diurnalvariations of radon and thoron activity ...

Radiation Protection Dosimetry Advance Access published June 25, 2013 Radiation Protection Dosimetry (2013), pp. 1–8

doi:10.1093/rpd/nct145

DIURNALVARIATIONS OF RADON AND THORON ACTIVITY CONCENTRATIONS AND EFFECTIVE DOSES IN DWELLINGS IN NISˇKA BANJA, SERBIA

*Corresponding author: [email protected] Received January 31 2013, revised May 10 2013, accepted May 14 2013 In Nisˇka Banja, a spa town in a radon-prone area in southern Serbia, radon (222Rn) and thoron (220Rn) activity concentrations were measured continuously for one day in indoor air of 10 dwellings with a SARAD RTM 2010-2 Radon/Thoron Monitor, and equilibrium factor between radon and its decay products and the fraction of unattached radon decay products with a SARAD EQF 3020-2 Equilibrium Factor Monitor. Radon concentration in winter time ranged from 26 to 73 100 Bq m23 and that of thoron, from 10 to 8650 Bq m23. In the same period, equilibrium factor and the unattached fraction varied in the range of 0.08 to 0.90 and 0.01 to 0.27, respectively. One-day effective doses were calculated and were in winter conditions from 4 to 2599 mSv d21 for radon and from 0.2 to 73 mSv d21 for thoron.

INTRODUCTION 222

220

Radon ( Rn, t1/2 ¼3.82 d) and thoron ( Rn, 55.6 s) originate from a-transformation of radium in the 238U and 232Th natural decay chains, respectively. Radon a-transformation is followed by a sequence of radon short-lived decay products (RnDP): 218Po (a, 3.11 min), 214Pb (b/g, 26.8 min), 214Bi (b/g, 19.9 min) and 214Po (a, 164 ms), and a-transformation of thoron, by thoron short-lived decay products (TnDP): 216 Po (a, 150 ms), 212Pb (b/g, 10.6 h), 212Bi (a and b/g, 60.5 min) and 212Po (a, 298 ns)(1). Levels of decay products in ambient air are most often expressed by a collective quantity based on their half-lives and potential a-decay energies and are called equilibriumequivalent activity concentration of RnDP (CRnDP) and TnDP (CTnDP). They are given by individual activity concentrations of the sequence members, as in the following equations(1, 2): CRnDP ¼ 0:105C218Po þ 0:515C214Pb þ 0:380C214Bi CTnDP ¼ 0:913C212Pb þ 0:087C212Bi :

ð1Þ ð2Þ

Because of air movement and plateout of decay products, the secular equilibrium between radon and RnDP and thoron and TnDP can hardly be reached in ambient air, and its degree is expressed by the so-called

equilibrium factors FRn and FTn, respectively, which are defined as(1, 3):

FRn ¼

CRnDP C and FTn ¼ TnDP ; CRn CTn

ð3Þ

with CRn and CTn as radon and thoron activity concentrations. For decades, great concern was devoted to radon survey in various environments worldwide, while thoron was mostly ignored, based on an assumption that at usual 232Th levels in the ground and building material, its activity concentration in indoor air is negligible, because of its short half-life as compared with that of radon. This assumption appeared to be wrong when papers started to report thoron levels, comparable(4) or even higher than those of radon, such as in traditional Japanese houses(5), in Italian buildings made of volcanic material(6), or in cave dwellings in China(7, 8). It is now well known that on the world average, radon, thoron and their decay products contribute more than half to the effective dose a member of the general public receives from all natural radioactivity(2) and are a major cause of lung cancer, second only to cigarette smoking(9). In this issue, thoron may contribute significantly and therefore its inclusion in radon survey is highly recommended(10 – 13).

