First results of measurement of equilibrium factors F and unattached fractions f p of radon progeny in Czech dwellings

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1 NUKLEONIKA 2010;55(4): ORIGINAL PAPER First results of mesurement of equilibrium fctors F nd unttched frctions f p of rdon progeny in Czech dwellings Krel Jílek, Josef Thoms, Ldislv Tomášek Abstrct. The unttched frction of rdon decy product clusters f p nd equilibrium fctor F re dose relevnt prmeters in ll dosimetric pproches to dose clcultion. In the pst, three yer continuous weekly mesurements of unttched nd ttched ctivity of rdon dughter product nd ir exchnge rte were crried out during heting seson in 30 occupied typicl Czech fmily houses. The results indicted significntly different weekly verges of equilibrium fctor F nd f p for houses locted in towns compred those in villges. Due to this fct, pproximtely 10% verge increse of equivlent lung dose rte ws estimted in the detriment in towns. Averge vlues of equilibrium fctor F nd f p were 0.40 nd 8.6% in urbn houses nd 0.32 nd 10.7%, respectively in rurl houses. Bsed on the mesurements of men vlues of f p, verge effective dose conversion fctors (DFC) in units of msv per working level month (WLM) were estimted to be 15.0 msv/wlm in urbn houses nd 15.9 msv/wlm in rurl houses, respectively. Key words: unttched frction f p dose ir exchnge rte indoor erosol concentrtion rurl/urbn re dy/night Introduction K. Jílek, J. Thoms Ntionl Rdition Protection Institute, 28 Brtoškov Str., Prh , Czech Republic, Tel.: , Fx: , E-mil: krel.jilek@suro.cz L. Tomášek Institute of Computer Science, Acdemy of Sciences of the Czech Republic, 28 Brtoškov Str., Prh , Czech Republic Received: 30 June 2009 Accepted: 30 December 2009 The unttched frction of rdon decy product clusters f p nd equilibrium fctor F re dose relevnt prmeters in ll dosimetric pproches to dose clcultion. Direct mesurements of f p nd F under different mbient conditions llow comprison of the influence of mbient conditions to reltive dose contribution [4]. The results of sensitivity nlysis pplied to the clssicl Jcobi-Porstendörfer (J-P) room model [5], generlly describing dynmics of both unttched nd ttched short-lived rdon progeny in the room, indicte ir exchnge rte (ACH), deposition q f nd ttchment X rte of unttched rdon dughter product s the key quntities influencing most strongly the behviour of observed vlues f p nd equilibrium fctor F in room. The most importnt role in processes ffecting vlues of deposition q f nd ttchment X rte plys, besides ACH, predominntly erosol size distribution nd totl erosol concentrtion. In the pst, three yer weekly continuous mesurements of unttched nd ttched ctivity concentrtion of ech rdon progeny nd of ACH were performed during heting seson in thirty occupied typicl Czech fmily houses. Primrily, the purpose of the present pper is to introduce the results of direct mesurements of F nd f p under different living conditions nd to estimte the influence of these conditions to dose contribution. We lso focused on the clcultion of the key model prmeters X, q f nd ACH in mesured houses in order to independently estimte the most probble vlues of

2 440 K. Jílek, J. Thoms, L. Tomášek F nd f p by mens of the room model nd to compre them with those obtined from direct mesurements. Since there re well known prcticl nd principl problems rising from long-term mesurements of erosols in occupied dwellings with ny scnning mobility prticle sizer (SMPS) cused by evporting of wide spred butyl lcohol sturtor used in prticle condenstion nuclei counters into the room, erosol prticle size distribution nd erosol concentrtion were not systemticlly directly mesured. Nevertheless, erosol concentrtion Z could be estimted indirectly from mesured vlues f p by mens of Porstendörfer s pproximte formul (f p = 414/Z) [6]. Mteril nd methods All mesurements were conducted in thirty occupied typicl Czech or two-storey fmily houses since October to Mrch during heting seson. The houses were chosen rndomly nd they were equipped with centrl heting system. Originlly, we intended to divide them ccording to ssumed different mbient living conditions from the erosol concentrtion