Significant reduction in indoor radon in newly built houses.

Results from two national surveys of radon in newly built homes in Norway, performed in 2008 and 2016, were used in this study to investigate the effect of the 2010 building regulations introducing limit values on radon and requirements for radon prevention measures upon construction of new buildings. In both surveys, homes were randomly selected from the National Building Registry. The overall result was a considerable reduction of radon concentrations after the implementation of new regulations, but the results varied between the different dwelling categories. A statistically significant reduction was found for detached houses where the average radon concentration was almost halved from 76 to 40 Bq/m. The fraction of detached houses which had at least one frequently occupied room with a radon concentration above the Action Level (100 Bq/m) fell from 23.9% to 6.4%, while the fraction above the Upper Limit Value (200 Bq/m) was reduced from 7.6% to 2.5%. In 2008 the average radon concentration measured in terraced and semi-detached houses was 44 and in 2016 it was 29 Bq/m, but the reduction was not statistically significant. For multifamily houses, it was not possible to draw a conclusion due to insufficient number of measurements.