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J. Vaupoticˇ1, *, T. Streil2, S. Tokonami3 and Z. S. Zˇunic´4 1 Radon Center, Department of Environmental Sciences, Jozˇef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia 2 SARAD GmbH, Wiesbadener Strasse 20, Dresden 01159, Germany 3 Department of Radiation Physics, Institute of Radiation Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan 4 ECE Lab, Vincˇa Institute of Nuclear Sciences, University of Belgrade, Mike Alasa 12-14, Belgrade 11000, Serbia

ˇ ET AL. J. VAUPOTIC

EXPERIMENTAL Description of sites For this study, ten houses were chosen in which previous measurements had shown indoor air radon concentrations higher than 1000 Bq m23(17), while thoron has not been measured. They were small onefamily one-storey buildings, located on the travertine bedrock and made of brick, wood and concrete (with local sand and gravel). There is no air conditioning and no central heating; heating is based on burning wood in kitchen and using electric heaters in rooms. During measurement, the room was not attended and not heated. In each house, one-day measurements were carried out in selected rooms, mostly the living room and bedrooms at the ground floor. The room was closed a day prior to measurement and, except the authors for air sampling and setting instruments, nobody entered it until the end of the measurement. The survey was performed in summer time (end of

June and beginning of July) and winter time (end of November and beginning of December). The instruments were placed in the middle of the room, 50 cm above the floor. Measurements lasted about one day. Measuring technique For radon and thoron detection, a portable RTM 2010-2 Radon/Thoron Monitor (Sarad, Germany) was used. Air is pumped through the chamber at a flow rate of 0.5 dm3 min21. The high voltage between the chamber wall and the silicon detector causes the positively charged 218Po and 216Po, created by 222Rn and 220Rn decays, respectively, to deposit on the detector. Based on a-spectrometry, activity concentrations of 222Rn and 220Rn are calculated, stored in the internal memory and later transferred to a personal computer for data evaluation. In several rooms, also EQF 3020-2 Equilibrium Factor Monitors were used (Sarad, Germany) to measure in addition to radon individual activity concentrations of 218Po, 214Pb, and 214Bi/214Po. Air is pumped for 6 min at a flow rate of 2.4 dm3 min21 over a metal mesh grid on which particles smaller than 5 nm (considered as associated with the unattached RnDP) are separated from those above this size (considered as associated with the attached RnDP), and the two fractions are deposited electrostatically on two separate 150-mm2 silicon surface barrier detectors. Gross alpha activity is measured during three consecutive intervals within 110 min after the end of pumping, and, applying the Markov method(21, 22), individual activity concentrations of 218 Po, 214Pb and 214Bi in the unattached and attached fractions are obtained. The device also gives radon activity concentration, FRn and fraction ( fun) of unattached RnDP. In order to comply with the Quality Assurance and Quality Control recommendations, the EQF and RTM devices were calibrated and checked for radon and thoron by the manufacturer every two years and calibrated for radon in a radon chamber(23). In addition, during every measurement, radon was also measured with three 0.7 dm3 alpha scintillation cells(24), which are calibrated regularly every six months by a standard 226RaCl2 solution [National Institute of Standards and Technology (NIST Standard Reference Material 4953D)], according to the Rushing procedure(25, 26). Cell efficiencies are around 1.41023 s21 Bq21 m3, which gives a lower limit of detection of 10–30 Bq m23 at a 1–2 min21 background and 30 min counting time. Air was sampled directly into cells, cells transported to the field laboratory, and gross alpha radiation counted after three hours, when secular equilibrium between radon and its short-lived decay products was reached. Radon concentrations measured with the EQF and