nd its size distribution stndpoint in structure s follows: towns-villges, smokers-non smokers. Unfortuntely, fter finishing ll mesurements we hd to distinguish only bsic living conditions ccording to the house loction, i.e. town-villge nd newly ccording to dily time period. To detect the ssumed influence of different occupnt s indoor ctivities nd outdoor chnges (trffic density, wether conditions etc.) on behviour of the observed prmeters f p nd F we divided ech dily 24 h tking mesurement time period into dy from 6.m. to 6 p.m. nd into night tking from 6 p.m. to 6.m., respectively. Exposure durtion took minimlly one week in ech house. Mesurement instruments For mesurement of unttched nd ttched ctivity concentrtion of ech short-lived rdon dughter progeny, we used continuous monitor Fritr 4 (J. Plch SMM, CZ). The monitor Fritr 4 hs built-in memory buffer llowing more thn 4000 records tking ech 120 min. This mesuring intervl included 10 min smpling nd 110 min tking computtionl procedure with implemented 4 lph counting intervls providing minimum detectble ctivity (MDA) for ech rdon progeny in the following rtio ( 218 Po: 214 Pb: 214 Bi) = (1: 0.6: 0.5) stepwise 40 Bq m 3, 12 Bq m 3, 15 Bq m 3. The MDA for mesured rdon concentrtion ws estimted to be bout 15 Bq m 3. Unttched ctivity concentrtion of ech rdon progeny ws mesured from the screen with mesh 300 nd cut-off dimeter d 50 = 4 nm nd the ttched ctivity concentrtion of ech rdon progeny from Millipore filter type AA, 0.8 μm plced behind the screen. The description of the monitor Fritr 4 lone in more detils nd qulity ssurnce from the mesurement of unttched nd ttched ctivity concentrtion for ech rdon progeny nd from the mesurement indoor verge erosol concentrtion stndpoint is given elsewhere [2]. Rdon mesurements were performed by mens of the well-known continuous monitor Alphgurd (Genitron, D) certified in Germn reference chmber t the Physiklisch-Technische Bundesnstlt (PTB) in Brunschweig. During ll mesurements, we used its 60 min mesuring diffusion mode. Both the monitors were plced mostly in living rooms t ech house. Theoreticl bckground Equilibrium fctor F nd f p s dose relevnt prmeters The lung equivlent dose H from inhltion of short- -lived rdon progeny cn be written by mens of mesured time integrl of rdon concentrtion s follows: (1) H = DFC F where: DFC is the dose conversion fctor [msv/wlm]; F is the equilibrium fctor; v is the verge rdon concentrtion [Bq m 3 ]; T exp is the exposition durtion [h]. According to clcultions by Mrsh nd Birchll [4], the DFC in Eq. (1) cn be expressed s liner function of unttched prt of equilibrium equivlent concentrtion of rdon progeny f p s follows: (2) DFC(f p ) = f p where f p expressed s the percentge of totl potentil lph energy concentrtion (PAEC) of the rdon progeny mixture. Finlly, considering Eqs. (1) nd (2) nd ssuming the sme brething rte, the influence of two different mbient conditions to dose cn be expressed in terms of reltive chnge of equivlent dose rte H 1 /H 0 s follows: H F 1 1 ( k+ fp,1) (3) = H F ( k+ f ) with subscripts 0 nd 1 representing two different mbient conditions nd k representing the rtio 11.35/0.43 from Eq. (2). Attchment rte X, deposition rte q f nd ir exchnge rte ACH The ttchment rte X nd deposition rte q f (plte-out) nd ACH cn be clculted, in principle, simultneously from the mesured unttched nd ttched ctivity concentrtion of ech rdon dughter progeny by mens of n lgebric inversion of the model [9] expressed for room in stedy stte s follows: (4) T exp v 0 0 p,0 j, f =λ i-1( j-1, f+ Rj-1j-1, )/(ACH +λ j+ qf+ X) (ACH jo, + (1 Rj-1) λ jj-1, + Xj-1, f) j, = (ACH +λ j + q) = ( Q + ACH ) / ( λ + ACH) 0, i Rn 0, o 0

