Exposure to radon is considered the second most important cause of lung cancer next to smoking and is estimated to account for about 500 lung cancer cases per year in Sweden. For this reason, it is important to lower radon concentrations in indoor air, particularly where levels exceed the reference level 200 Bq/m³.
In order to be able to update calculations on number of lung cancer cases, which are caused by radon, it is necessary to estimate the average radon level in indoor air. It is also essential to estimate how many dwellings and workplaces there are which exceed the reference level 200 Bq/m³. This study focuses on single-family houses but also includes analysis for some workplaces and multi-family houses. The radon measurement data for single-family houses in the database of the company Radonova consists of about 340 000 measurements.
In the database, there are more measurements in areas with well-known radon problems and in houses which include the radioactive building material blue concrete. However, in the analysis it is possible to compensate for a number of such background factors to reduce bias. This enables estimation of a national average for the radon level in dwellings, which should resemble results based on a representative sample.
In order to be able to prioritize supervision of both dwellings and workplaces it is important to understand which background factors are associated with radon levels exceeding the reference level. Suggested factors for such analysis include building year, ventilation type, building material (blue concrete), region, uranium level in the ground and type of soil.
Based on the analysis of a large amount of measurements the national average of radon concentration in single-family houses was estimated. For the measurement season 2007/2008 it was assessed to 128 Bq/m³ and for 2008/2009 to 136 Bq/m³. These two time periods were believed to provide the most reliable results since offers for measurements were distributed widely to house-owners during those years. The results in the study for average radon level in Swedish single-family houses are in good agreement with a previous national survey within the so-called BETSI project. That project reported a national average of 124 Bq/m³ for the corresponding time period and type of dwelling.
It was also estimated that about 19% of single family houses during the time period 2007-2009, corresponding to 370 000 units, have radon levels above the reference level 200 Bq/m³. This number is considerably higher than in some other studies (BETSI and ELIB studies). For radiation protection purposes the number of houses exceeding 200 Bq/m³ is important since it shows how many single-family houses there are that need to lower radon concentrations. Based on the result 370 000 and analysis of number of mitigated houses and new houses since 2008, it was estimated that in 2021 there are about 330 000 single-family houses with radon levels exceeding 200 Bq/m³. This corresponds to 16% of the number of single-family houses in 2021.
In the study there are also some results for workplaces, but since these are based on long-term average measurements around the clock, these results are usually not representative for working hours. This is because ventilation often is reduced during non-working hours causing increased radon levels during those times. Thus radon levels during working-hours can be substantially lower than the levels reported in this study.
In this study, it was shown that single family houses built in more recent years have substantially lower radon levels than in older houses. The same clear pattern of decreasing radon levels in newer buildings was however not observed for workplaces.
The radon concentration in single-family houses was highly correlated to the uranium level in the ground based on a limited subset of the database. In the corresponding analysis for workplaces some correlation was also observed but less pronounced.
New regulations regarding radon in workplaces was implemented 2018 in response to the revised European Basic Safety Standards (BSS). Since then the Swedish Radiation Safety Authority has a new responsibility to perform supervision of workplaces regarding radon. In order to prioritize efforts for supervision it is important to know which background factors are most important for risk of elevated radon concentrations in buildings. The analysis in this research project includes quantitative estimation of a number of such potential risk factors.
For the purpose of risk assessment for lung cancer in the population due to radon exposure it is necessary to determine the national average radon concentration in indoor air. In this study this average is estimated for single-family houses. Thus additional analysis of studies in other indoor environments is necessary to determine a comprehensive assessment of the national average radon level.
The number of single family houses with radon levels exceeding the reference limit 200 Bq/m³ is of great interest as an indicator of the size of the radon problem in Sweden. The corresponding assessment for workplaces is also important, but this study is based on long-term measurements around the clock, which often results in overestimation of radon levels during working-hours.
Need for further research
It would be worthwhile to initiate a new study to assess the current national average of radon level and fraction of buildings exceeding the reference level 200 Bq/m³. This kind of study should be based on selecting a representative sample of dwellings and workplaces.
In the large dataset of measurement results from Radonova it was indeed possible to estimate a national average. However, despite efforts to control for factors that bias results to resemble a representative selection, it is difficult to rule out some remaining bias. This is particularly valid for recent measurement years but presumably also for possible future studies.
For workplaces, it would be very useful to add follow up measurements to determine radon levels during working hours to complement current results based on long-term measurements. This is because long-term measurements around the clock in workplaces often overestimate the radon levels during working hours.
It would also be of interest to study the long-term sustainability of measures to reduce radon concentrations in buildings. Furthermore, studies to provide additional scientific background for current recommendations regarding methods to measure radon in both dwellings and workplaces would be valuable.