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Following this direction, thoron was included in the national indoor radon survey in Serbia, covering 200 places in rural communities(14), using UFO(15) and RADUET(16) dosemeters based on solid state nuclear track detectors. Among them was also Nisˇka Banja, a spa town with high natural radioactivity, in which in a previous study(17) in more than half of 200 dwellings surveyed (with 1100 inhabitants), indoor air radon concentrations of more than 200 Bq m23 were found, with the highest value of 13.4 kBq m23(18). Extremely high were also radon concentrations in soil gas and radon exhalation rates, the highest values being more than 2 MBq m23 and 1.5 Bq m22 s21, respectively(17). These high levels have been attributed to elevated 226Ra contents in soil, ranging from 26 to 1809 Bq kg21, the highest values observed in the travertine bedrock(19). In contrast, 228 Th content was not high, ranging from 11 to 53 Bq kg21(19), as compared with the world averages of 35 Bq kg21(2). Indoor air thoron concentrations measured in 26 dwellings ranged from 26 to 866 Bq m23 with a geometric mean and geometric standard deviation of 165 Bq m23 and 2.70, respectively(14). Elevated radon levels in air and water have been also found in spa facilities of several hotels. Annual effective doses of employees working at the pools or at therapeutic baths using thermal water and mud may reach up to 18 mSv(20). In this work, one-day continuous monitoring of radon and thoron in indoor air was carried out in 10 dwellings in that part of Nisˇka Banja where thoron was not measured yet and previous measurements had shown highest radon levels. Based on average daily concentrations obtained, effective doses received by residents from the exposure both to radon and thoron were calculated and are discussed.

RADON AND THORON IN DWELLINGS

RTM devises always agreed within less than 10 % with the values shown by alpha scintillation cells. For thoron, RTM device was checked by simultaneous measurement with an RAD7 monitor (Durridge, USA), and the readings of the two instruments did not deviate for more than 15 %.

RESULTS AND DISCUSSION Radon and thoron levels and their daily changes

Table 1. Ranges and arithmetic mean values (AM) of activity concentrations of radon (CRn) and thoron (CTn) calculated from one-day continuous monitoring with the RTM device, and ratios of their AM values. House

1 2 3 4 5 6 7 8 9

Room

Living room Living room Living room Living room 1 Living room 1 Living room 2 Living room Living room Living room Living room Bedroom 1 Bedroom 2 Guest room Kitchen Kitchen

Season

Summer Winter Winter Summer Winter Winter Winter Winter Winter Winter Summer Winter Winter Winter Winter

CRn/Bq m23

CTn/Bq m23

CTn/CRn

Range

AM

Range

AM

Range

AM

450– 7540 6200–44 600 5200–27 700 88–5900 5800–37 700 2300–6700 1760–7700 730– 12 800 9500–19 900 1180–9500 1660–11 500 5900–14 200 10 800– 22 500 26–107 2000–6760

3100+250 23 500+300 12 000+200 1700+170 22 700+300 4300+200 4900+200 6600+200 15 500+300 4800+150 8000+400 9800+300 15 700+300 64+16 4600+150

10– 1190 2230– 8650 910–5200 10– 609 1380– 7200 585–1270 350–1950 278–2680 2080– 4600 223–2380 10– 1720 830–2630 2360– 4560 10– 29 412–1470

490+135 4780+200 2250+130 205+80 4410+180 880+90 1020+90 1390+100 3170+160 1170+100 906+185 2090+140 3160+160 16+10 960+90

0.01 –0.67 0.15 –0.49 0.10 –0.26 0.01 –1.25 0.14 –0.41 0.17 –0.27 0.16 –0.36 0.16 –0.38 0.14 –0.25 0.12 –0.67 0.01 –0.19 0.14 –0.38 0.15 –0.24 0.12 –0.77 0.14 –0.26

0.16 0.20 0.19 0.12 0.19 0.20 0.21 0.21 0.20 0.24 0.11 0.21 0.20 0.25 0.21

Table 2. Ranges and arithmetic mean values (AM) of activity concentrations of radon (CRn), equilibrium factor (FRn) and fraction of unattached radon short-lived decay products ( fun) calculated from one-day continuous monitoring with the EQF device. House

1 2 3 4 5 6 7 8 9 10

Room

Guest room Children’s room Bedroom Bedroom Bedroom Bedroom Living room Kitchen Living room Living room Children’s room

