Purpose: To present the resultsudof the annualudaverage indoor radonudconcentrationsudfrom Nationaludsurvey in Bulgarian regions. Variabilityudofudmeasureduddata betweenud28udregions wasudinvestigated in order toudachieveudthe preliminaryudassessmentudtheuddistributionudofudindoorudradonudconcentrationudthroughudBulgarianududterritory.ududArgument: The National survey on average radon concentrationudinudBulgarianuddwellingsudwas carried outudfromud2015udtoud2016,udwith theududco-operationudofudtheudRegionaludHealth InspectorateudunderudtheudNationalududRadonudProgram.udData were collected through personaludinterviews withudresidentsudand deployment of alpha track detectorsudforudoneudyearudthroughudtwoududphases. The first phase was from March 2015 to December 2015udand second from December 2015 to April 2016 approximately. For eachudregion 100 detectors were distributed and spread to villages in regionsuddepending on the population. The measurements were completed inud2775 dwellings. The distribution of indoor radon levels has been foundudto be log-normal with a geometric mean (GM) of 81.6 Bq•m−3 and audstandard deviation of 2.1. The highest value of the radon concentrationudfound was 1314 Bq•m−3 in the Sliven region. Region with high GM of 184udBq•m−3 and a standard deviation of 1.7 is Yambol in Southeast Bulgaria.udIn about 6% of the houses, the radon concentration was found to be inudexcess of the recommended reference value of 300 Bq•m−3, suggestedudby the Council of the European UnionCommission in in new EU Directiveud2013/59/Euratom. Conclusion: The data from National survey are the firstudsystematic results and are received to complete the National Program,udwhich will finish in 2017. The preliminary results clearly show that the nationaludaction planudto addressudlong-term risksudfrom radonudexposures should be developed taking into account the results achieve in this programudand in accordance with the new Directive 2013/59/Euratom.
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