Radiation Protection Dosimetry Advance Access published May 2, 2015 Radiation Protection Dosimetry (2015), pp. 1–5
doi:10.1093/rpd/ncv218
AN OVERVIEW OF RADON RESEARCH IN CANADA Jing Chen1,*, Jeff Whyte1 and Ken Ford2 1 Radiation Protection Bureau, Health Canada, 775 Brookfield Road, Ottawa, Canada 2 Geological Survey of Canada, Natural Resources of Canada, 601 Booth Street, Ottawa, Canada *Corresponding author: [email protected]
INTRODUCTION Recent scientific studies have conclusively linked an increased risk of developing lung cancer to levels of radon found in homes. These studies prompted the Canadian federal government to collaborate with provincial and territorial governments to review the Canadian radon guideline. Based on new scientific information and a broad public consultation, the guideline was lowered from 800 to 200 Bq m23 in June 2007(1). In addition to residential homes, the new guideline also applies to public buildings with a high occupancy rate by members of the public, and workplace exposure limits were harmonised with this new guideline as shown in the newly revised Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials(2). The new radon guideline recommends techniques be employed in new home construction to minimise radon entry and facilitate post-construction radon reduction, should this subsequently prove necessary. The revised radon guideline provides advice that is more broadly applicable and more protective than the previous guideline. To support the implementation of the revised guideline, a national radon program (NRP) was developed. The NRP consists of five components: (1) establishment of a national radon laboratory, (2) radon testing projects, (3) radon database and mapping, (4) radon research and (5) education and public awareness. Some of the achievements accomplished in the past 7 y and on-going activities in the radon research component of the NRP are highlighted here. SOIL RADON AND GROUND NATURAL RADIOACTIVITY SURVEYS Radon research under the NRP started with soil gas radon surveys across Canada. From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon, as shown in Figure 1. Long-term monitoring of soil radon variations was conducted at two reference sites in Ottawa(3), from thawing of the ground in the late # Crown copyright 2015.
spring to 1 d before the first snowfall in early winter. Results showed that during the normal field survey period from June to September in Canada, a single field survey with multiple measurements of soil gas radon concentrations at a depth of 80 cm could characterise the soil gas radon level of a site within a deviation of +30 %. During the 4 y of soil radon surveys, multiple in situ soil gas radon and soil permeability measurements were performed to characterise soil gas radon concentrations and radon availability for each site. The average soil gas radon concentrations varied significantly from site to site and ranged from below detection limit to several hundreds of kBq m23. Among the sites surveyed, 476 sites were in cities (represented as black dots in Figure 1). In the 2010 soil radon survey in four cities(4), the soil radon potential index was determined to be 20+16, 12+11, 8+9 and 12+10 for Montreal, Gatineau, Kinston and Toronto, respectively. Currently, studies are underway to establish models or correlations between measured indoor radon concentration and geoscience data, including data for soil gas radon, soil permeability, airborne gamma-ray surveys, surficial geology and bedrock geology, in various geological zones. Soil gas radon surveying is limited by road accessibility of individual sites. Canada is a large country with a relatively small population spread; there are large areas that are inaccessible to this type of surveying. Therefore, airborne gamma-ray spectrometric (AGRS) surveys for natural ground radioactivity had become a key contributor to the mapping component of the NRP. In Canada, most publically available AGRS data were collected between 1970 and 2007 to support geological mapping and mineral exploration in areas of high mineral potential. Therefore, prior to the NRP, there was very little coverage in populated areas. The existing data covered about 2 530 000 km2 or 28 % of Canada’s landmass. Most coverage was flown with reconnaissance of 5-km line spacing. As an important component of the NRP, AGRS surveying started in 2008 and continued to 2010. The AGRS surveys added over 200 000 line kilometres of
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
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 programme in 2007. Major achievements in radon research accomplished in the past 7 y are highlighted here.
J. CHEN ET AL.
new data covering an additional 725 000 km2 in more densely populated regions, as shown in Figure 2 with circled areas.
