Radiation Protection Dosimetry 7:49-54 (1984)
© 1984 Oxford University Press
Soil Gas Radon - A Source for Indoor Radon Daughters
In Sweden, 400 Bq.m-3 has been taken as the limit for the permitted radon daughter concentrations in dwellings. Out of a total of about 1.6 million houses in Sweden, it is estimated that some 40,000 houses have radon daughter concentrations exceeding the limit of 400 Bq.m-3. In the majority of these houses, the radon has come from the ground. The relationship between radium and radon contents in the ground and radon concentrations indoors has been studied in a research project financed by the Swedish Radon Commission. 253 houses were included in the study, some situated in areas where the bedrock and overlying soils consist of granite with enhanced uranium contents (12 to 30 ppm U), others in areas of uranium-rich black alum shale (120 to 250 ppm U). 105 of the houses were selected for detailed studies which included geological mapping of bedrock and soils, measurement of the radium and thorium contents of the bedrock and soils by gamma spectrometry, measurement of soil gas radon adjacent to and under foundations, measurement of indoor radon concentrations and investigations of the building structure, ventilation etc. Indoor radon concentrations varied from 20 to 200,000 Bq.m-3 and soil gas radon contents, at one metre depth, from 5,000 Bq.m-3 in sand to 700,000 Bq.m-3 in alum shale rich soils. The results of the investigations show that: (i) soil gas containing radon leaks into houses through cracks or holes in the foundations because of lower air pressure indoors compared to outside, (ii) the radon content of soil gas is always sufficient to give higher radon daughter levels than 400 Bq.m-3 if sufficient soil gas leaks into a house, and if the volume of air in the soil is sufficient to maintain active transport indoors, (iii) the radon content in the soil gas is determined by the content of radium-226 in the soil particles, by emanation and by the moisture content of the soil, (iv) the possibility for transport of radon-bearing soil gas is determined by the permeability of the soil, (v) radon in soil gas is almost always produced by the immediate soil layers. Only in exceptional circumstances is radon transported up from depth, (vi) high risk soils are those with enhanced radium contents and those with high permeability, for example, gravel in eskers, gravelly sandy till and infillings of crush rock or gravel even if the radium content of the material is low.
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