Deborah Moir

Canadian population risk of radon induced lung cancer: a re-assessment based on the recent cross-Canada radon survey

Exposure to indoor radon has been determined to be the second leading cause of lung cancer after tobacco smoking. Canadian population risk of radon induced lung cancer was assessed in 2005 with the radon distribution characteristics determined from a radon survey carried out in the late 1970s in 19 cities. In that survey, a grab sampling method was used to measure radon levels. The observed radon concentration in 14 000 Canadian homes surveyed followed a log–normal distribution with a geometric mean (GM) of 11.2 Bq m–3 and a geometric standard deviation (GSD) of 3.9. Based on the information from that survey, it was estimated that ∼10 % of lung cancers in Canada resulted from indoor radon exposure. To gain a better understanding of radon concentrations in homes across the country, a national residential radon survey was launched in April 2009. In the recent survey, long-term (3 month or longer) indoor radon measurements were made in roughly 14 000 homes in 121 health regions across Canada. The observed radon concentrations follow, as expected, a log–normal distribution with a GM of 41.9 Bq m–3 and a GSD of 2.8. Based on the more accurate radon distribution characteristics obtained from the recent cross-Canada radon survey, a re-assessment of Canadian population risk for radon induced lung cancer was undertaken. The theoretical estimates show that 16 % of lung cancer deaths among Canadians are attributable to indoor radon exposure. These results strongly suggest the ongoing need for the Canadian National Radon Program. In particular, there is a need for a focus on education and awareness by all levels of government, and in partnership with key stakeholders, to encourage Canadians to take action to reduce the risk from indoor radon exposure.

Determination of thoron equilibrium factor from simultaneous long-term thoron and its progeny measurements.

With 3-month simultaneous measurements of thoron and its progeny concentrations in the lowest floors of 109 homes, the geometric mean (GM) of thoron equilibrium factor was determined to be 0.019 with a geometric standard deviation (GSD) of 3.63. Combined with the analysis from results obtained from a previous study in 138 homes, the GM of thoron equilibrium factor was determined to be 0.022 with a GSD of 3.02. The results indicate that the F value of 0.02 for thoron recommended by UNSCEAR is a reasonable value for those Canadian homes tested.

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.

An estimation of the annual effective dose to the Canadian population from medical CT examinations

This study was carried out to assess the annual per capita effective dose from medical diagnostic procedures using computed tomography (CT) in Canada. Relevant data concerning the nature and the frequency of various diagnostic CT examinations were obtained from the reports on Medical Imaging in Canada and Diagnostic Services in Ontario. Doses associated with examinations of different types were based primarily on typical effective doses used in the National Council on Radiation Protection and Measurements Report 160 with considerations of limited dose information surveyed in Canada. The results show that the per capita annual effective dose from diagnostic CT exams was 0.74 mSv in 2006, up from 0.19 mSv in 1991. This significant increase in population radiation dose from CT scans is due mainly to a more than doubling in the examination rate and to a higher radiation dose per procedure from the newer generation of multi-detector CTs.

Measurement of 236U in human tissue samples using solid phase extraction coupled to ICP-MS

236U is present at ultra-trace levels in typical environmental and biological samples. Typically, it has been measured by highly sensitive techniques, such as accelerator mass spectrometry. This paper reports the measurement of 236U in 20 human tissue samples using a sector field ICP-MS following automated SPE separation. The tissue samples were selected from one USTUR case, representing tissues/organs that are important for internal radiation assessment. Another uranium isotope, 235U, was also measured in the samples. The results for 235U were compared with those obtained by alpha spectrometry. For most cases, results from the two methods were comparable, indicating that the measurement of 236U in the samples is reliable.

Achievements and current activities of the Canadian radon program.

Based on new scientific information and broad public consultation, the Government of Canada updated the guideline for exposure to indoor radon and launched a multi-year radon program in 2007. Major achievements accomplished in the past 3 y and current activities underway are highlighted here.

Preliminary assessment of thoron exposure in Canada

Radon has been identified as the second leading cause of lung cancer after tobacco smoking. (222)Rn (radon gas) and (220)Rn (thoron gas) are the most common isotopes of radon. In this study, thoron exposure in Canada was assessed based on three community radon/thoron surveys conducted recently. It was confirmed that thoron was detectable in most homes and thoron progeny were present in every home surveyed. Results demonstrated that thoron concentrations varied more widely than radon. No clear correlation between (222)Rn and (220)Rn concentrations was observed in simultaneous measurements. It is estimated that thoron contributes to about 7 % of the radiation dose due to indoor radon exposure based on measurements in about 260 individual homes. Because indoor measurements and geological gamma-ray surveys did not support a reasonable association between (222)Rn and (220)Rn, thoron concentrations could not be predicted from widely available indoor radon information. In order to better assess thoron exposure in Canada and thoron risk to the Canadian population in various geographic locations, more thoron progeny measurements are required.

An updated assessment of radon exposure in Canada.

Based on data from a national residential radon survey performed in 18 cities in Canada in the 1970s, an annual effective dose to the Canadian population due to indoor radon exposure was estimated at 0.71 mSv. An updated estimate of radon exposure in Canada has been made using additional indoor radon data from recent surveys in Ontario and Nova Scotia, and in 28 communities of British Columbia and 15 regions of Quebec. The associated annual effective dose to the Canadian population is now estimated to be 1.15 mSv. The percentage of homes in Canada with radon concentrations above the Canadian Radon Guideline of 200 Bq m(-3) is estimated to be about 3.3 %. As might be expected, this number varies significantly (from a low of 1 % of homes above the Guideline to a high of 19 %) from region to region. Because more radon data are included in the current assessment, and the data set covers broader geographical areas, the current assessment better represents the radon exposure in Canada.

Soil radon measurements in the Canadian cities

Radon has been identified as the second leading cause of lung cancer after tobacco smoking. Information on indoor radon concentrations is required to assess the lung cancer burden due to radon exposure. Since radon in soil is believed to be the main source of radon in homes, measurements of soil gas radon concentrations can be used to estimate variations in radon potential of indoor environments. This study reports surveys of natural background variation in soil radon levels in four cities, Montreal, Gatineau, Kingston and the largest Canadian city of Toronto. A total of 212 sites were surveyed. The average soil gas radon concentrations varied significantly from site to site, and ranged from below detection limit to 157 kBq m(-3). For each site, the soil radon potential (SRP) index was determined with the average soil radon concentration and average soil permeability measured. The average SRP indexes are 20±16, 12±11, 8±9 and 12±10 for Montreal, Gatineau, Kingston and Toronto, respectively. The results provide additional data for the validation of an association between indoor and soil radon potentials and for the development of radon potential map of Canada.

A study on the thoron sensitivity of radon detectors available to Canadians

Radon and its decay products have been identified as the second leading cause of lung cancer after tobacco smoking. Thoron is an isotope of radon. With increased awareness of radon, questions related to thoron are arising from the public. Currently, only radon detectors are commonly available to Canadian homeowners. A study on the thoron sensitivities of those radon detectors was undertaken. The average thoron sensitivity relative to radon varied from a factor of 0.012 to 0.74 for the five commonly available types of alpha track radon detectors. The potential impact of thoron sensitivity on radon test results is discussed.