3 First results of mesurement of equilibrium fctors F nd unttched frctions f p of rdon progeny where initil conditions 0, i = 0,i,f, 0,i, = 0, j,o,f = 0; j ctivity concentrtion of j-th rdon dughter product [Bq m 3 ]; λ j decy constnt of j-th rdon dughter product [h 1 ]; R j frction of recoiled toms of j-th nuclide from erosol prticles; 1st subscript j = 0, 1, 2, 3, 4 stepwise 222 Rn, 218 Po, 214 Pb, 214 Bi, 214 Po; 2nd subscript i, o denotes indoor/outdoor; 3rd subscript f, denotes unttched/ttched ctivity j-th rdon dughter product; Q Rn sum of rdon entry rtes into house [Bq m 3 h 1 ]. Attchment rte Under relistic indoor conditions, the observed ttchment rte X in room in stedy stte cn be pproximtely clculted only on the bse of mesurement of unttched nd ttched ctivity concentrtion 218 Po s follows [5]. (5) 1i X = λ1 where: 1i,, 1i,f re the ctivity concentrtion ttched, unttched respectively 218 Po [Bq m 3 ]; λ 1 is the decy constnt 218 Po [h 1 ] or in the cse of mesurement the proper erosol size distribution is s follows: (6) where: β(d) ttchment coefficient of ttched rdon progeny to erosol s function of prticle dimeter d [cm 3 h 1 ]; Z(d) erosol size distribution [cm 3 ]. According to [1], ttchment coefficient β(d) cn be clculted s follows: 2πDd (7) 0 β ( d) = 8D0 d + dv 2 δ where: D o is the diffusion coefficient cm 2 s 1 ; v 0 is the men therml velocity cm 1 ; δ o = d/2 + s o where s o = cm is the men free pth of the unttched decy product clusters. Plte-out According to the model, the deposition rte q f, of unttched dughter products cn be clculted pproximtely under relistic indoor conditions (ACH << X + λ 1 ) s follows: 0 (8) qf =λ1 ( λ 1+ X) with ll symbols s mentioned in previous text Eqs. (4) (5). Air exchnge rte 0 1if X = β( d) Z( d)dd 1f 0 0 Under the ssumptions of the model introduced by set of Eq. (4), the ACH cn be clculted both by mens of n lgebric inversion given previously, nd by mens of numericl itertions pplying the set of Eq. (4). On the bsis of our previous experiments crried out in rdon chmber of Ntionl Rdition Protection Institute (NRPI), Prh we developed code clculting ACH, in principle, s follows. Input dt for the code were mesured t lower nd upper limits of rtio ctivity concentrtions, stepwise Rn/ 218 Po, 214 Pb/ 218 Po, 214 Bi/ 218 Po nd t lower nd upper limit estimtes of X, q f nd ACH. The code provides stepwise outputs in form of sets of required prmeters X, q f, ACH nd recoil R fulfilling ll input conditions. The most probble vlue of ACH is chosen from the set where vlues X nd q f re the closest to the corresponding vlue X nd q f, clculted ccording to Eqs. (5) nd (8), respectively. A lrge number of tests under the defined ACH in the rdon chmber indicted overll greement of the clculted nd true vlues of ACH up to cceptble 30%. The differences between mesurements were evluted using the Student-test nd ssuming log-norml distribution. Results nd discussion The results of confirmtory dt nlysis from ll mesured houses (N = 30) in the form of weekly geometric mens (GM), corresponding geometric stndrd devition (GSD), 95% confidence intervl (95% CI) for desired quntities of interest nd p-vlue re summrized in Tbles 1 nd 2. In Tble 1 re shown the results of the nlysis pplied to mesured dt for compred time periods dy/night. From Tble 1, by mens of p-vlues cn be seen tht ll quntities of interest re not significntly different, except ACH. The observed sttisticlly significnt pproximte verge 30% increse of ACH during dy compred to night ws not surprising. Due to this fct, higher indoor erosol concentrtion coming from outdoor ir is ssumed nd implicitly higher ttchment rte X (see Eq. (6)) is expected nd lso mesured. The results of ttchment rte X clculted ccording to Eq. (5) cn be pproximted by their bsic prmeters (GM = 24 h 1, GSD = 1.63) for time period dy nd (GM = 20 h 1, GSD = 1.41) for time period night, respectively. These results gree firly well with published dt. According to [1], ttchment rte X is rnging from 20 to 50 h 1 in poor ventilted houses (ACH < 0.5 h 1 ) without dditionl erosol sources. Considering Eq. (6) nd the verge ttchment coefficient β(d) = cm 3 h 1 published by Porstendörfer in cse of poorly ventilted houses (ACH < 0.5 h 1 ) without dditionl erosol sources, we cn expect n verge indoor erosol concentrtion Z 4800 cm 3 during dy nd Z 4000 cm 3 during night, respectively. The results of mesured deposition rte q f clculted ccording Eq. (8), cn be pproximted by their bsic prmeters (GM = 23 h 1, GSD = 1.48) for time period dy nd (GM = 22 h 1, GSD = 1.35) for time period night, respectively. These mesured results gree reltively well with dt published by other uthors 40 h 1 [7], 10 h 1 [3], 30 h 1 [10].