RADON MEASUREMENTS OF BUILDING MATERIALS Radon measurements of building materials started with the determination of radon diffusion coefficients for vapour barrier membranes used in Canadian building construction(5). Ten commonly used vapour barrier membranes were tested. The results indicate that all of the tested membranes can serve as a barrier against soil gas radon. In general, membranes of higher density display better radon proof properties. Since it is believed that the prevention of radon from entering new dwellings is one of the most effective ways to reduce population burden from radon exposure, the most recent edition of the National Building Code of Canada (NBC)(6) requires that all new homes in Canada be built with an aggregate layer and a sealed vapour barrier under the slab, plus a capped rough-in pipe set through the slab to make the home ready for active sub-slab depressurisation, if required. With the implementation of the revised NBC, the percentage of homes above the guideline value is expected to decline in the future, even though more
and more new constructions are expected to enter the market every year. Radon exhalation rates were also determined for different types of building materials, such as various aggregates, drywalls, tiles and granites available on the Canadian market (7). The radon exhalation rates ranged from non-detectable to 312 Bq m22 d21. Slate tiles and granite slabs have relatively higher radon exhalation rates than other decorative materials, such as ceramic or porcelain tiles. The average radon exhalation rates were 30 Bq m22 d21 for slate tiles and 42 Bq m22 d21 for granite slabs of various types and origins. Generally speaking, building materials used in home construction or decoration make no significant contribution to indoor radon for a house with adequate air exchange. RADON TESTING Before starting the testing, radon measurement protocols were developed for homes(8) and public buildings(9). The major radon testing activity started with the testing of federal buildings. Up to now, .15 000 buildings have been tested for radon. Roughly 3.5 % of buildings tested have an average radon value of .200 Bq m23 and have been found in all areas of Canada. To gain a better understanding of radon concentrations in homes across Canada, a national residential
Page 2 of 5
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
Figure 1. Activities of soil gas radon survey in 2007–2010. Black dots represent locations of the cities surveyed.
RADON RESEARCH IN CANADA
radon survey was launched in April 2009. The survey used alpha track detectors deployed for a minimum of 3 months during the heating season (October– April) with the objective of testing 18 000 homes in all 127 health regions in Canada(10). The 18 000 target homes were roughly uniformly distributed in all health regions with more sampling occurring in northern communities and in regions where radon concentrations were yet unknown. The survey was completed in 2011 with a return rate of 77 %. Longterm radon results were obtained from a total of 13 814 homes. It showed that 7 % of Canadians are living in homes with radon concentration of .200 Bq m23. The highest indoor radon result obtained from this survey was 5600 Bq m23. The populationweighted AM concentration of radon and the standard deviation were 72 and 95 Bq m23, respectively. RESEARCH RELATED TO THE BUILDING ENVELOPE Health Canada in collaboration with its partner the National Research Council (NRC) has begun to focus
radon research on ways of minimising radon ingress into new homes and buildings and the effectiveness of mitigation measures. This includes studying the risk of radon re-entrainment from active soil depressurisation systems (ASD) having a variety of exhaust point locations, risk of combustion appliance back-drafting from radon mitigation systems and energy consumption of ASD systems. The degree of leak-tightness of ASD fans is another area of research that will be studied in the near future with NRC. Currently, Health Canada is conducting a field study on ASD systems having indoor mounted ASD fans with sidewall discharge near ground level. The goal of this study is to measure long-term indoor radon levels in roughly 50 homes to further verify that this geometry reduces indoor radon levels efficiently. In a subset of these homes, outdoor radon measurements will also be made to measure how quickly radon concentrations in the ASD exhaust stream dissipate to background levels. Some of this research has provided data to support two national radon mitigation standards (one for new construction, and one for existing construction) currently being developed in conjunction
Page 3 of 5
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
Figure 2. Equivalent uranium ( ppm) map of Canada compiled from airborne gamma-ray spectrometry surveys flown between 1969 and 2011. New data acquired by the GSC as a contribution to Health Canada’s new Radon Strategy for Canada indicated by circled areas. This compilation grid and all data for individual surveys are available from Natural Resources Canada’s Geoscience Data Repository for Geophysical and Geochemical Data at http://gdr.agg.nrcan.gc.ca/ geodap/home/Default.aspx?lang=e.
