Indoor radon concentrations in urban settlements on the Montenegrin Coast

Radiation Measurements 42 (2007) 1573 – 1579 www.elsevier.com/locate/radmeas

Indoor radon concentrations in urban settlements on the Montenegrin Coast N. Antovic a,∗ , P. Vukotic a , R. Zekic b , R. Svrkota c , R. Ilic d a Faculty of Natural Sciences and Mathematics, University of Montenegro, Cetinjski put b.b., 81000 Podgorica, Montenegro b Center for Ecotoxicological Research, Put Radomira Ivanovica 2, 81000 Podgorica, Montenegro c Center for Geological Research, Cetinjski put b.b., 81000 Podgorica, Montenegro d Institute Jozef Stefan, Ljubljana, Slovenia

Received 14 November 2006; received in revised form 28 February 2007; accepted 19 June 2007

Abstract The first systematic indoor radon measurements on the Montenegrin Coast were carried out in the period 2002–2003, when 107 randomly selected homes in urban settlements were surveyed using CR-39 track-etch detectors, twice a year, each time for about 6 months. None of the measured radon concentrations exceeded the action level of 400 Bq m−3 . The annual average radon concentrations were found to be lognormally distributed (GM = 25.5 Bq m−3 , GSD = 2.1) within the range from 3 to 202 Bq m−3 , with arithmetic mean of 31.8 Bq m−3 , and median of 25.1 Bq m−3 . The average effective dose due to exposure to radon in urban homes on the Montenegrin Coast is estimated to be 0.50 mSv y−1 . © 2007 Elsevier Ltd. All rights reserved. Keywords: Dosimeter; Indoor radon concentration; Effective dose

1. Introduction Radon is an inert and radioactive gas. Its three natural isotopes are produced by the decay of uranium and thorium that occur in Earth’s crust. Among these isotopes, 222 Rn is the most significant and usually called “radon”. Emanating from the ground, radon gas builds up in buildings because of indoor underpressure and poor air ventilation in them. Although the ground is usually the major source of indoor radon, the other sources are building materials, water, natural gas and outdoor air. The main reason for growing interest in radon concentrations at homes and workplaces is of health nature: indoor radon is the main contributor to the exposure of human population to natural sources of ionizing radiation, and one of the main causes of lung cancer. This is why many countries put a lot of efforts to make national surveys of indoor radon and to identify radon-prone areas, as well as to adopt a number of related regulations. The development of radon program in the UK is a very illustrative ∗ Corresponding author. Tel.: +381 81 266 150; fax: +381 81 244 608.

E-mail addresses: [email protected], [email protected] (N. Antovic). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.06.003

example (Kendall et al., 2005). However, a recent overview of radon surveys in Europe (Dubois, 2005) reveals clearly that the Western Balkan region is behind this trend, being characterized with scarce available data on radon. Montenegro is one of the Western Balkan countries. It has an area of 13 812 km2 , population of 620 527 and 173 887 inhabited dwellings (Census of Population, Households and Dwellings in the Republic of Montenegro in 2003, Statistical Office of the Republic of Montenegro, 2005). The radon action levels adopted in Montenegro, 400 Bq m−3 for existing houses and 200 Bq m−3 for future buildings, are in line with international recommendations (ICRP Publication 65, 1994). Although radon measurements in Montenegro started 10 y ago (Uvarov et al., 1997; Vukotic et al., 1997, 1998; Antovich et al., 2002), the first systematic long-term radon survey commenced in 2002, in the frame of the ongoing national radon program. As a part of this program, during the period 2002–2003, radon was surveyed in urban homes on the Montenegrin Coast, a very attractive region and the best-known tourist area in the country. The purpose of the survey was to get, for the first time, a systematic insight into the indoor radon levels in this region—to obtain annual average radon concentrations and

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estimate the related effective doses to the population, and potentially to identify houses and areas with elevated radon concentrations. This paper presents the obtained results.