4 442 K. Jílek, J. Thoms, L. Tomášek Tble 1. Results of confirmtory nlysis of weekly verge vlues f p, F, X, q f nd ACH over time periods dy nd night Quntity (units) Period GM GSD 95% CI p-vlue dy f p night p = F dy night p = X (h 1 ) dy night p = q f (h 1 ) dy night p = ACH (h 1 ) dy night p = F, f p denote non-dimensionl equilibrium fctor nd unttched frction of equivlent equilibrium concentrtion, respectively. X, q f denote ttchment rte nd deposition rte of unttched rdon progeny. ACH denotes ir exchnge rte. According to lredy cited [1] typicl published verge vlues F nd f p in indoor ir rnge between nd , respectively for norml erosol conditions Z ( ) cm 3 with men vlues of F = 0.3 nd f p = These results re in good greement with our mesured results f p (GM = 0.099, GSD = 1.58), F (GM = 0.35, GSD = 1.31) found for time period dy nd f p (GM = 0.107, GSD = 1.47), F (GM = 0.36, GSD = 1.32) found for time period night. In Tble 2 re shown the results of the nlysis pplied to mesured dt for compred loctions town- -villge. From p-vlues in Tble 2, it cn be seen tht ll quntity of interest re significntly different except deposition q f nd the fct tht observed men vlues F, f p, X nd q f gree very well with the published dt mentioned previously. In our opinion, the higher verge vlue F nd logiclly lower verge vlue f p in indoor ir in towns compred to villges re cused minly by higher verge erosol concentrtion probbly in consequence of different indoor ctivities. This fct is indicted by drmticlly higher vlues of ttchment rte X in towns compred to villges nd sttisticlly significnt lower verge vlue ACH pproximtely bout 30%. If we consider the bove-mentioned verge ttchment coefficient β(d) = cm 3 h 1 published by Porstendörfer, Eq. (6) nd mesured mens of ttchment rtes X from Tble 2, we re ble to estimte the verge indoor erosol concentrtion Z 5200 cm 3 in the cse of towns nd Z 3600 cm 3 in the cse of villges. From Tble 2, it cn be lso seen tht the mesured verge vlue ACH is rnging round ( ) h 1 in the cse of both towns nd villges. If we consider the Porstendörfer s pproximte formul f p = 414/Z mentioned previously (see Introduction) nd the mesured verge vlues of f p clculted for different observed conditions from Tbles 1 nd 2 we cn estimte verge vlue of indoor erosol concentrtion s follows: Z 4800 cm 3 in the cse of towns nd Z 4000 cm 3 in the cse of villges, respectively, Z 4200 cm 3 for time period dy nd Z 3900 cm 3 for time period night. These results indicte very good greement of both pproches used to estimte verge erosol concentrtion Z, i.e. by mens of the mesured ttchment rte X nd f p, respectively nd lso implicitly good ccurcy of mesurements of unttched nd ttched ctivity concentrtions of rdon dughter product by mens of the monitor Fritr 4. Since the verge vlue f p in the mesured urbn houses were 8.6% nd 10.6% in rurl houses, then, with respect to Eq. (2), the lung DFC = 15.0 msv/wlm should be pplied in the cse of urbn houses nd DFC = 15.9 msv/wlm in the cse of rurl houses, respectively. Further, ccording to [4], the lung dose sensitivity per unit exposure to rdon progeny contributes to more Tble 2. Results of confirmtory nlysis weekly verge vlues of f p, F, X, q f nd ACH for loction town nd villge Quntity (units) Period GM GSD 95% CI p-vlue town f p villge town F villge town X (h 1 ) villge town q f (h 1 ) villge town ACH (h 1 ) villge The quntity f p, F, X, q f nd ACH hve the sme mening s in the previous Tble 1. p = p = p = p = p = 0.046