J. CHEN ET AL.
Hirosaki University, results of the Feq determined from those long-term measurements are expected in near future.
RESEARCH IN RADON DOSIMETRY
THORON RESEARCH
Radon gas itself delivers a relatively small dose to the human airways. It is the inhalation of the short-lived solid radon decay products and their subsequent deposition to the walls of the airways of the bronchial tree that provide the main radiation dose to human lungs. To assess radon dose to the population, accurate radon progeny measurements are required. Although radon decay products, primarily 218Po and 214 Po, are responsible for most of the radiation dose, radon gas concentration is generally considered a good surrogate for the radon progeny concentration. Radon gas measurements are usually preferred to radon progeny measurements because of their relative simplicity and cost-effectiveness(11). Radon progeny concentrations are often estimated from considerations of equilibrium or disequilibrium between radon and its progeny. An equilibrium factor, Feq, is then defined as the ratio of the radon progeny equilibrium equivalent concentration (EEC) to the radon gas concentration. The radon gas concentration times the Feq determines the EEC, which can be used to directly determine the bronchial dose. In Canada, a radon and radon progeny survey was carried out in the 1970s in 19 cities(12). To the authors’ knowledge, this is the only large survey of simultaneous radon and radon progeny measurements up to the present time. From the summary results of this large simultaneous radon and radon progeny survey, the characteristics of radon equilibrium factor were assessed(13). The average Feq assessed from this survey in 12 576 houses was 0.54; however, city-wide mean Feq values ranged from 0.20 to 0.82. The assessment may indicate that the typical Feq value of 0.4 recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)(14) and the International Commission on Radiological Protection (ICRP)(15) could lead to a downward bias in the estimation of radon doses to the lung. Indoor radon equilibrium factors were determined from long-term measurements (77 –162 d) in two laboratories and six residential houses(16). The arithmetic mean Feq value was 0.75 with a range from 0.59 to 0.95. The results suggest that, at least in some Canadian indoor environments, the radon dose would be about twice as high as normally estimated from the conventional so-called typical Feq value of 0.4. Because of the importance of the Feq in radon dosimetry and its large variation observed in limited studies, .300 radon progeny detectors were deployed side-by-side with radon detectors in the 2013 radon/ thoron survey. A total of 242 progeny detectors were returned for analysis. Jointly with scientists from
Research in thoron exposure started at the very beginning of the NRP. The first simultaneous radon and thoron measurements were conducted in 93 homes in the national capital area(17). This preliminary survey showed that thoron is present in most of the Ottawa homes surveyed. Both radon and thoron concentrations follow log–normal distributions. In order to assess the thoron contribution to the combined indoor radon and thoron exposure, a survey of residential radon and thoron concentrations was designed for 4000 homes in all 33 census metropolitan areas specified by Statistics Canada, which cover 70 % of the Canadian population. As in the previous residential radon survey, the radon/thoron survey followed the procedure outlined in Health Canada’s guide for radon measurements in residential dwellings(6). To determine the concentrations of radon and thoron, a passive integrated radon –thoron discriminative detector developed at the National Institute of Radiological Sciences in Japan (commercially known as RADUET) was used in this survey. The detector analysis system was calibrated by exposing RADUETs to three different known radon and thoron concentrations at the National Institute of Radiological Sciences and Hirosaki University in Japan. The radon/thoron survey was completed in 2013 and had a return rate of 79 %. A total of 3215 test results were reported directly to the survey participants. Radon was present in all homes in varying concentrations with the highest measured concentration of 2100 Bq m23. The population-weighted AM concentration of radon and the standard deviation were 96 and 87 Bq m23, respectively. On average ( population weighted), 48 % of homes surveyed had thoron concentration below the detection limit. The population-weighted AM concentration of thoron and the standard deviation were 9 and 11 Bq m23, respectively. It was estimated that thoron contributes 3 % of the radiation dose due to indoor radon and thoron exposure in Canada(18). CONCLUSIONS Exposure to indoor radon has been determined to be the second leading cause of lung cancer after tobacco smoking. The recently completed national radon survey provided better understanding of radon concentrations in homes across Canada. 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(19). It is
Page 4 of 5
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with the Canadian General Standards Board. These two national standards are expected to be completed sometime in 2015.
