Radon exposure of the Greek population

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Radon exposure of the Greek population D. Nikolopoulos, A. Louizi, A. Serefoglou, J. Malamitsi Medical Physics Department, University of Athens, Mikras Asias 75, Goudi, 11527 Athens, Greece

A radon survey has been carried out from 1995 to 1998 in Greece in order to study the exposure of the Greek population. The total sample size was 1137 dwellings. Values of residential potential alpha energy exposure ranged between (0.024 ± 0.009) and (8 ± 1) WLM per year (p < 0.05). Values of effective dose ranged between (0.09 ± 0.04) and (28 ± 4) mSv (p < 0.05). The mean lifetime risk of the Greek population was found to be equal to 0.4%.

1. Introduction Small-scale measurements of radon gas concentration within dwellings in Greece have been reported [1–5]. Continuing the radon investigation conducted by the Medical Physics Department of University of Athens (MPD-UOA) since 1988, a large-scale nation-wide residential radon survey in Greece was designed and performed, using dosimeters of MPD-UOA construction, fully calibrated and tested by the MPD-UOA [6]. The main scope was to obtain an adequate estimation of the annual radon concentration distribution indoors, to assess the average risk, to determine the percentage of dwellings in which radon concentrations exceed certain reference levels and to investigate some factors that affect indoor radon concentrations.

2. Materials and methods 2.1. Statistical data The most recently published statistical data is the 1991 census, according to which Greece had a population of 10.4 million people [7]. The population was highly concentrated in urban areas, and mainly in Athens where about 37% of the total population resided. The total number of buildings was 3.8 million. Approximately 75% (2.85 million) of the buildings were used as dwellings. The buildings were classified by the National Statistical Service of Greece (NSSG) according to their use. Building data were provided nation-wide according to an administrative RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07049-4

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partition proposed by the NSSG. For those used as residencies, no data about the number of stories, building attributes and occupational status of each separate dwelling was provided by the NSSG, except for a part of the capital (Athens). For the rest of the country, the data were given only in an accumulative manner. Moreover, the NSSG provided no maps through which the geographical coordinates of each house could be found. 2.2. Sampling design Due to the limitations placed by the statistical data and taking into account the financial and work power of the MPD-UOA, the radon survey was not based on a grid division but was administratively designed. The sampling design was based on the partition proposed by the NSSG. Ten [10] administrative districts, called regions, were divided into prefectures, called departments, which were subdivided into provinces. These were organized in municipalities and communes, which included villages and city quarters. The sampling design was based on the following procedure: a sampling density of 1 per 1000 dwellings was adopted, balancing both feasibility and precision of estimation. The number of samples was calculated for a department, according to the total number of its dwellings. This number was further allocated to each province, in proportion to the fraction of the number of dwellings of the province over the total number of dwellings of the department. Continuing, the sample number of each province was allocated to its municipalities and communes, in proportion to the number of the dwellings of the municipality or commune over the total number of dwellings of the province. The procedure was followed so as to allocate an adequate number of samples to each village or city quarter [8]. 2.3. Experimental apparatus The experimental apparatus was the MPD radon dosimeter [9]. The dosimeter consisted of a cylindrical non-conductive plastic cup of 5 cm height and 1.5 cm radius. The cover had a 3 mm hole in the center and a filter that prevented radon daughters from entering. Radon was detected by a 2 × 2 cm CR-39 nuclear track detector placed at the bottom of the cup. The overall uncertainty of radon measurement in the 95% confidence interval was below 10% [6]. The 12-month exposure period was selected due to the best estimation of the average value it provides. One detector was installed in each sampled dwelling, placed in the bedroom 1 m above the ground, near the wall. 2.4. Measurement procedure Detectors were installed by trained personnel. A door-to-door approach was selected, so as to minimize non-response and bias. This scheme was generally followed and changed only by restrictions placed at the implementation stage (i.e. refusals and other difficulties). Within every sampling location, dwellings were selected by the personnel, so as to sample nation-wide all types of buildings. In each case, a questionnaire was filled and the inhabitant was given informative brochures. At the end of the 12-month period, the dosimeters were collected, either via door-to-door approach or by post.

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3. Results The survey was carried out between July of 1995 and August 1998 with the installation of 1500 MPD dosimeters in 834 locations (i.e. villages and city quarters) resulting in the sampling of 950 dwellings in 722 locations. The data included an additional 216 samples in 12 locations collected between 1988 and 1994 by other MPD-UOA investigators [1–3], resulting in a total of 1137 samples in 734 locations within Greece. Broad sampling was performed in South Greece, i.e. Attica Department, Peloponnese and the island of Crete covering about 40% of the Greek territory and about 50% of the Greek population, while local sampling occurred in all other investigated areas. Sample density ranged between 1/271 dwellings and 1/10003 dwellings with an average of 1/2405 dwellings excepting the capital province within the West Attica prefecture where the sample density was 1/46018 dwellings and the four provinces where no dosimeters were finally collected. With the above exceptions, the sample density is representative according to [10] and is comparable to that of the other similarly designed surveys based on statistically representative sampling [11–15]. The results are given as a frequency distribution histogram in Fig. 1. Introducing the χ 2 test, the overall results follow the log-normal distribution (p < 0.01). Residential radon concentration ranged between 200 and 400 Bq m−3 in 22 dwellings (1.9%), 400 and 1000 Bq m−3 in 8 (0.7%) dwellings, and above 1000 Bq m−3 in 4 (0.4%) dwellings. In the full data set, the arithmetic mean was found to be 55 Bq m−3 and the geometric mean 44.0 Bq m−3 with a geometric standard deviation of 2.4 Bq m−3 . In only a small percentage (1.1%) of dwellings in Greece did the measured radon concentrations exceed the action level proposed by [16] (400 Bq m−3 ). Through the questionnaires it was found that from the full data set 527 dwellings located were on the ground floor, 334 on the first floor, and 89 above the first floor of a building. Among these categories the one-way analysis of variance (ANOVA) method was applied to the logarithms of the radon concentrations, which follow a Gaussian distribution. Ground floor dwellings presented statistically significant higher radon concentrations but for the dwellings