Table 1 Number of residents and dwellings in urban settlements on the Montenegrin Coast Municipality

Number of inhabitants in urban settlements

Number of dwellings permanently inhabited

Herceg Novi Kotor Tivat Budva Bar Ulcinj

21 12 10 13 17 10

7293 4123 3242 4416 5633 2864

Total

86 754

2. Experimental 2.1. Coastal region—demography and geology The Montenegrin Coast is a narrow strip of land, separated from the inland by high and steep mountains. It is up to 10 km wide and 100 km long by air distance, but with 293 km long coastline and 53 km of beaches. There are six municipalities in this region, named after their largest towns: Herceg Novi, Kotor, Tivat, Budva, Bar and Ulcinj. Table 1 gives information on the number of inhabitants in urban settlements of these municipalities and the number of permanently inhabited dwellings, which are based on the mentioned Census 2003. The total number of dwellings in these settlements is much higher, because many of them are used only for vacation and recreation purposes. For instance, the total number of dwellings in urban settlements in the Budva municipality is 10 840, and only 4416 of them (or 40.7%) are permanently inhabited. Geological composition of Montenegrin coastal area includes carbonatic, clastic and volcanic rocks of Triassic, Jurassic, Cretaceous, Tertiary and Quaternary age. The coastal area is divided into two geotectonic units: Budva–Cukali (B–C) zone and Adriatic–Ionian (A–I) zone. B–C zone encompasses urban areas of Bar, Budva, Kotor and partly Tivat and Herceg Novi. Geological structure of this zone consists of Mesozoic carbonates and eruptive rocks, Anisian and Paleogen flysch. The urban areas of Ulcinj and partly of Tivat and Herceg Novi belong to A–I zone, which is composed of Upper Cretaceous limestones, dolomites and dolomitic limestones; Middle Eocene foraminifer limestone and flysch; Middle and Upper Eocene flysch sediments and Middle Miocene sands, sandstones, clays and lithothamnium limestones. The most of the sampled urban localities are situated on terrains made of Quaternary formations, whose material is composed of different rock parts and particles conveyed from the immediate surrounding by erosiveaccumulation processes. The surface part of these terrains is predominantly made of three types of soil (Djuretic et al., 1966–1983): (1) anthropogenized Mediterranean cambisols on flyshe, which is characteristic of the Herceg Novi, Tivat and Bar urban settlements; (2) anthropogenized cambisols on calcareous–silicate rocks, present in urban areas of Kotor and Ulcinj and (3) Alluvial and alluvial–diluvial calcareous gleyed and poorly saline soils, in urban regions of Herceg Novi, Budva, Tivat and Bar.

735 657 024 599 950 789

27 571

Table 2 Summary of key data about radon surveyed houses Category

Statistics

House type

83% Single-family detached house 13% Multi-story apartment house 4% One-story apartment house

House age

51% Less than 25 y old 32% Between 25 and 40 y old 12% Between 40 and 100 y old 5% More than 100 y old

Building materials

55% 20% 13% 11%

Concrete pillars and brick walls Concrete hollow blocks Reinforced concrete Stones

Fig. 1. Schematic drawing of a radon dosimeter.

2.2. Sampling Sampling of urban homes on the Montenegrin Coast for radon survey was based on a regular grid, covering whole territory of the coastal urban settlements. In order to obtain a sampling ratio of about 1:200 compared to the whole dwelling stock, in each 500×500 m grid square one house was randomly selected and one dwelling in the house. The total number of

dwellings selected in this way was 154. Only dwellings with permanent occupancy were surveyed on radon. Table 2 summarizes some key data about the houses selected for radon survey, which are obtained from questionnaires. It can be seen that most of these houses are single-family houses (83%), mostly built in the last 25 y (51%) and made predominantly of concrete pillars and brick walls (55% of the houses).

