An update on thoron exposure in Canada with simultaneous ²²²Rn and ²²⁰Rn measurements in Fredericton and Halifax.

Naturally occurring isotopes of radon in indoor air are identified as the second leading cause of lung cancer after tobacco smoking. Radon-222 (radon gas) and radon-220 (thoron gas) are the most common isotopes of radon. While extensive radon surveys have been conducted, indoor thoron data are very limited. To better assess thoron exposure in Canada, radon/thoron discriminating detectors were deployed in 45 homes in Fredericton and 65 homes in Halifax for a period of 3 months. In this study, radon concentrations ranged from 16 to 1374 Bq m(-3) with a geometric mean (GM) of 82 Bq m(-3) and a geometric standard deviation (GSD) of 2.56 in Fredericton, and from 4 to 2341 Bq m(-3) with a GM of 107 Bq m(-3) and a GSD of 3.67 in Halifax. It is estimated that 18 % of Fredericton homes and 32 % of Halifax homes could have radon concentrations above the Canadian indoor radon guideline of 200 Bq m(-3). This conclusion is significantly higher than the previous estimates made 30 y ago with short-term radon measurements. Thoron concentrations were below the detection limit in 62 % of homes in both cities. Among the homes with detectable thoron concentrations, the values varied from 12 to 1977 Bq m(-3) in Fredericton and from 6 to 206 Bq m(-3) in Halifax. The GM and GSD were 86 Bq m(-3) and 3.19 for Fredericton, and 35 Bq m(-3) and 2.35 for Halifax, respectively. On the basis of these results, together with previous measurements in Ottawa, Winnipeg and the Mont-Laurier region of Quebec, it is estimated that thoron contributes ∼8 % of the radiation dose due to indoor radon exposure in Canada.