00 0.10 0.00 b) /un = 0.0022Rtf- 0.0124 R2 = 0.11 o o1 oq Jo 08 0.00 20 22 24 26 7V°C 28 30 Figure 2. Dependence of fun on a) equilibrium factor (F), b) relative humidity of indoor air (RH) and c) indoor air temperature (T) in the LJ-S2-01b kindergar ten. Vaupotič J., Kobal I. The role of nanosized aerosols of radon decay products in radon dosimetry 135 8000 7 1.00 0J 4000-r 0.00 1.00 940 - 930 A 60 50 trt i—i m >-h N -H (S H oooooooooooooooooooooooooooo 00 00 0\ » 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o 00 o o o ^H -1 CN es m m ¦št- t in in o *o Date and time § 1 rs rs rs rs# rs rs rs rs rs rs rs rs rs rs rs rs rs rs 'O ^d ^o *d ^d ^o <^o ^ *o ^o *o ^o 'o^o Figure 5. Continuous measurement of concentrations of Rn (CRJ and RnDP (CRnDP), equilibrium factor (F), and unattached fraction of RnDP (fun) in the winery at VK-P-02, using the Sarad EQF3020-2 instrument in the period from May 30 to June 14, 2002. 138 Zdrav Var 2007; 46 Based on f values in Table 1 and using eauations 1 un -i and 2, DCFDm and DCFDnvvere calculated together with their values divided by 5 (Table 1). Table 2 summarises the results obtained in ali kindergartens surveved and shovvs that, on average, DCFDm is higher than DCFE by a factor of 4.0 and DCFDn by a factor of 1.7. According to ICRP criteria (9), ali persons in kindergartens, both personnel and children, are considered as nasal breath-ers. 4.2 Postojna Cave As expected, higher F values are accompanied by lower f values (24, 28). VVhile Fig. 4a shows the f - F un un relationship for summer and vvinter at the lovvest point (together described as f = 3.7e~6 3F) and this relation- un ship is compared at the railway station and lovvest point (together described as f = 1.9e~37F) (Fig. 4b), Fig. 4c shows the best fit of ali the points from Figs. 2a and 2b, approximated by f = 2.0e~4 2F. Due to the chimnev un J effectVne air draught from the cave to the outdoor at-mosphere is stronger in vvinter than in summer, thus vve vvould expect the cave air to be more stagnant, and thus f lovver, in summer than in vvinter. Nonetheless, un the opposite vvas observed, vvith much higher fun values in summer (f = 0.58) in the period August 10-18, 1998 than in vvinter (f = 0.10) in the period December un 14-22, 1998. The chimney effect appears to be domi-nated by the air mixing produced by the visitors mov-ing through narrovv corridors. A lovv f value in a stag-nant air (32) during the night vvas rapidly increased at the start of the visits in the morning, and started to decrease in the afternoon. Fluctuations of f are much un more pronounced in summer than in vvinter, most prob-ably because of much larger number of visitors in summer. Dose conversion factors are summarized in Table 3 vvhich shovvs that average f values are much higher un than in kindergartens. The resulting DCFDm is higher by factors of 11.5-14.2 in summer and 3.6-4.0 in vvinter than DCFE (= 5 mSv WLM~1), and similarly, DCFDn is higher by factors of 3.1-3.5 and 1.6-1.7, respectively . According to the ICRP criteria (9), only maintenance vvorkers carrying out heavy physical vvork in the cave may be considered as mouth breathers, vvhile ali the others are nasal breathers. 4.3 Wineries At VK-P-02, the cellar system is more than 200 years old and is ven/ complex, composed of hundreds of metres of underground tunnels vaulted vvith stone and clay brick. The CRn run (Fig. 5a) does not shovv the typical diurnal variations, vvith maxima during nights and minima during days, as observed at other vvork-places (33). Neither are the values constantly higher during vveekends (June 1-2 and June 8-9). Similarly, at VK-S-05, neither typical diurnal CRn fluctuations nor elevated values during non-vvorking days (First May holiday: May 1-5 and vveekend: May 11-12) vvere observed (Fig. 6a). This winery is composed of several large underground halls, vaulted vvith stone. It is lo-cated in the karst region vvhere the majority of elevated indoor Rn levels have been observed vvithin the na-tional radon programme (34), due to underground cracks, fissures and faults vvhich enable Rn to travel long distances and accumulate in closed rooms (35). Elevated Rn levels in this winery vvere therefore ex-pected but not observed, because of effective ventila-tion. Similar C runs have also been observed in other Rn vvineries, not presented here. In the