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Table 3 Statistics for radon measurements in 126 dwellings in the summer period Municipality

NM

Herceg Novi Kotor Tivat Budva Bar Ulcinj Total

AM (Bq m−3 )

SD (Bq m−3 )

GM (Bq m−3 )

GSD

32 14 8 22 30 20

19.5 26.8 15.5 21.7 27.5 22.1

11.9 22.6 9.2 17.3 32.4 27.3

15.1 16.6 13.1 13.8 17.4 11.3

2.5 3.1 1.9 3.5 2.8 3.6

126

22.2

20.1

14.6

2.9

MIN (Bq m−3 )

MAX (Bq m−3 )

Median (Bq m−3 )

 1.5  1.5  1.5  1.5

60 65 33 62 175 99

17.2 18.0 13.0 14.1 19.7 10.7

 1.5

175

15.4

3 4

NM = number of measurements, AM = arithmetic mean, SD = standard deviation, GM = geometric mean, GSD = geometric standard deviation, MIN = minimum measured radon concentration, MAX = maximum measured radon concentration.

Fig. 2. Frequency distribution of indoor radon concentrations in urban settlements on the Montenegrin Coast in the summer period (126 dwellings).

2.3. Measurements The solid-state nuclear track detectors are most suitable for the determination of the annual exposure of the general public to indoor radon (George, 1996). Radon monitoring device of the Nuclear Center Karlsruhe (KfK), Karlsruhe (Urban and Piesch, 1981), has been successfully used in many European countries, such as Germany, Belgium, Luxemburg, Austria, Switzerland, the Netherlands (Put et al., 2000), and was found to have the best performance among several different types of passive dosimeters employing solid-state nuclear track detectors (Jamil et al., 1997). Having these facts in mind, and for economy sake, it was decided that the home-made dosimeter which is alike to the KfK device should be used for nationwide long-term radon measurements in Montenegro. Schematic drawing of this home-made dosimeter is shown in Fig. 1. It presents a diffusion chamber equipped with a 25 × 25 mm CR-39 solid-state nuclear track detector (INTERCAST, Italy) and a glass-fiber filter, which allows radon gas to enter the chamber volume but retains radon progeny and moisture.

This dosimeter was regularly located in the living room or a bedroom on the ground floor or the first floor, in a place which is away from windows and doors, and about 1.5 m above the floor and 0.5 m away from the wall. Indoor radon concentration was measured twice a year at the same place, each time during approximately 6 months. The detectors were exposed in the period from April to September 2002 (the “summer” period) and from October 2002 to March 2003 (the “winter” period), under normal living conditions. Some of the detectors placed in the selected 154 dwellings were lost during the campaign of radon measurement, so that finally we obtained radon concentrations for 126 dwellings in the summer period, for 116 in the winter period and for 107 dwellings in both the radon measurement periods. In order to control the consistency and accuracy of dosimeter response, at each 10th measuring location two of our dosimeters were placed together and, again at each 10th (but the other) location, a passive radon monitoring device of the J. Stefan Institute, Ljubljana, Slovenia (which is described by Sutej et al., 1988; Humar et al., 1992, and will be referred to as IJS radon dosimeter), utilizing CR-39 detector, was placed beside our dosimeter. The comparison of track densities in the paired detectors confirmed a good consistency and accuracy of the dosimeters which we used for radon measurements. Generally, these results were mutually consistent within a discrepancy range less than 10%, which grew to about 20% only in the cases when radon concentrations were very low. Latent image of -particle tracks in a detector foil was transformed to visible tracks with etching technique (6.25 N NaOH, 70◦ C, 7 h), while track density was determined by the TRACOS automatic image analysis system (Skvarc, 1993), developed at the J. Stefan Institute, Ljubljana. Calibration of our dosimeter was carried out using the radon chamber at the J. Stefan Institute. In a standard procedure (Sutej et al., 1988), the response was determined as k1 = 0.167 tracks cm−2 Bq−1 m3 d−1 . Then, we performed intercalibration of our dosimeters with the IJS radon dosimeter, which was subjected to several proficiency tests in standard radon chambers in the UK, and obtained k2 = 0.165 tracks cm−2 Bq−1 m3 d−1 . Finally, we adopted the value k=0.166 tracks cm−2 Bq−1 m3 d−1 , as the arithmetic mean (AM) of the two mentioned values.

