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The study presents the analysis of the results for indoor radon concentration in the kindergartens of two Bulgarian districts. The study presents the analysis of indoor radon concentration results in kindergartens of two Bulgarian districts. Measurements were performed with a passive method for the period from February to May 2015, in 411 children’s rooms situated in 157 kindergartens. The measured radon concentrations in the children’s rooms were between 10 and 1087 Bq per m. The evaluation of the results showed that in 10% of the kindergarten rooms, the radon concentration is above the national reference level of 300 Bq per m. The influence of the presence of a basement and the implementation of building renovation on the radon concentration were examined. The presence of a basement is a prerequisite for reducing the concentration of radon in the building. It has been proven that renovating the building increases the radon concentration in it. The analysis confirms the necessity of measuring the indoor radon concentration before the renovation and repair of the building, especially the implementation of energy efficiency measures.

Bulgaria is a country in the eastern part of the Balkan Peninsula, in South-eastern Europe. The presence of many mountains and karst areas is a prerequisite for a large number of caves in the country, but only some of them are managed and accessible to tourists. The purpose of the study is continuous measurement of radon concentration in six popular Bulgarian tourist caves (Bacho Kiro, Magura, Venetsa, Saeva dupka, Snejanka, and Uhlovica), and analysis of the diurnal variation. The direct, continuous radon measurements were performed with TERA (TSR4) system (Tesla, Czech Republic). 29 radon concentration time series were considered for evaluation of radon variations during the day and night. The period of the measurement was the autumn of 2019, and only one cave (Bacho Kiro Cave) was measured during the winter. The measurements were performed at different points (gallery, halls, or other cave formations) in each of the caves. The average diurnal radon concentration in the different caves varied from 531 Bq m−3 (Magura cave) to 5472 Bq m−3 (Venetsa cave). The variation of radon concentration between distinct places in most caves was approximately 1–2 k Bq m−3. No significant difference in radon concentrations was observed between night and day in a 24–96 h period measurement. The large fluctuations between the values measured at the entry (10–640 Bq m−3) and exit (165–927 Bq m−3) compared to those inside the cave were found only in cave Magura. The radon exposure of workers and visitors was assessed, using long-term measurements in the caves, based on the ICRP recommendation.

Radon (222Rn) is a radioactive gas and formed as a results of the radioactive decay of radium. Radon relieved from ground could accumulate in the building and contribute to human exposure. Radon is recognized as a carcinogenic agent by the World Health Organization and is the second leading cause of lung cancer after tobacco smoke. Radon in buildings is considered to be the most important indoor air pollutant, with harmful effects on the health of the population. The purpose of this survey is to analyze the influence of building type and year of construction on indoor radon concentrations in Pernik city, Bulgaria. The measurements of the indoor radon concentration in buildings located in the town of Pernik (housing, schools and kindergartens) from national database is used. The results for annual radon concentration in 70 dwellings, in 28 public schools and in 34 kindergartens is applied. The nuclear track detectors (CR-39) were used for measurements. The annual radon concentration in schools and kindergartens was evaluated using the seasonal factor for Pernik district. The descriptive statistic is presented according to type of building and year of construction in the article. The arithmetic means of radon concentrations are: for dwellings AM = 133 Bq/m3, for schools AM = 123 Bq/m3 and for kindergartens AM = 136 Bq/m3. The group of buildings built during the period 1950 – 1969 has the highest average value of indoor radon concentration (AM=152 Bq/m3). In order to evaluate the difference between of the radon concentration in the groups separated by type of buildings and year of construction the Kruskal Wallis test was applied. No statistically significant difference was found between the types of buildings, unlike between the groups on years of construction. The influence of year of construction on the radon level was analyzed by applying the discriminative analysis. The results shown that 7.2% of the radon variation could be explain by year of construction of the building

Bulgaria is rich in geothermal water. The radon level in spas buildings depends on soil underneath, thermal water and the ventilation conditions. The aim of the paper is to analyses influence of ventilation system on indoor radon in air in the spa buildings. Direct measurements of radon in the air were performed in treatment rooms (treatment with thermal water and without) and mineral pools. A statistically significant difference was found between the indoor radon levels in treatment rooms using water and those without water, located in branches with high radon concentration (MW, p = 0.032). The spectra of the rooms for 24 hours were considered for analysing the indoor radon daily variations. The radon arithmetic mean value in rooms with operating ventilation system was 148 Bq /m3, and in those without - 756 Bq /m3 and the statistically difference is significant (MW, p

Новите препоръки на МКРЗ и нашето законодателство включват контрол на облъчването от радон и установено национално референтно ниво за средногодишна активност от 300 Bq.m 3 за жилища и сгради с обществен достъп. Целта на настоящото проучване е извършване на оценка на облъчването на българското население, свързана с инхалацията на радон, която се основава на данните от Националното проучване на нивата на радон в жилища и анализ на фактори, които влияят на обемната активност на радон за територията на България. Установените средна геометрична стойност е 81 Bq.m-3 с геометрично стандартно отклонение 2,15 и аритметичната средна стойност - 111 Bq.m- 3 със стандартно отклонение 105 Bq.m- 3 са сравнима с повечето стойности за европейските страни. Потвърдено е лог - нормално разпределение на измерените обемни активности на радон в проучените жилища (p ˃ 0.01). Извършени са оценки на влиянието на: геоложкия и скален състав, вида на населеното място, сградни характеристики и навици на обитателите на измерените жилища върху вариацията на радон. Установени са статистически значими разлики за влияние на вид тектонска зона (KW, p ˃ 0.0001), скален състав (KW, p ˃ 0.0001), вид населено място (KW, p ˃ 0.0001), наличие на мазе (KW, p ˃ 0.0001), вид помещение (KW, p ˃ 0.01) и тютюнопушене (KW, p = 0.06) върху обемната активност на радон. Потвърдена е пространствената вариация на обемната активност на радон за всичките области на страната (KW, p