Season

Winter Winter Winter Winter Winter Winter Summer Winter Winter Winter Summer

CRn/Bq m23

F

fun

Range

AM

Range

AM

Range

AM

2500–26 800 9500–73 100 2200–8500 3200–17 800 2600–25 400 930–12 600 840–3000 9900–21 600 36– 187 2200–7200 11– 180

13 300+200 37 600+400 4600+100 11 900+200 15 400+300 6900+200 2000+100 17 100+300 89+19 5200+200 87+29

0.08–0.80 0.13–0.36 0.42–0.50 0.26–0.46 0.24–0.86 0.16–0.31 0.06–0.24 0.27–0.42 0.26–0.90 0.44–0.62 0.34–0.90

0.59 0.24 0.46 0.39 0.43 0.29 0.15 0.33 0.63 0.54 0.60

0.01– 0.27 0.01– 0.04 0.01– 0.02 0.02– 0.03 0.01– 0.03 0.02– 0.09 0.14– 0.43 0.03– 0.10 0.03– 0.07 0.03– 0.08 0.05– 0.16

0.08 0.02 0.02 0.02 0.02 0.07 0.23 0.06 0.05 0.05 0.08

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Ranges and average values of the measured quantities are collected in Tables 1 and 2. While very high radon levels were found also in previous surveys(17 – 19), the highest thoron concentration of 886 Bq m23 previously

measured(14) is exceeded by some values in Table 1 by almost a factor of 10. Figure 1 shows the time variation of radon and thoron activity concentrations measured with the RTM device in summer time in living rooms of two houses. In house 2, both concentrations were highest overnight and lowest at noon (Figure 1a), as expected for both radon(27) and thoron(7, 28), while in house 5, both concentrations were higher in the afternoon than during night time (Figure 1b). The former situation was observed in the majority of rooms. A comparison between summer and winter radon and thoron concentrations is shown in Figure 2 for the living room in house 1. Average values in winter were higher than in summer by a factor of 7.6 and 9.8, respectively. Although this seasonal trend was expected(29 – 32), such


Figure 3. Relationship between thoron (CTn) and radon (CRn) activity concentrations in the living room of house 2 in: (a) summer and (b) winter.

Figure 2. Time variation of radon (CRn) and thoron (CTn) activity concentrations in the living room of house 1 in: (a) summer and (b) winter.

high differences are surprising. They explain why the concentrations are markedly higher than those obtained previously in summer in Nisˇka Banja, with maximum values of 1040 and 886 Bq m23, respectively(14).

A good correlation was observed between thoron and radon concentrations in both summer and winter, as exemplified in Figure 3a and b for the living room in house 2. For all rooms, correlation coefficients (r) ranged from 0.61 to 0.95, except in the kitchen of house 8 with only 0.06. Such high coefficients have rarely been observed(32, 33). Generally, the correlation is weak(7, 34) or even missing(12, 35, 36), because the properties and behaviour of radon and thoron are obviously so much different that a good correlation can hardly be expected. Presumably, this correlation appeared to be so good because the investigated area was very small, within the same geology and with very similar construction practice and building material. Therefore, also the average CTn/CRn ratio varied in a very narrow range between 0.11 and 0.25 (Table 1), as compared with the ranges of 0.93 –2.0 in other places in Serbia(14), 0.11 –0.72 in Slovenia(37), 0.05 –7.4 in Hungary(38, 39) and 0.6 –6 in Japan(40). However, range of individual CTn/CRn values was from 0.01 to 1.25. Time variations of radon activity concentration, equilibrium-equivalent activity concentration of RnDP, equilibrium factor between radon and RnDP and fraction of unattached RnDP measured with the EQF device in winter time are shown in Figure 4 for the guest room in house 1 and in Figure 5 for the

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Figure 1. Time variation of radon (CRn) and thoron (CTn) activity concentrations in the bedroom of (a) house 2 and (b) house 5.


except at noon (Figure 5b), they were changing markedly in house 1 (Figure 4b). The former pattern was more frequently observed than the latter one. The abrupt changes in Figure 4b would be understood if a window were opened for a while, or air humidity varied substantially; it was in this case between 32 and 54 %, being thus insufficient to affect the size of aerosol particles and hence the attachment of RnDP significantly(41, 42). It was suspected that somebody must have entered the room, despite the residents were asked not to do so. Negative correlation between FRn and fun, although expected(43, 44), was not seen in all rooms.

