The Norwegian indoor radon mapping strategy

The indoor radon concentrations in Norway are among the highest in Europe. This is partly explained by the geology due to the large occurrences of radium rich soil and bedrock, large occurrences of highly permeable unconsolidated sediments, and the construction of buildings, including heating methods. Based on results of nation-wide surveys it has been estimated that 8.9 % and 3.3 % of the present housing stock has an annual mean level of radon higher than 200 Bq/m3 and 400 Bq/m3, respectively. The populations weighted annual mean radon concentration is estimated to 88 Bq/m3. Results of surveys show that there are significant geographical variations — between counties and municipalities — as well as within each municipality and even between homes in the same residential area. In some areas more than 2/3 of the housing stock exceeds the recommended action level for existing homes of 200 Bq/m3, and in some homes very high levels have been measured (up to 50,000 Bq/m3).
This paper describes the Norwegian indoor radon mapping strategy as recommended by NRPA and applied in several extensive municipal surveys during the last ten years. The main objective of the municipal radon surveys is to identify dwellings exceeding the action level (annual average > 200 Bq/m3) by as few measurements as possible. The mapping strategy is divided into two phases; an initial Phase (I) where measurements are made in a randomly selected sample of dwellings and a follow-up Phase (II). The aim of the Phase I survey is to obtain information on the distribution of indoor radon concentrations and identify radon prone areas. The aim of Phase II is to target as many homes as possible above the action level based on the results of Phase I. In phase II it is recommended that all dwellings are measured in areas where more than 20% of the measurements in phase I are higher than 200 Bq/m3. In areas where 5 to 20 % of the measurements are higher than 200 Bq/m3, a Phase II survey is recommended by taking into account additional information from the questionnaires — e.g. type of dwelling, year of construction, use of light expanded clay aggregate blocks in the foundation walls etc. — as well as more detailed information on geology. No further surveys are recommended in areas where less than 5 % exceeds 200 Bq/m3 provided that no single measurement is above 400 Bq/m3. However, measurement services are offered commercially in Norway and it is generally recommended that radon measurements are made in every home.
By the end of 2007 radon measurements have been carried out in approximately 80,000 homes, which is close to 4 % of the Norwegian housing stock. So far, nearly 200 out of the 430 Norwegian municipalities have carried out Phase I radon surveys. By the NRPA radon mapping strategy it is assumed that it should be possible to identify nearly ¾ of the homes above the action level by making measurements in 1/3 of the housing stock.