Sensitivity to thoron of an SSNTD-based passive radon measuring device: Experimental evaluation and implications for radon concentration measurements and risk assessment

Passive devices based on SSNTDs (Solid State Nuclear Track Detectors) are widely used to measure radon concentration in indoor air. These devices often include a filter or other types of barrier to prevent the sampling of decay products. However, such filters and barriers have different degrees of effectiveness in preventing thoron from entering the sensitive volume of the passive device, with the result that in some cases the measured track density is affected by thoron concentration, especially if devices are placed very close to walls exhalating thoron. This can produce a bias in epidemiological studies aimed to evaluate the risk from radon. A radon measuring device with LR 115 detectors enclosed in a heat-sealed 35 μm low density (0.92 g/cm3) polyethylene bag has been largely used in Italy and other countries. Moreover, it was used in an epidemiological study carried out in an Italian region where a large fraction of dwellings are built with materials containing high thorium concentration and exhalating a remarkable quantity of thoron. The sensitivity to thoron of this device was experimentally evaluated by exposing groups of 10 devices each in the NIRS (Japan) radon/thoron chamber to three different thoron exposures, i.e. 500, 1000 and 2000 kBq/m3 h. The sensitivity to thoron of these devices resulted to be about 0.4% of their sensitivity to radon. In conclusion, radon concentrations measured with the evaluated passive device are not significantly affected by the presence of thoron. Therefore both the distribution of radon concentration obtained in the national and regional surveys in Italy and the risk of lung cancer from radon exposure in dwellings, as estimated in an epidemiological study carried out in an Italian region with high radon and thoron, have not been biased by the presence of thoron.