Figure 4. Time variation of: (a) radon activity concentration (CRn) and equilibrium-equivalent activity concentration of RnDP (CRnDP) and (b) equilibrium factor between radon and radon short-lived decay products (FRn) and the fraction of unattached radon decay products ( fun) in the guest room of house 1.

Figure 5. Time variation of: (a) radon activity concentration (CRn) and equilibrium-equivalent activity concentration of RnDP (CRnDP) and (b) equilibrium factor between radon and radon short-lived decay products (FRn) and the fraction of unattached radon decay products (CRn) in the bedroom of house 4.

bedroom in house 4. In both rooms, CRn and CRnDP were lowest in the afternoon when they start to increase to reach maximum overnight. While in house 4, FRn and fun were almost constant over the day

Based on the average values of CRn, CRnDP and CTn, obtained from continuous measurements, the effective doses received by a resident in one day in a room due Rn Þ and Tn and to breathing in Rn and RnDP ðEeff Tn Þ were calculated. For that purpose, the TnDP ðEeff values of dose conversion factors ( fDC, nSv per Bq m23 h), proposed by UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) (2) as 9 for RnDP and 40 for TnDP, and occupancy fraction of 0.80 were used in the equations below (for simplicity reasons, units are omitted): Rn ¼ 9 CRnDP 24 0:80 Eeff

ð4Þ

Tn Eeff ¼ 40 CTnDP 24 0:80:

ð5Þ

Because CRnDP was not measured always and CTnDP not at all, they were expressed by (cf. Eqs. 2 and 3) CRn FRn and CTn FTn, respectively, with FRn ¼0.40 and FTn ¼0.02(45, 46). In addition, for rooms in which also the EQF device was used, the measured CRnDP values were also used directly in Eq. 1 and the calcuRnDP . lated effective doses are denoted asEeff The one-day effective doses are shown in Table 3. Except in house 8, doses due to RnDP in winter range from 297 to 2599 mSv d21, being thus in some rooms by more than ten times higher than previously measured in summer time (highest value of 26 mSv a21 8 71 mSv d21)(14), for the same reason as emphasised above when commenting on very high radon concentrations in Table 1. On the other hand, doses due to TnDP in winter ranging from 14 to 73 mSv d21 (with one exception of 0.3 mSv d21 in house 8) are comparable in both surveys, with highest value measured previously of 24 mSv a21 (66 mSv d21)(14), simply because in the previous study, FTn ¼0.10 was used in dose calculation and in ours, FTn ¼0.02, and therefore, these values are by a factor of 4.5 lower. Tn appears to be lower in summer The fraction of Eeff (0.025 –0.036) than in winter (0.041–0.051) (last column in Table 3). Actually, thoron doses are high,

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Effective doses

ˇ ET AL. J. VAUPOTIC Rn Table 3. One-day effective doses due to radon and radon short-lived decay products, using both (i) CRnDP 50.403CRn ðEeff Þ RnDP Þ in Eq. 4, one-day effective doses due to thoron and thoron short-lived decay products, and (ii) measured CRnDP values ðEeff Tn Tn ) in Eq. 5, and the fraction of Eeff . using CTnDP 50.023CTn (Eeff