Fig. 1. Frequency distribution histogram of indoor radon concentrations in Greece. Sample size 1137.

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of the first floor and above, the differences were not significant (p < 0.001). Moreover, 529 dwellings were constructed from brick and concrete, 109 from concrete only, 254 from stone and 71 from mixed building materials. By application of the same method it was found that the dwellings constructed with stone presented statistically significant higher concentrations (p < 0.005). Applying the same method to the ground floor dwelling, radon concentration data of each surveyed area it was found that some areas presented statistically significant differences in radon concentrations (p < 0.001). In some of these areas, the residential radon concentrations lie in the tail (p < 0.01) of the log-normal distribution (Fig. 1). From these only two (2), i.e. Arnea Chalkidikis and Vrisses Apokoronou Chanion, are “radon prone” areas according to [17]. Residential Potential Alpha Energy Exposure (PAEE) and effective dose values may be calculated from the above data set by using appropriate values for the equilibrium, occupancy and dose conversion factor. Since no such values are available for Greece, a mean equilibrium value of 0.4 [18] and an occupancy factor of 0.8 [10], respectively, were used to estimate risks. PAEC values were calculated using 72 WLM y−1 /(Bq m−3 ) of mean annual equivalent radon concentration, and effective doses using 6 nSv h−1 /(Bq m−3 ) of mean annual equivalent radon concentration as a dose conversion factor. In South Greece, where broad area sampling was performed, residential PAEC values ranged between (0.024 ± 0.009) and (2.8 ± 1.0) WLM per year (p < 0.05) with a mean of 0.2 WLM per year. Effective doses were between (0.09 ± 0.04) and (11 ± 4) mSv per year (p < 0.05), with a mean of 0.8 mSv per year. These mean values lie far beyond the maximum values of (8 ± 1) WLM per year and (28 ± 4) mSv per year (p < 0.05) that occurred in the radon prone area of Arnea Chalkidikis. Using the risk factor of 2.8 × 10−4 per WLM [18] according to epidemiological data, a mean PAEC value of 0.2 WLM per year assuming that this is representative for Greece, due to the broad sampling performed there and the mean life expectancy of 74 y for men and 77 y for women in Greece [19], the mean lifetime risk in Greece due to residential radon is 0.4% (0–1.1% in the 95% confidence interval). This means that on average 40 out of every 10 000 inhabitants of Greece would die due to lung cancer caused by residential radon exposure. Since Greece has a population of 10.4 million people, it may be calculated that on average 400 mortal lung cancers due to residential radon are expected to occur each year in Greece. Mean lifetime risk was calculated excluding the data from the rest of the country because the sampling there was not statistically representative according to [10]. The uncertainties of PAEC and effective dose values were calculated taking into account the instrumental uncertainty of the MPD radon dosimeter and the statistical fluctuations of the recorded concentrations within every surveyed area. Mean lifetime risk uncertainty was calculated taking into account the fluctuations of the calculated PAEC values in South Greece. Both uncertainties are biased by the uncertainties in the dosimetric conversion factors [20,21]. Moreover, mean lifetime risk is biased by age, smoking habits [20] and by uncertainties in the mean life expectancy in Greece.

4. Conclusions According to the results obtained, it was found that only a small percentage of dwellings appeared to have annual average radon levels above 400 Bq m−3 , which is the action level

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proposed by the European Community. The survey supports the recommendation of testing mainly ground floor or first floor dwellings, since there were not found to be significant differences in radon concentrations among the dwellings of the upper floors. In addition, the radon estimates have shown geographical differences, leading to support for the strategy of focusing on areas with high radon potential. The survey is still in progress, because on the one hand, different results may be obtained elsewhere and on the other hand, a better estimation of the national average should be determined. Moreover, geological and other relevant data are being collected by MPD-UOA. These will be combined in future with the questionnaire data, in order to investigate the factors that affect indoor radon concentrations in Greece. The calculations of the mean nation-wide annual risk due to residential radon were based only on broad area sampling (about 40% of the Greek territory and about 50% of the Greek population) because the use of data from local sampling may have introduced a systematic error if these had represented over- or under-estimations of the mean radon concentration of each surveyed area. Nevertheless, elevated residential radon concentrations may be found in the non-broadly surveyed part of Greece.

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