A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities
Introduction
Radon (222Rn) is a naturally occurring radioactive gas generated by the decay of uranium bearing minerals in rocks and soils. A certain fraction of the radon escapes from the ground into the air. In the open air, radon is diluted to low concentrations and is not considered a health concern. However, radon that enters an enclosed space, such as a family house or a school building, can sometimes accumulate to concentrations above the Canadian Action Level (200 Bq m−3). Soil gas radon contributing to the indoor environment has been identified as the main source of natural radiation exposure to the population (UNSCEAR, 2006). Radon has been identified as the second leading cause of lung cancer after tobacco smoking (WHO, 2009).
Soil gas radon measurement is a useful tool for the assessment of environmental radon potential and for the prediction of potential indoor radon concentrations in a geographic area, as demonstrated by many studies in various geographic locations (Keller et al., 1992, Mose et al., 1992, Neznal et al., 1996, Akerblom and Mellander, 1997, Vaupotic et al., 2002, Sundal et al., 2004, Reimer and Szarzi, 2005, Neznal et al., 2006, Kemski et al., 2006, Kemski et al., 2009, Chen et al., 2009a, Minda et al., 2009, Barnet, 2012, Cinelli et al., 2015, Lara et al., 2015). Various studies have shown that in addition to radon levels in the soil, the soil permeability is another important factor in determining the radon potential of a site or an area, because higher permeability enables the increased migration of soil gas radon from soil into houses. For example, in a German case study in 1988–1989, Keller et al. found that high radon levels in the soil cause high indoor concentrations, if there are easy pathways into houses, and positive radon anomalies in the soil gas coincide with the locations of houses showing the highest concentrations (Keller et al., 1992). In northern Virginia and southern Maryland, Mose et al. attempted to predict which geographic areas should be associated with a high percentage of homes with unusually high indoor radon levels based on estimates of soil radon and soil permeability for geological units (Mose et al., 1992). They concluded that predictions of indoor radon based on estimates of home site soil radon and soil permeability are very useful. Similar conclusions were obtained in a recent study in Brazil (Lara et al., 2015) when using soil features to predict indoor radon concentration. Barnet (2012) compared two different methods (real and calculated) for evaluating the relationship of soil gas radon and probability of indoor radon exceeding 200 Bq m−3 based on data sets from the Czech Republic, the results showed minor differences.
Since radon in soil is believed to be the main source of radon in Canadian homes, it is of public interest to study the correlation between radon in soil and radon indoors in different geographic locations. Based on measurement data collected from various soil and indoor radon surveys in the past 7 years, a summary report on the correlation between soil radon potential and indoor radon potential in Canadian cities is presented here.
Section snippets
Soil radon and permeability measurements
Soil gas radon concentration was determined by measuring the radioactivity of soil gas samples taken from a depth of 80 cm below the ground surface (Radon VOS, 2007a). Because the boulder-rich nature of glacial tills made it difficult to reach the target depth at some sites, the sample depth was occasionally reduced to 60 cm. Samples of soil gas were collected using a 150 ml syringe. The soil gas samples were then introduced into ionisation chambers for measurement. Background control
Results and discussion
From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon (Chen et al., 2008b, Chen et al., 2009c, Chen et al., 2012, Ford and Chen, 2008, Goodwin et al., 2009, Goodwin et al., 2010, Ford et al., 2010, Ford et al., 2015, Friske et al., 2012, Friske et al., 2013), as shown in Fig. 1. Among the sites surveyed, 476 sites were in cities where data could be correlated with indoor radon measurements (represented as black dots in Fig. 1; a few cities with less than 7 sites surveyed
Conclusions
Soil radon survey was conducted in 14 Canadian cities where information of indoor radon distribution was available. Among the 14 cities, indoor radon potential (percentage of homes above 200 Bq m−3) varied from 1.5% to 42% while the SRP index ranged from 8 to 26. Generally speaking, indoor radon potential correlates reasonably well with soil radon potential index. For 5 cities (Ottawa, Halifax, Fredericton, Winnipeg and Regina) where detailed in-situ soil permeability was measured in more than
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