VK-P-02 winery, the CRn - F relationship can be approximated by CRn = 302e~114F, though vvith a lovv correlation coefficient (Fig. 7a). Although expected (29) no dependence of f on F could be seen (Fig. 7b). In contrast, in the VK-S-05 winery no dependence of CRn on Fvvas observed (Fig. 8a) vvhile the f - F relationship vvas described, though un vvith lovv correlation coefficient, by f = 0.13F025 (Fig. un 8b). In the majority of vvineries the situation resembles that of the VK-S-05 winery. Average f values and dose conversion factors are un summarized in Table 4. Both are similar to those in kindergartens, although vvorkplaces surveved vvere underground and therefore values similar to those in the Postojna Cave vvere expected. Clearly, the concen-tration of aerosols in the cave air is much lovver than in a winery (26). The resulting DCFDm is higher than DCFE = 5 mSv WLM"1 by a factor of 2.8-5.1 and DCFDn by a factor of 1.5-1.9. According to the ICRP criteria (9), aH vvorkers in the vvineries may be considered as nasal breathers. 5 Conclusions The measurements of f in the three environments have un shovvn the follovving situation. In kindergartens, f un ranged from 0.03 to 0.24 vvith a geometric mean of 0.14. The resulting dose conversion factors for mouth (DCFDm) and nasal breathing (DCFDn) are higher than DCFE (= 5 mSv WLM"1) by factors of 4.0 and 1.7, re-spectively. fun values in the Postojna Cave vvere much higher than in kindergartens, ranging from 0.54 to 0.68 in summer and from 0.12 to 0.14 in vvinter at the lovvest point. The resulting DCFDm is higher than DCFE by fac- Vaupotič J., Kobal I. The role of nanosized aerosols of radon decay products in radon dosimetry 139 200 200 150 100 o" S Date and tirne Figure 6. Continuous measurement of concentrations of Rn (CRn) and RnDP (CRnDP), equilibrium factor (F), and unattached fraction of RnDP (fuJ in the winery at VK-S-05, using the Sarad EQF3020-2 instrument in the period from April 23 to May 13, 2002. 140 Zdrav Var 2007; 46 500 400 'g 300 o- QQ ~^ i u 100 a) CRn = 302e U4F 0 0 "-«62 rP^ o (STj^Ss O o o R2 = 0.28 oŠ§& ^oc °0 |^5o^ ° o° "^o—s- 0.00 0.20 0.40 0.60 0.80 1.00 0.25 0.20 0.15 = < 0.10 0.05 0.00 0.00 0.20 0.40 0.60 F 0.80 1.00 Figure 7. Dependence of: a) Rn concentrations (CRJ on equilibrium factor (F), and b) unattached fraction of RnDP (fun) on equilibrium factor (F), in the winery at VK-P-02, using the Sarad EQF3020-2 instrument in the period from May 30 to June 14, 2002. Vaupotič J., Kobal I. The role of nanosized aerosols of radon decay products in radon dosimetry 141 200 0.00 0.10 0.20 0.30 F 0.40 0.50 0.60 B 0.50 0.40 0.30 0.20 0.10 0.00 b) f^ = 0A3F^25 O R2 = 0.07 o 0 o o ^ o o /0L&m tLh tO ^6—cr€ o o o o o°V^8 w jOoO o o o o 0.00 0.10 0.20 0.30 F 0.40 0.50 0.60 Figure 8. Dependence of: a) Rn concentrations (CRn) on equilibrium factor (F), and b) unattached fraction of RnDP (fun) on equilibrium factor (F), in the winery at VK-S-05, using the Sarad EQF3020-2 instrument in the period from April 23 to May 13, 2002. 142 Zdrav Var 2007; 46 tors of 11.5–14.2 in summer and 3.6–4.0 in winter, while these factors are 3.1–3.5 and 1.6–1.7 respectively for DCFDn. fun values in wineries are similar to those in kin-dergartens, ranging from 0.08 to 0.20. The resulting DCFDm and DCFDn are higher than DCFE by factors of 2.8–5.1 and 1.5–1.9, respectively. These results pro-vide experimental evidence for the disagreement be-tween dose conversion factors (and hence effective doses) suggested by the epidemiologic studies and those calculated applying the dosimetric approach for the above environments, and as such provide a good basis for continuing the discussion as to how to tackle this problem and eventually solve it. Acknowledgement The authors thank Dr. Thomas Streil for fruitful discus-sions. The cooperation of managements and person-nel of kindergartens, the Postojna Cave, and wineries is appreciated. The authors also thank Prof. Dr. Roger Pain for his lin-guistic corrections. Contributors: Janja Vaupoti~, as the head of the Radon Center at the Jo‘ef Stefan Institute, designed the programme and, together with her co-workers, ran the measurements, evaluated the data obtained and prepared the paper. Ivan Kobal contributed to data evaluation and writing the paper. Conflict of interest: none. 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