Framework for evaluating legacy high activity building material based on indoor gamma radiation surveys

In Europe, people spend most of their time in buildings. The indoor gamma radiation has the highest contribution to our natural exposure after the inhalation of radon and its daughter elements and its level can reach sometimes very high values. But there is no reference level which can support the decision about the necessity of mitigation actions. Our purpose was to characterise the usual level of indoor gamma radiation in the Hungarian homes and to originate a Reference Level by the adoption of legislation applied for building materials. In this article, a statistical evaluation was made from the results of outdoor and indoor gamma radiation surveys made between 1995 and 2024 using passive and active detectors and gamma-ray spectrometric results of building materials. The average outdoor and indoor gamma radiation level was found to be 97 nSv/h and 149 nSv/h, respectively. According to our calculation, 250 nSv/h ambient gamma dose rate corresponds to the 1 mSv annual effective dose. Therefore, the Reference Level of indoor gamma radiation should be 350 nSv/h for the Hungarian homes. Such kind of elevated levels was detected only in buildings where bottom ash weas used as floor space filling material or the main walls were made from bottom ash concrete blocks. Additionally, a direct link was found between the gamma dose rate levels near to the surface of the building materials and its radioactivity.