Indoor Radon Concentrations of School Buildings in Varna District, Bulgaria

Radon is a radioactive gas that is naturally produced by the breakdown of uranium in soil,rock and water.Recent International Commission on Radiological Protection(ICRP)and World Health Organization(WHO)requirements include controlling indoor radon exposure and settings of a reference level for the annual average activity concentration of up to 300 Bq/m3 for dwellings and public buildings.Radon measurements in school buildings have long been carried out in many countries.Systematic measurements in schools in Bulgaria have started and are progressing.The study concerns the measurements of radon concentration(CRn)in all state school buildings in one of the Black Sea regions,Varna district,and the analysis of indoor variation.The survey of 1,185 premises of 107 schools was conducted from December 2019/January 2020 to March/May 2020 using a passive method.The AM(Arithmetic Mean)of CRn in the premises of the studied schools on the territory of Varna district is 130 Bq/m^(3),and the geometric mean is 97 Bq/m^(3).The number of premises exceeding the national reference level of 300 Bq/m^(3) is 72 located in 38 schools(about 36% of schools).A statistically significant difference(KW(Kruskal-Wallis),p<0.0001)was found between the radon data,grouped by municipalities.Furthermore,the radon data were classified into two groups based on municipality location:on the Black Sea coast and inland.Schools located near the sea have a higher value of CRn.In order to more accurately assess the exposure of pupils and teachers,the results are divided into four groups,depending on the type of use of the premises and the floor on which they are located.The highest CRn value was found in the study rooms used for specialized education such as physics,chemistry,music etc.(AM=155 Bq/m3),which can be explained by their smaller size and less frequent use.A statistically significant difference(KW,p<0.0001)was found,which means that the radon exposure in the examined rooms depends on the type of use.The key to reducing children’s exposure in schools is to measure CRn and then apply corrective action to the high levels of radon.