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Table 4 Statistics for radon measurements in 116 dwellings in the winter period Municipality

NM

Herceg Novi Kotor Tivat Budva Bar Ulcinj Total

AM (Bq m−3 )

SD (Bq m−3 )

GM (Bq m−3 )

GSD

MIN (Bq m−3 )

MAX (Bq m−3 )

Median (Bq m−3 )

37 7 6 18 29 19

39.8 47.8 29.2 61.4 46.6 31.4

60.7 46.2 20.0 52.7 22.4 25.0

24.6 31.7 21.7 44.7 42.0 21.3

2.4 2.6 2.6 2.3 1.6 2.7

5 11 4 12 17 4

344 128 61 201 104 92

22.3 25.3 27.8 45.2 37.1 25.4

116

42.7

37.8

31.0

2.4

4

344

30.5

Indoor radon concentration CRn was calculated from the track density  using the formula  = 0 + kC Rn t,

(1)

where t is the detector exposure time and 0 is the background. For an estimation of the lowest detectable radon concentration, the following formula is used: CRn,min = LD (kSt)−1 ,

(2)

where LD is the lower limit of counted tracks for a qualitative detection and S is the scanned area of the detector. For a well-known blank, defined as the signal resulting from an experiment in which conditions are identical to the experiment in question except that no radon gas is present, the definition of LD is (Ilic et al., 1990) LD = 2.71 + 3.29B ,

(3)

where B is the standard deviation of the blank, which is defined as (Ilic and Rant, 1991)  B = (b + b t) · S + (s · S)2 , (4) where b is the background track density, b is the mean rate of background radiation tracks and s is the constant that describes variation of background track density due to variations in detector sensitivity, etching and counting technique. In our case, an average detector exposure time was t = 180 d, b = 0 = 422 tracks cm−2 , b 0, s = 2 cm−2 , S = 2.25 cm2 , B 31, LD 105, k = 0.166 tracks cm−2 Bq−1 m3 d−1 . With these values we obtain that CRn,min 1.5 Bq m−3 , which indicates a good sensitivity of our dosimeter. 3. Results 3.1. Radon levels Radon was measured in the summer period in 126 dwellings on the Montenegrin Coast. The results of these measurements are presented in Table 3 and Fig. 2. Summary of the results obtained by radon measurements in 116 dwellings during the winter period is presented in Table 4 and Fig. 3.

Fig. 3. Frequency distribution of indoor radon concentrations in urban settlements on the Montenegrin Coast in the winter period (116 dwellings).

Based on the two measurements, in the summer and in the winter period, the annual average concentrations of indoor radon are calculated for 107 dwellings on the Montenegrin Coast, and their statistics is presented in Table 5 and Fig. 4. Table 6 gives the AM of annual radon concentrations in dwellings classified by house type, age and building material. 3.2. Effective dose The annual effective dose due to exposure to indoor radon, or more precisely due to inhalation of 222 Rn and its decay products, can be estimated from the known annual average radon concentrations using the formula E = dC Rn F T ,

(5)

where E is the average effective dose in Sv, d is the dose conversion factor for radon in Sv per Bq h m−3 , CRn is the average radon (222 Rn) concentration in the air in Bq m−3 , F is the equilibrium factor between 222 Rn and its decay products and T is the time spent annually inside dwelling expressed in hours. There is no consensus in the scientific community on the value of the dose conversion factor, because of the complex physical and biological issues involved. While the ICRP 1994 recommends a dose factor of 1.1 mSv per mJ h m−3 for members of the general public, which corresponds to 6 nSv per

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Table 5 Statistics for the annual average concentrations of indoor radon Municipality

NM

Herceg Novi Kotor Tivat Budva Bar Ulcinj Total

AM (Bq m−3 )

SD (Bq m−3 )

