According to the Radiation Protection Act (StrlSchG), according to the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, by December 31, 2020, all federal states had to identify the areas in which reference values were frequently exceeded in buildings. Said reference value applies nationwide and is 300 Bq / m3 in residential buildings, schools and at workplaces. High radon-222 activity concentrations occur, for example, in the German low mountain ranges and former mining areas in the federal states of Saxony, Thuringia, Bavaria and Baden-Württemberg. Targeted reduction measures must be implemented there to protect residents. Schools and employers are also obliged to check compliance with the reference value and, if necessary, to implement protective measures.