Large-scale radon hazard evaluation in the Oslofjord region of Norway utilizing indoor radon concentrations, airborne gamma ray spectrometry and geological mapping

Abstract

We test whether airborne gamma ray spectrometer measurements can be used to estimate levels of radon hazard in the Oslofjord region of Norway. We compile 43,000 line kilometres of gamma ray spectrometer data from 8 airborne surveys covering 10,000 km² and compare them with 6326 indoor radon measurements. We find a clear spatial correlation between areas with elevated concentrations of uranium daughters in the near surface of the ground and regions with high incidence of elevated radon concentrations in dwellings. This correlation permits cautious use of the airborne data in radon hazard evaluation where direct measurements of indoor radon concentrations are few or absent. In radon hazard evaluation there is a natural synergy between the mapping of radon in indoor air, bedrock and drift geology mapping and airborne gamma ray surveying. We produce radon hazard forecast maps for the Oslofjord region based on a spatial union of hazard indicators from all four of these data sources. Indication of elevated radon hazard in any one of the data sets leads to the classification of a region as having an elevated radon hazard potential. This approach is inclusive in nature and we find that the majority of actual radon hazards lie in the assumed elevated risk regions.

Introduction

Radon gas is the largest natural source of human exposure to ionising radiation and most of that exposure occurs at home (UNSCEAR, 2000). Recent studies in Europe (Darby et al., 2005, Darby et al., 2006), North America (Krewski et al., 2005, Krewski et al., 2006) and China (Lubin et al., 2004) present overwhelming evidence in support of the traditionally held view that prolonged exposure to radon is responsible for many new cases of lung cancer each year, and offer estimates of the magnitude of the risk. The European study revealed a statistically significant relationship between levels of exposure to radon and the incidence of lung cancer that persists down to radon concentrations of 200 Bq m^− 3 or lower — the radon action level in most countries. This verifies studies based on cancer cases among miners exposed to radon and its progenies at work (BEIR VI, 1999) and further suggests that long term exposure to radon represents a risk to health even at low concentrations.

Although there are several epidemiological studies of possible relationships between radon and other forms of cancer such as leukaemia (Raaschou-Nielsen et al., 2008), and between radon and other diseases like multiple sclerosis (Bølviken et al., 2003), the analyses have not been able to demonstrate any clear associations between the diseases and radon. Therefore it is probable that lung cancer represents by far the most significant risk to human health associated with exposure to radon.

Norway has some of the highest concentrations of radon in indoor air in the world. Extensive mapping of radon in indoor air suggests that approximately 175,000 dwellings in Norway, 9% of the total number, have annual average radon concentrations above the action level of 200 Bq m^− 3 (Strand et al., 1991, Strand et al., 1992, Strand et al., 2001, Strand et al., 2003). It is further estimated that around 27,000 Norwegians live in dwellings with average radon concentrations above 1000 Bq m^− 3 (Lunder Jensen et al., 2004). In the studied area around the capital city of Oslo and Oslofjord (Fig. 1), where approximately half of the Norwegian population live, radon concentrations in dwellings can exceed 5000 Bq m^− 3 (Strand et al., 2001, Strand et al., 2003).

The substrate beneath dwellings is the principal source of radon in Norwegian homes (Stranden, 1986). The amount of radium in the ground determines the amount of radon generated there. The concentration of radium in different rock types varies from under 10 Bq kg^− 1 in sandstone and limestone to several thousand Bq kg^− 1 in alum shale (Andersson et al., 1985, Nordic, 2000). Radon is a special problem in the investigated area because alum shale and soils derived from it occur in well populated areas (Stranden and Strand, 1988). Also, much of this most densely populated part of Norway is underlain by the Oslo Rift, containing large volumes of radium bearing magmatic rocks (Ramberg and Larsen, 1978, Ro et al., 1990, Neumann et al., 2004). Adding to the hazard profile of the region, large bodies of radium rich granitic gneiss occur in populated areas outside the rift (Slagstad, 2006, Bingen et al., 2008).

Transport of radon in pore spaces in the ground up to a building is through convection or diffusion depending on factors such as water content and permeability of the ground (Tanner, 1964, Nero and Nazaroff, 1984, Nazaroff, 1992). In low permeability materials such as marine silt and clay, occurring locally in the investigated area, radon emanation can reach 70% but transport of radon is minimal so these materials are generally associated with a low radon hazard level. Emanation is lower in glacio-fluvial sand and gravel deposits, common in the investigated area, but their higher permeability permits the transport of much more of the emanated fraction of radon towards a building's foundations (Stranden et al., 1985, Åkerblom et al., 1990, Peake, 1988, Hutri and Mäkeläinen, 1993, Sundal et al., 2004a). Therefore glacio-fluvial deposits make a significant contribution to the radon hazard profile of the investigated area.

Programmes for the mapping of radon in the housing stock of the Oslo/Oslofjord region have been orchestrated by the Norwegian Radiation Protection Authority over many years as most recently documented by Strand et al., 1991, Strand et al., 1992, Strand et al., 2001, Strand et al., 2003. The programmes are largely founded on measurements of radon concentrations in indoor air in homes selected at random from the housing stock. However, given the strong spatial variability in geological conditions governing radon emanation and transport in the region, geologically focused investigations have also been carried out (e.g. Stranden and Strand, 1986, Stranden and Strand, 1988, Sundal et al., 2004b).

In 2004 the Norwegian Radiation Protection Authority joined forces with the Geological Survey of Norway to examine new ways to combine available geological data with indoor radon measurements to establish a new overview of spatially variable radon hazard levels across the Oslo/Oslofjord region. That initiative comprised the construction of a geographic information system containing information on radon measurements in the housing stock, bedrock geology, drift geology (superficial deposits) and airborne gamma ray spectrometer data. The data sets were analyzed and their implications for radon hazard combined into radon awareness maps with an accompanying explanation (Smethurst et al., 2006 [in Norwegian]; maps available from http://www.ngu.no).

The present contribution presents the findings of that cross-institution initiative. We go further than that investigation by taking the hazard indications from the individual data sets and compounding them into a single radon awareness map theme layer that records awareness level (high or moderate) and which data types triggered the assignment of elevated hazard. This theme is currently being made available to Norwegian planning authorities through the national AREALIS interactive map service co-ordinated by the Norwegian Mapping Authority (http://www.ngu.no/kart/arealis [in Norwegian]).