RADON RESEARCH IN CANADA
REFERENCES
9.
10.
11.
12.
13.
14.
1. Government Canada radon guideline (2009). Available on http://www.hc-sc.gc.ca/ewh-semt/radiation/radon/ guidelines_lignes_directrice-eng.php. (1 September 2014, date last accessed). 2. Canadian guidelines for the management of naturally occurring radioactive materials (NORM) (2014). Available on http://www.hc-sc.gc.ca/ewh-semt/alt_formats/pdf/ pubs/contaminants/norm-mrn/norm-mrn-eng.pdf. (1 September 2014, date last accessed). 3. Chen, J., Falcomer, R., Ly, J., Wierdsma, J. and Bergman, L. Long-term monitoring of soil gas radon and permeability at two reference sites. Radiat. Prot. Dosim. 131, 503– 508 (2009). 4. Chen, J., Moir, D., MacLellan, K., Leigh, E., Nunez, D., Murphy, S. and Ford, K. Soil radon measurements in the Canadian cities. Radiat. Prot. Dosim. 151, 172–174 (2012). 5. Chen, J., Ly, J., Schroth, E., Hnatiuk, S., Frenette, E. and Blain, M. F. Radon diffusion coefficients of vapourbarrier membranes used in Canadian building construction. Radiat. Environ. Biophys. 48, 153–158 (2009). 6. National Reseach Council of Canada. National Building Code of Canada, 13th edn. (2010). 7. Chem, J., Rahman, N. M. and Atiya, I. A. Radon exhalation from building materials for decorative use. J. Environ. Radioact. 101, 317–322 (2010). 8. Health Canada. Guide for radon measurements in residential dwellings (homes) (2008). Available on http:// www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/
15. 16. 17. 18.
19.
20.
21.
pubs/radiation/radon_homes-maisons/radon_homesmaisons-eng.pdf. (1 September 2014, date last accessed). Health Canada. Guide for radon measurements in public buildings (2008). Available on http://www.hc-sc.gc.ca/ ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/ radon_building-edifices/radon_building-edifices-eng.pdf. (1 September 2014, date last accessed). Health Canada. Cross-Canada survey of radon concentrations in homes (2012). Available on http://www.hc-sc.gc. ca/ewh-semt/alt_formats/pdf/radiation/radon/surveysondage-eng.pdf. (1 September 2014, date last accessed). World Health Organization. WHO Handbook on Indoor Radon (2009). Available on http://whqlibdoc. who.int/publications/2009/9789241547673_eng.pdf. (1 September 2014, date last accessed). Letourneau, E. G., McGregor, R. G. and Walker, W. B. Design and interpretation of large surveys for indoor exposure to radon daughters. Radiat. Prot. Dosim. 7, 303–308 (1984). Chen, J. and Marro, L. Determine radon equilibrium factor from distribution parameters of simultaneous radon and radon progeny measurements. Radiat. Environ. Biophys. 50, 597–601 (2011). United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Effects of ionizing radiation. Volume I: sources-to-effects assessment for radon in homes and workplaces. United Nations, New York (2006). International Commission on Radiological Protection. Lung cancer risk from radon and progeny and statement on radon. ICRP Publication 115. Ann. ICRP 40(1), (2010). Harley, N., Chen, J., Chittaporn, P., Sorimachi, A. and Tokonami, S. Long-term measurements of indoor radon equilibrium factor. Health Phys. 102(4), 459– 462 (2012). Chen, J., Tokonami, S., Sorimachi, A., Takahashi, H. and Falcomer, R. Results of simultaneous radon and thoron tests in Ottawa. Radiat. Prot. Dosim. 130, 253–256 (2008). Chen, J., Bergman, L., Falcomer, R. and Whyte, J. Results of simultaneous radon and thoron measurements in 33 metropolitan areas of Canada. Radiat. Prot. Dosim. 163, 210– 216 (2015). Chen, J., Moir, D. and Whyte, J. Canadian population risk of radon induced lung cancer—a reassessment based on recent cross Canadian radon survey. Radiat. Prot. Dosim. 152, 9 –13 (2012). Health Canada. Radon—reduction guide for Canadians (2014). Available on http://www.hc-sc.gc.ca/ewh-semt/alt_ formats/pdf/pubs/radiation/radon_canadians-canadiens/ radon_canadians-canadien-eng.pdf. (1 September 2014, date last accessed). Health Canada. Reducing radon levels in existing homes: a Canadian guide for professional contractors (2014).
