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This article examines the results of a study of radon concentrations in kindergartens in three districts of north-western Bulgaria. For the period from December 2019 to May 2020, passive radon measurements were accomplished in 1490 premises of 130 kindergartens. The highest arithmetic mean (AM) value of 219 Bq m -3 and geometric mean (GM) value of 156 Bq m -3 radon concentration were found in the state kindergartens of the Vratsa district. The radon values for the remaining two districts are as follows: Lovech-AM = 156 Bq m -3 and GM = 114 Bq m -3 ; Montana-AM = 125 Bq m -3 and GM = 88 Bq m -3 . The effects of various factors on the radon concentration including district, year of building construction, presence of basement, place of premise, wall, and floor interior covering materials, and presence of a ventilation system were investigated. Factor Analysis was used to assess the combined effects of those factors on indoor radon concentration. The results revealed three combinations of the investigated factors: the first combined the district in which the kindergartens were located and the materials used for the floor of the premises, the second combined the year of construction of the building and the presence of a building foundation, and the third combined the rest of the investigated factors. It is concluded that a regional assessment of predictors of radon variability is needed. The walls in kindergartens should not be covered by gypsum, and floors should not be made from materials that can easily develop cracks such as terra cotta. The installation of a ventilation system reduces the radon concentrations in the premises, which is of particular importance in cases where the measured value is above the national reference levels., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Mineral springs are used in spa resorts throughout the world. Radon is a natural radioactive source, which can dissolve, accumulate, and be transported by water. This study investigates the radon concentration in air and water in 12 Bulgarian rehabilitation hospitals and presents the assessment of the exposure to radon in them. The measurements were performed at 401 premises within 21 buildings, using two types of passive detectors for a dry and wet environment that were exposed from February, 2019 to June, 2019. The radon concentration varied from 19 to 2550 Bq/m 3 with an arithmetic mean and a standard deviation of 102 Bq/m 3 and 191 Bq/m 3 , respectively. The hypothesis that in hospitals the source of radon, besides soil under the buildings, is also the mineral water that is used for treatment was tested. Thermal water samples were procured sequentially from a spring and baths to analyse the reduction of radon concentration in them till reaching the premises. The results show that the concentration of radon decreased by approximately 50%. Further, the correlation analysis applied to the data proved the relation of the levels of indoor radon in the treatment rooms with those in the water. Mineral water used in rehabilitation hospitals have radon transfer coefficients ranging from 4.5·10 -4 to 8.4·10 -3 . In addition, an analysis of the exposure of patients and workers to radon in rehabilitation hospitals based on the indoor radon levels and period of exposure was performed. The doses of workers do not exceed the limit of the annual effective dose for the population from all sources (1 mSv/year)., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

This paper presents the methodology and results of the national radon survey in Bulgaria and its spatial variability. The measurements were carried out in 2778 dwellings using CR-39 track detectors over two successive 9 and 3-month periods from April 2015 to March 2016. The arithmetic (AM) and geometric (GM) means of annual indoor radon concentration were 111 ± 105 Bq/m 3 and 81 Bq/m 3 (GSD = 2.15), respectively. The distribution of data has been accepted to be log-normal. Two hypotheses have been investigated in the paper. The first one was a spatial variation of indoor radon concentration and the second was spatiality of the factor that influences radon variation. The indoor radon concentrations in the 28 districts have been significantly different, which prove the first hypothesis. The influence of the factors, geology (geotectonic unit, type of rock, and faults distance of the measuring site), type of the region, and the presence of the basement in the building on radon spatial variation, was examined. The analyses have been shown that they significantly affect radon variations but with a relatively small contribution in comparison to the radon variation between district. Furthermore, the significance and contribution of the investigated factors were different in each district, which confirmed the second hypothesis for their spatiality.

In the light of the new ICRP requirements the first National radon survey has been conducted in Bulgaria in the period 2015-16. Indoor radon concentrations were measured for 1 y in 2778 dwellings of whole Bulgarian territory in order to obtain the systematic data for the distribution of radon. The arithmetic mean is 111 Bq m-3 and the geometric mean is 81 Bq m-3 with geometric standard deviation of 2.15. On the base of the data, an annual effective dose to the Bulgarian population due to indoor radon exposure was estimated at 5.2 mSv and excess lifetime cancer risk at about 2%. The evaluation has been made using the ICRP dose coefficient of 6.7 × 10-6 (mSv h-1)/(Bq m-3), 7000 h occupancy per year and value of 0.4 for the equilibrium factor. This preliminary assessment allows consideration and planning the long-term appropriate radon and exposure reduction measures in the new national action plan.

As a part of the systematic survey of indoor radon in Bulgaria, the indoor radon concentration was measured in 296 kindergarten buildings of Sofia city during 3 months (February to April 2013) using the CR-39 nuclear tract detectors. In 256 buildings 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 an 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 that in Sofia city dwellings obtained from a previous study. A detailed statistical analysis of the building factors was presented., (© The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

The first systematic indoor radon survey in four districts of Bulgaria was performed. Three hundred and seventy-three selected ground floor dwellings were measured using the nuclear track detectors during 6 months, from October 2011 to May 2012. It was found that indoor radon concentration varied between 20 and 3560 Bq m(-3) with median value of 90 Bq m(-3). The fractions of dwellings in four districts: Sofia city, Sofia districts, Plovdiv and Varna above the reference levels of 300 Bq m(-3) were 3, 9, 14 and 5 %, respectively. Each data set does not follow a log-normal distribution at a significance level of 95 %. The results of the analysis of the variance showed statistically significant differences among the indoor radon concentrations for the regions between urban and rural municipalities as well for the building with and without basement. These results may be utilised to set up the methodology for a more systematic survey in Bulgaria.