House 1

2

3

5 6 7

8 9 10

Season

Rn Eeff mSv d21

Living room Living room Bedroom Guest room Living room 1 Living room 1 Living room 2 Children’s room Living room Bedroom Living room Bedroom Living room Bedroom Living room Bedroom Living room Bedroom 1 Bedroom 2 Guest room Kitchen Living room Kitchen Living room Kitchen Children’s room

Summer Winter Winter Winter Summer Winter Winter Winter Winter Winter Winter Winter Winter Winter Winter Winter Summer Summer Winter Winter Winter Winter Winter Winter Winter Summer

214 1624 829 919 118 1569 297 2599 339 318 456 823 1071 1064 332 477 138 553 677 1085 1182 6 4 359 318 6

bearing in mind that 10 mSv d21 would result in 3.65 mSy a21, and are only relatively low because compared with radon doses. In 6 rooms of 16 surveyed, FRn higher than 0.40 Rn was found (Table 2) and therefore the calculated Eeff values in Table 3 are under-estimated with respect to RnDP Þ, and, analogously in 5, the actual exposure ðEeff they are overestimated. Nonetheless, in 9 rooms, deviation from 0.40 in both directions was less than 30 %. Applying the empirical Porstendo¨rfer’s(47) relationships between dose conversion factor ( fDC in mSv WLM21) for nasal breathing and fun for 0.8-nm unattached RnDP aerosol particles: fDC ¼ 101 fun þ 6:7 ð1 fun Þ;

ð6Þ

the above doses can be commented with respect to the measured fun values. Based on the highest average value of 0.23 obtained in the living room of house 7 in summer (Table 2), a conversion factor of 28 mSv WLM21, equivalent to 64 nSv (Bq m23 h)21, is obtained, and consequently the dose would be increased accordingly. In winter time, the highest average fun value was 0.08 in the guest room of house 1, resulting in an fDC value of 32 nSv (Bq m23 h)21. Nevertheless, these fDC values are merely of scientific

RnDP Eeff mSv d21

53 801 1356 139 343 1559

Tn Eeff mSv d21

Tn Tn Rn Eeff =ðEeff þ Eeff Þ

8 73 35

0.036 0.043 0.041

3 68 14

0.025

16

0.042 0.045 0.045

21

0.044

49

0.044

18

0.051

14 32 49

0.025 0.045 0.043

0.2

0.048

15

0.045

366 802 1144 346 52 1559 975 10 485 9

dosimetric interest and not for actual radiation protection purposes. Annual effective doses have neither been calculated, because one-day CRn, CRnDP and CTn averages, obtained under the so-called closed conditions in winter time, are far from representing the annual average concentrations. Therefore, the values in Table 3 may not be discussed with respect to the actual exposure of residents to radon and thoron, but rather to be used to point out how extremely high doses may be received in areas of elevated natural radioactivity. CONCLUSION Average one-day radon activity concentrations in 21 rooms of 10 houses in Nisˇka Banja in winter ranged (Tables 1 and 2) from 4300 to 37 600 Bq m23, except in a living room (Table 2) and a kitchen of the house 8 (Table 1) with less than 100 Bq m23, and those of thoron, from 205 to 4780 Bq m23, except in a kitchen of 16 Bq m23 (Table 1). Average one-day values of equilibrium factor between radon and its short-lived decay products in 9 rooms in winter time were in the range of 0.24– 0.63, with 5 rooms exceeding 0.40, and those of the fraction of unattached radon short-lived decay products in the range of 0.02 –0.08. One-day

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4

Room


effective doses due to radon decay products were in winter time between 297 and 2599 mSv d21, except in both rooms in house 8 with 4 and 6 mSv d21, and those due to thoron decay products, between 14 and 297 mSv d21, except in one room in house 8 with 0.2 mSv d21. The contribution of the latter dose in winter ranges from 4.0 to 5.3 %. FUNDING

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The study was funded by the Slovenian Research Agency within the project contract BI-SC/04-05-002 and by the Ministry of Education and Science of Republic of Serbia within the project contract P-41028.

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29. 30.

31. 32.

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