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estimated that roughly 16 % of lung cancers in Canada are related to radon exposure. The annual average dose from indoor radon and thoron exposure in Canada is estimated to be 2.5 mSv, more than double the worldwide average dose of 1.2 mSv estimated by the UNSCEAR(14). It is clear that continued efforts are needed to further reduce the exposure and effectively reduce the number of lung cancers caused by radon. Current activities of radon research are, therefore, focused on radon mitigation techniques, strategies and standards. Information on radon control was developed for homeowners(20) and radon professionals(21). A follow-up study has been initiated in order to gauge the extent to which Canadians have taken action to reduce the number of homes with radon levels above the new 200 Bq m23 radon guideline. The success of any NRP will rely on the necessary actions taken by the population.
doi:10.1093/rpd/ncv218
AN OVERVIEW OF RADON RESEARCH IN CANADA Jing Chen1,*, Jeff Whyte1 and Ken Ford2 1 Radiation Protection Bureau, Health Canada, 775 Brookfield Road, Ottawa, Canada 2 Geological Survey of Canada, Natural Resources of Canada, 601 Booth Street, Ottawa, Canada *Corresponding author: [email protected]
INTRODUCTION Recent scientific studies have conclusively linked an increased risk of developing lung cancer to levels of radon found in homes. These studies prompted the Canadian federal government to collaborate with provincial and territorial governments to review the Canadian radon guideline. Based on new scientific information and a broad public consultation, the guideline was lowered from 800 to 200 Bq m23 in June 2007(1). In addition to residential homes, the new guideline also applies to public buildings with a high occupancy rate by members of the public, and workplace exposure limits were harmonised with this new guideline as shown in the newly revised Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials(2). The new radon guideline recommends techniques be employed in new home construction to minimise radon entry and facilitate post-construction radon reduction, should this subsequently prove necessary. The revised radon guideline provides advice that is more broadly applicable and more protective than the previous guideline. To support the implementation of the revised guideline, a national radon program (NRP) was developed. The NRP consists of five components: (1) establishment of a national radon laboratory, (2) radon testing projects, (3) radon database and mapping, (4) radon research and (5) education and public awareness. Some of the achievements accomplished in the past 7 y and on-going activities in the radon research component of the NRP are highlighted here. SOIL RADON AND GROUND NATURAL RADIOACTIVITY SURVEYS Radon research under the NRP started with soil gas radon surveys across Canada. From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon, as shown in Figure 1. Long-term monitoring of soil radon variations was conducted at two reference sites in Ottawa(3), from thawing of the ground in the late # Crown copyright 2015.