GM (Bq m−3 )

GSD

MIN (Bq m−3 )

MAX (Bq m−3 )

Median (Bq m−3 )

32 6 6 17 27 19

32.0 30.8 21.1 43.3 37.4 26.5

36.4 23.3 11.2 27.9 21.8 20.1

23.5 25.6 17.3 35.9 32.7 17.9

2.0 1.8 2.2 1.9 1.6 2.8

9 14 4 12 13 3

202 76 37 106 107 65

20 22.4 22.1 34.8 31.2 19.9

107

31.8

23.4

25.5

2.1

3

202

25.1

Table 6 Effective doses on the Montenegrin Coast due to radon in dwellings Municipality

Average effective dose (mSv y−1 )

Herceg Novi Kotor Tivat Budva Bar Ulcinj

0.50 0.48 0.33 0.68 0.59 0.42

Whole region

0.50

Table 7 Arithmetic mean (AM) of annual radon concentrations in dwellings depending on house type, age and building material

Fig. 4. The annual average concentrations of indoor radon in urban settlements on the Montenegrin Coast (107 dwellings): (a) histogram of frequency distribution, lognormal fit and percentages, (b) probability–probability plot (for lognormal distribution  = 3.25,  = 0.751).

Bq h m−3 , the UNSCEAR 2000 suggests an increased radon risk per unit exposure and recommends the dose conversion factor of 9 nSv per Bq h m−3 . The average effective dose is estimated by using the AM of indoor radon concentrations (ICRP Publication 65, 1994),

Category

Classification

AM (Bq m−3 )

House type

Single-family detached house Apartment house

33.1 25.4

House age

Less than 25 y old Older than 25 y

31.4 32.2

Building materials

Concrete pillars and brick walls Concrete hollow blocks Reinforced concrete Stones

31.4 34.5 27.5 32.8

so that CRn = AM in Eq. (5). Assuming the typical value of equilibrium factor F = 0.4 for radon indoors (ICRP Publication 65, 1994; UNSCEAR, 2000), then the more stringent recommended value for the dose conversion factor d = 9 nSv per Bq h m−3 and the mean residence time on the Montenegrin Coast of 12 h daily, we calculated the average effective doses due to exposure to radon in urban homes on the Montenegrin Coast. These doses are given in Table 7 . 4. Discussion None of the measured indoor radon concentrations on the Montenegrin Coast exceeded the action level of 400 Bq m−3 . As Fig. 3 shows, indoor radon concentrations in the winter period were mostly (82%) below 60 Bq m−3 . The radon concentrations above 100 Bq m−3 were found at seven