As a very mountainous country, Bulgaria has a lot of caves, but only some of them are developed, managed and available for tourist visits. The main sources of radon are rocks and a high concentration can be measured in caves. Caves have been of interest because radon concentration in these environments may sometimes reach high values. A rather large number of investigations have been carried out all over the world, from the point of view of both researches on radon behavior in a cave environment and radiation protection. There are no systematic investigations of radon in the caves in Bulgaria in order to assess the health risk. The paper presents the sample design of the radon concentration survey in Bulgarian caves for the evaluation of the health risk of people. The sample design covers the method of selection, the sample structure and the plan for analyzing and interpreting the results. The method selected to define the representative sample corresponds to the following criteria: it covers all types of caves in different mountains in Bulgaria; it is consistent over time; it includes changes to the definition over time; it constitutes the selection of the sample design for the caves and organization of survey. The samples of caves are defined on the basis of mountains in Bulgaria in order for the definitions to remain constant over time. Groups of caves according to the mountains are: caves in Stara Planina, caves in Rhodope Mountains and caves in other mountains. The design of the samples of caves is built around a random selection from the most visited caves in Bulgaria. According public information, there are 65 of these caves. The calculated sample size for a confidence level of 95% and a margin of error of 30% is 10 caves. The numbers of samples have been weighed according to the percent of the caves in the groups with 5 per cent precision at 95 per cent confidence. The way of the selection of the sample and the measurement method is presented in the paper.

This study discusses the sampling frames of the buildings with public access to radonconcentration surveys on the territory of Bulgaria in order to assess the health risk for the population.The sampling frame is the list from which the sample is selected, so the quality of the sampling frameaffects the quality of the sample. The general list of different types of buildings with public access hasbeen prepared in accordance with the list of the Ministry of the Regional Development and PublicWorks in Bulgaria and international experience in the field of radon investigation. The types ofbuildings with public access have been classified in nine general groups. These groups of buildingswith public access are defined as follows: educational institutions; buildings for commerce and/orservices; health and social care buildings; buildings in the field of culture; sports facilities; buildingsin the field of transport; prisons; cult and religious buildings and post offices. Multicriteria analysiswas applied to select the types of buildings in order to assess the health risk from radon exposure inbuildings with public access. The main criteria for the analysis are: the probability of high radonconcentration; time spent in building and the number of people accessing the place. For the purposeof the precise definition of the target buildings, it is necessary to ensure that the all the places wherethe population could be exposed to radon are adequately covered. To define the whole list of thetarget buildings, the official available information has been used from: the Ministry of Education andScience for the schools and universities; Ministry of Health for the hospitals and polyclinics; Ministryof Culture for the theatres, cinemas and museums and the Ministry of Tourism for hotels, caves, etc.

A data set of long-term radon measurements in approximately 2800 houses in Bulgaria has been collected in National radon survey (2015-2016) under National Radon Program. The distribution of all radon data sets has been found to be log-normal. The main results of measured annual radon concentration in each for the 28 bulgarian districts are summarized. The arithmetic mean is 111 Bq/m3 and is approximately equal to the weighted average population on 108 Bq/m3.The object of the study is to assess the impact of building characteristics on variations in radon concentration. The article discusses the following building features: year of construction, type of base and facade of the measured dwellings and type of window frames in the buildings surveyed. Because more radon data are included in the current assessment, and the data set covers broader building factors, the current assessment better represents the radon exposure in Bulgaria. In conclusion, the complete results of National Survey provided a reliable estimate of the radon national distribution in bulgarian dwellings, and represent the basis for more detailed estimates of the impact of different factors on the indoor radon concentration.

As a part of the systematic survey of indoor radon in Bulgaria, indoor radon concentration was measured in 292 kindergartens buildings of Sofia city during 3 month (January – April), 2013 using the CR-39 nuclear tracts detectors. In any building at least two frequently occupied rooms (mainly playrooms) were observed. Altogether 922 measurements were performed. The frequency distribution was well described by the lognormal function. The measured radon concentrations range between 9 and 1415 Bq m-3 with a geometric mean of 101 Bq m-3 (2.08) and arithmetic mean 132 Bq m-3 with a standard deviation of 118 Bq.m-3. The radon concentrations obtained in this survey were compared with radon concentration in Sofia dwellings obtained from previous study. A detailed statistical analysis of the building factors was present.

This work presents the results of continuous radon measurements in open atmosphere, including measurements of air humidity, temperature, pressure and gamma dose rate, in 11 different settlements of Bulgaria. Using AlphaGuard equipment, measurements were made in mountain, spas, plain, sea level and uranium mining environments in the summer period, in series of 10-min intervals over 24 hours., . The analysis of results showed different daily variation among the outdoor radon measurements in different measuring sites. The outdoor radon concentrations ranged from 22 Bq.m−3 (city in lowland) to 180 Bq.m−3 (uranium mining site Eleshnica). Outdoor radon levels were found to be increasing during the night by a factor of 1.5 (in 9 locations), factor 2 (location at sea level) and factor 1 (location in the plain). The relation between outdoor radon concentrations and meteorological factors as well with gamma dose rate was observed.