spring to 1 d before the first snowfall in early winter. Results showed that during the normal field survey period from June to September in Canada, a single field survey with multiple measurements of soil gas radon concentrations at a depth of 80 cm could characterise the soil gas radon level of a site within a deviation of +30 %. During the 4 y of soil radon surveys, multiple in situ soil gas radon and soil permeability measurements were performed to characterise soil gas radon concentrations and radon availability for each site. The average soil gas radon concentrations varied significantly from site to site and ranged from below detection limit to several hundreds of kBq m23. Among the sites surveyed, 476 sites were in cities (represented as black dots in Figure 1). In the 2010 soil radon survey in four cities(4), the soil radon potential index was determined to be 20+16, 12+11, 8+9 and 12+10 for Montreal, Gatineau, Kinston and Toronto, respectively. Currently, studies are underway to establish models or correlations between measured indoor radon concentration and geoscience data, including data for soil gas radon, soil permeability, airborne gamma-ray surveys, surficial geology and bedrock geology, in various geological zones. Soil gas radon surveying is limited by road accessibility of individual sites. Canada is a large country with a relatively small population spread; there are large areas that are inaccessible to this type of surveying. Therefore, airborne gamma-ray spectrometric (AGRS) surveys for natural ground radioactivity had become a key contributor to the mapping component of the NRP. In Canada, most publically available AGRS data were collected between 1970 and 2007 to support geological mapping and mineral exploration in areas of high mineral potential. Therefore, prior to the NRP, there was very little coverage in populated areas. The existing data covered about 2 530 000 km2 or 28 % of Canada’s landmass. Most coverage was flown with reconnaissance of 5-km line spacing. As an important component of the NRP, AGRS surveying started in 2008 and continued to 2010. The AGRS surveys added over 200 000 line kilometres of
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
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 programme in 2007. Major achievements in radon research accomplished in the past 7 y are highlighted here.
J. CHEN ET AL.
new data covering an additional 725 000 km2 in more densely populated regions, as shown in Figure 2 with circled areas.
RADON MEASUREMENTS OF BUILDING MATERIALS Radon measurements of building materials started with the determination of radon diffusion coefficients for vapour barrier membranes used in Canadian building construction(5). Ten commonly used vapour barrier membranes were tested. The results indicate that all of the tested membranes can serve as a barrier against soil gas radon. In general, membranes of higher density display better radon proof properties. Since it is believed that the prevention of radon from entering new dwellings is one of the most effective ways to reduce population burden from radon exposure, the most recent edition of the National Building Code of Canada (NBC)(6) requires that all new homes in Canada be built with an aggregate layer and a sealed vapour barrier under the slab, plus a capped rough-in pipe set through the slab to make the home ready for active sub-slab depressurisation, if required. With the implementation of the revised NBC, the percentage of homes above the guideline value is expected to decline in the future, even though more
and more new constructions are expected to enter the market every year. Radon exhalation rates were also determined for different types of building materials, such as various aggregates, drywalls, tiles and granites available on the Canadian market (7). The radon exhalation rates ranged from non-detectable to 312 Bq m22 d21. Slate tiles and granite slabs have relatively higher radon exhalation rates than other decorative materials, such as ceramic or porcelain tiles. The average radon exhalation rates were 30 Bq m22 d21 for slate tiles and 42 Bq m22 d21 for granite slabs of various types and origins. Generally speaking, building materials used in home construction or decoration make no significant contribution to indoor radon for a house with adequate air exchange. RADON TESTING Before starting the testing, radon measurement protocols were developed for homes(8) and public buildings(9). The major radon testing activity started with the testing of federal buildings. Up to now, .15 000 buildings have been tested for radon. Roughly 3.5 % of buildings tested have an average radon value of .200 Bq m23 and have been found in all areas of Canada. To gain a better understanding of radon concentrations in homes across Canada, a national residential
Page 2 of 5
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
Figure 1. Activities of soil gas radon survey in 2007–2010. Black dots represent locations of the cities surveyed.