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locations, but all of them are below the action level for existing houses. As it was expected, radon concentrations in the summer period were lower than in the winter months (in average twice lower—see Tables 3 and 4). Only one of these concentrations was above 100 Bq m−3 (but lower than 200 Bq m−3 ), while 95% of them were below 60 Bq m−3 (Fig. 2). Fig. 4b clearly shows that the distribution of annual average concentrations of indoor radon in urban settlements on the Montenegrin Coast is approximately lognormal (with geometric mean of 25.5 Bq m−3 and geometric standard deviation of 2.1), which is in accordance with general findings for dwellings worldwide (UNSCEAR, 2000). From the results presented in Table 5 and Fig. 4 it follows that all of these radon concentrations are much below the action level. There were only four of the surveyed dwellings with an annual average concentration above 100 Bq m−3 , and all of them belong to single-family houses (two in the municipality of Herceg Novi, one in Budva and one in Bar). The maximum annual average concentration is 202 Bq m−3 and was found in a dwelling in the Djenovici settlement (municipality of Herceg Novi), in the living room, which is wood-heated and naturally ventilated. This ground floor dwelling belongs to a twostory detached house which is built of concrete and bricks on a sloping terrain with good porosity and drainage. Radon concentration measured in that home was 60 Bq m−3 in the summer period, and 344 Bq m−3 in the winter period, the latter being the absolute maximum measured throughout the radon survey. The results of radon measurements in urban homes on the Montenegrin Coast (AM = 31.8 Bq m−3 , GM = 25.5 Bq m−3 , GSD = 2.1), summarized in Table 5, are evidently lower than the worldwide average indoor radon concentrations: AM = 40 Bq m−3 , GM = 30 Bq m−3 , GSD = 2.3 (UNSCEAR, 2000). One of the main reasons for relatively low domestic radon concentrations on the Montenegrin Coast is the mild Mediterranean weather, with more than 2500 sunny hours annually, which enables keeping the windows open for hours almost every day year round—customary habit in this region. There are indeed only very few cold weeks in the winter season when the windows stay closed all day long, so that annual average of ventilation rate in the coastal homes is relatively high. The average effective dose for population in urban settlements on the Montenegrin Coast due to radon indoors, if adjusted on the occupancy time of 7000 h as recommended for global population by ICRP and UNSCEAR (ICRP Publication 65, 1994; UNSCEAR, 2000), would amount to 80% of the worldwide average of 1.0 mSv y−1 . The lowest average effective dose is found in the urban area of Tivat, and the highest in the urban area of Budva, twice higher than the first one (see Table 7). Both of them are about 35% different from the average effective dose for the whole coastal region. The reason for such a large difference between the radon induced effective doses in these two urban areas is not obvious, because both Tivat and Budva are situated on the Quaternary sediments composed of the alluvial deposits, and the houses in both places are mostly single-family houses built from materials which are common for the whole region. It could

be eventually connected to the age of the houses, because Budva has the fastest growing housing stock on the Montenegrin Coast (82% of sampled homes are built in the last 25 y, while in Tivat this percentage is the lowest—only 33%), and new homes are expected to be built more air-tight to conserve energy. However, a closer insight in the questionnaires shows that this is not a right conclusion, because radon results for the other coastal municipalities do not match it. Moreover, Table 7 does not reveal any significant difference in annual radon concentrations with respect to the age of surveyed houses. Results presented in Table 7 support the generally known fact that radon concentrations are higher in detached single-family houses than in apartment buildings, and show that, as to radon exposure indoors, the reinforced concrete is the best among the building materials, commonly used on the Montenegrin Coast, while the concrete hollow block is the worst. We can compare results of this radon survey with some results obtained in the winter season of 1994/1995, during an investigation of environmental radioactivity on the Montenegrin Coast (Vukotic et al., 1998). That investigation encompassed in situ measurements of external gamma exposure on the beaches and on their hinterlands, as well as radon and in situ gammaradiation measurements inside the hotels. The radon measurements of 1994/1995 showed low concentrations in 12 hotels on the Coast. Namely, winter radon concentrations, measured with track-etch detectors for 90 d in these hotels (multi-storied buildings, with frameworks made of reinforced concrete and inside walls of bricks), were in a range from 22 to 80 Bq m−3 , with an AM of 41 Bq m−3 . Those findings very well match the results of our radon measurements in coastal urban homes in the winter period, presented here in Table 4 and Fig. 3. 5. Conclusion Indoor radon concentrations in urban settlements on the Montenegrin Coast are much below the action level of 400 Bq m−3 adopted in the Republic of Montenegro. They are generally higher in detached single-family houses than in apartment houses, and do not depend significantly on the age of the houses. As to radon exposure indoors, reinforced concrete appears as the most favorable of all the building materials, commonly used on the Coast. The annual average concentrations of indoor radon on the Montenegrin Coast are about 20% lower than the corresponding worldwide population-weighted values, and the average effective dose for local population due to exposure to radon indoors amounts to 80% of the worldwide average. Acknowledgments This study was performed within the national program of indoor radon survey in Montenegro, and was funded by the Ministry of Environment. The authors wish to thank I. Kobal of the J. Stefan Institute, Slovenia, for his kind help in carrying out calibration of the radon dosimeters, and the staff of the Center for Ecotoxicological Research for assistance in the field measurements.

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