5 First results of mesurement of equilibrium fctors F nd unttched frctions f p of rdon progeny Fig. 1. Results of comprison of equilibrium fctor F clculted vi the room model nd directly mesured, respectively (with GSD). Fig. 3. Averge reltive contribution to lung equivlent dose rte H 1 /H 0 under different investigted mbient conditions nd the corresponding combined stndrd uncertinty. Conclusion Fig. 2. Results of comprison of f p clculted vi the room model nd directly mesured, respectively (with GSD). thn 99% of the effective dose, nd therefore the bove- -mentioned vlues of the lung DFC cn represent very well the effective DFC in relevnt units (msv/wlm). To verify the J-P model (see Eq. (4)) from the prediction of F nd f p point of view, in Figs. 1 nd 2 re illustrted results of comprison of the mesured vlues F nd f p with those clculted by mens of the room model on the bsis of input dt, i.e. verge vlues X, q f nd ACH (tken from Tbles 1 nd 2) for investigted mbient conditions. For ll clcultions, verge vlue of recoil R = 0.5 ws used [8]. The results indicte very good greement between the directly mesured vlues of F nd f p nd those clculted by mens of the room model. Assuming the sme brething rte, the influence of our different investigted mbient conditions (denoted 0 nd 1, respectively) to reltive chnges of the equivlent lung dose rte H 1 /H 0 is illustrted in Fig. 3. In Fig. 3 it is seen the verge vlue of H 1 /H 0 clculted for proper observed mbient conditions pplying Eqs. (1) (3) nd combined stndrd uncertinty cquired by ppliction of the Lw of Propgtion of Uncertinty to Eq. (3). About 10% significnt reltive increse of the equivlent lung dose rte in the cse of towns compred to villges ws found. On the other hnd, the difference in contribution to reltive lung dose rte in the cse of observed conditions dy/night ws not sttisticlly significnt. The results of weekly continuous mesurements of rdon concentrtion nd unttched nd ttched ctivity concentrtion of ech short-lived rdon progeny in thirty rndomly chosen Czech fmily houses during heting seson indicted sttisticlly significnt difference of weekly verges equilibrium fctor F nd f p for urbn houses compred to rurl. The log-norml distribution of weekly mens f p nd F is represented in the cse of towns s follows: f p (GM = 0.086, GSD = 1.58), F (GM = 0.40, GSD = 1.25) nd f p (GM = 0.107, GSD = 1.40), F (GM = 0.32, GSD = 1.31), respectively in cse of villges. Due to this fct, pproximtely 10% verge increse of equivlent lung dose rte ws clculted in the detriment of towns. The difference in men vlues f p nd F during observed time period dy/night ws not sttisticlly significnt. The effective dose conversion coefficients per WLM were estimted to 15.0 msv/wlm in the cse of towns nd 15.9 msv/wlm in the cse of villges, respectively. Both key prmeters of the J-P room model, i.e. ttchment rte X nd plte-out q f of unttched rdon progeny rnged from 10 to 60 h 1 with the men round (20 25) h 1. The ir exchnge rte rnged from 0.1 to 1 h 1 with the men pproximtely round ( ) h 1, nd its sttisticlly significnt verge 30% decrese during the night compred to dy ws observed. The verge indoor erosol concentrtion ws sttisticlly significnt higher in towns (Z 5000 cm 3 ) compred to villges (Z 4000 cm 3 ). On the other hnd, difference between men indoor erosol concentrtion during observed time period dy (Z 4200 cm 3 ) ws not sttisticlly significnt compred to night time period (Z 3900 cm 3 ). Acknowledgment. The present work ws supported by the Stte Office for Nucler Sfety under the Project no /11/2006.

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