RADON RESEARCH IN CANADA
radon survey was launched in April 2009. The survey used alpha track detectors deployed for a minimum of 3 months during the heating season (October– April) with the objective of testing 18 000 homes in all 127 health regions in Canada(10). The 18 000 target homes were roughly uniformly distributed in all health regions with more sampling occurring in northern communities and in regions where radon concentrations were yet unknown. The survey was completed in 2011 with a return rate of 77 %. Longterm radon results were obtained from a total of 13 814 homes. It showed that 7 % of Canadians are living in homes with radon concentration of .200 Bq m23. The highest indoor radon result obtained from this survey was 5600 Bq m23. The populationweighted AM concentration of radon and the standard deviation were 72 and 95 Bq m23, respectively. RESEARCH RELATED TO THE BUILDING ENVELOPE Health Canada in collaboration with its partner the National Research Council (NRC) has begun to focus
radon research on ways of minimising radon ingress into new homes and buildings and the effectiveness of mitigation measures. This includes studying the risk of radon re-entrainment from active soil depressurisation systems (ASD) having a variety of exhaust point locations, risk of combustion appliance back-drafting from radon mitigation systems and energy consumption of ASD systems. The degree of leak-tightness of ASD fans is another area of research that will be studied in the near future with NRC. Currently, Health Canada is conducting a field study on ASD systems having indoor mounted ASD fans with sidewall discharge near ground level. The goal of this study is to measure long-term indoor radon levels in roughly 50 homes to further verify that this geometry reduces indoor radon levels efficiently. In a subset of these homes, outdoor radon measurements will also be made to measure how quickly radon concentrations in the ASD exhaust stream dissipate to background levels. Some of this research has provided data to support two national radon mitigation standards (one for new construction, and one for existing construction) currently being developed in conjunction
Page 3 of 5
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
Figure 2. Equivalent uranium ( ppm) map of Canada compiled from airborne gamma-ray spectrometry surveys flown between 1969 and 2011. New data acquired by the GSC as a contribution to Health Canada’s new Radon Strategy for Canada indicated by circled areas. This compilation grid and all data for individual surveys are available from Natural Resources Canada’s Geoscience Data Repository for Geophysical and Geochemical Data at http://gdr.agg.nrcan.gc.ca/ geodap/home/Default.aspx?lang=e.
J. CHEN ET AL.
Hirosaki University, results of the Feq determined from those long-term measurements are expected in near future.
RESEARCH IN RADON DOSIMETRY
THORON RESEARCH
Radon gas itself delivers a relatively small dose to the human airways. It is the inhalation of the short-lived solid radon decay products and their subsequent deposition to the walls of the airways of the bronchial tree that provide the main radiation dose to human lungs. To assess radon dose to the population, accurate radon progeny measurements are required. Although radon decay products, primarily 218Po and 214 Po, are responsible for most of the radiation dose, radon gas concentration is generally considered a good surrogate for the radon progeny concentration. Radon gas measurements are usually preferred to radon progeny measurements because of their relative simplicity and cost-effectiveness(11). Radon progeny concentrations are often estimated from considerations of equilibrium or disequilibrium between radon and its progeny. An equilibrium factor, Feq, is then defined as the ratio of the radon progeny equilibrium equivalent concentration (EEC) to the radon gas concentration. The radon gas concentration times the Feq determines the EEC, which can be used to directly determine the bronchial dose. In Canada, a radon and radon progeny survey was carried out in the 1970s in 19 cities(12). To the authors’ knowledge, this is the only large survey of simultaneous radon and radon progeny measurements up to the present time. From the summary results of this large simultaneous radon and radon progeny survey, the characteristics of radon equilibrium factor were assessed(13). The average Feq assessed from this survey in 12 576 houses was 0.54; however, city-wide mean Feq values ranged from 0.20 to 0.82. The assessment may indicate that the typical Feq value of 0.4 recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)(14) and the International Commission on Radiological Protection (ICRP)(15) could lead to a downward bias in the estimation of radon doses to the lung. Indoor radon equilibrium factors were determined from long-term measurements (77 –162 d) in two laboratories and six residential houses(16). The arithmetic mean Feq value was 0.75 with a range from 0.59 to 0.95. The results suggest that, at least in some Canadian indoor environments, the radon dose would be about twice as high as normally estimated from the conventional so-called typical Feq value of 0.4. Because of the importance of the Feq in radon dosimetry and its large variation observed in limited studies, .300 radon progeny detectors were deployed side-by-side with radon detectors in the 2013 radon/ thoron survey. A total of 242 progeny detectors were returned for analysis. Jointly with scientists from
Research in thoron exposure started at the very beginning of the NRP. The first simultaneous radon and thoron measurements were conducted in 93 homes in the national capital area(17). This preliminary survey showed that thoron is present in most of the Ottawa homes surveyed. Both radon and thoron concentrations follow log–normal distributions. In order to assess the thoron contribution to the combined indoor radon and thoron exposure, a survey of residential radon and thoron concentrations was designed for 4000 homes in all 33 census metropolitan areas specified by Statistics Canada, which cover 70 % of the Canadian population. As in the previous residential radon survey, the radon/thoron survey followed the procedure outlined in Health Canada’s guide for radon measurements in residential dwellings(6). To determine the concentrations of radon and thoron, a passive integrated radon –thoron discriminative detector developed at the National Institute of Radiological Sciences in Japan (commercially known as RADUET) was used in this survey. The detector analysis system was calibrated by exposing RADUETs to three different known radon and thoron concentrations at the National Institute of Radiological Sciences and Hirosaki University in Japan. The radon/thoron survey was completed in 2013 and had a return rate of 79 %. A total of 3215 test results were reported directly to the survey participants. Radon was present in all homes in varying concentrations with the highest measured concentration of 2100 Bq m23. The population-weighted AM concentration of radon and the standard deviation were 96 and 87 Bq m23, respectively. On average ( population weighted), 48 % of homes surveyed had thoron concentration below the detection limit. The population-weighted AM concentration of thoron and the standard deviation were 9 and 11 Bq m23, respectively. It was estimated that thoron contributes 3 % of the radiation dose due to indoor radon and thoron exposure in Canada(18). CONCLUSIONS Exposure to indoor radon has been determined to be the second leading cause of lung cancer after tobacco smoking. The recently completed national radon survey provided better understanding of radon concentrations in homes across Canada. 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(19). It is
Page 4 of 5
Downloaded from http://rpd.oxfordjournals.org/ at United Arab Emirates University on November 14, 2015
with the Canadian General Standards Board. These two national standards are expected to be completed sometime in 2015.
RADON RESEARCH IN CANADA
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1. Government Canada radon guideline (2009). Available on http://www.hc-sc.gc.ca/ewh-semt/radiation/radon/ guidelines_lignes_directrice-eng.php. (1 September 2014, date last accessed). 2. Canadian guidelines for the management of naturally occurring radioactive materials (NORM) (2014). Available on http://www.hc-sc.gc.ca/ewh-semt/alt_formats/pdf/ pubs/contaminants/norm-mrn/norm-mrn-eng.pdf. (1 September 2014, date last accessed). 3. Chen, J., Falcomer, R., Ly, J., Wierdsma, J. and Bergman, L. Long-term monitoring of soil gas radon and permeability at two reference sites. Radiat. Prot. Dosim. 131, 503– 508 (2009). 4. Chen, J., Moir, D., MacLellan, K., Leigh, E., Nunez, D., Murphy, S. and Ford, K. Soil radon measurements in the Canadian cities. Radiat. Prot. Dosim. 151, 172–174 (2012). 5. Chen, J., Ly, J., Schroth, E., Hnatiuk, S., Frenette, E. and Blain, M. F. Radon diffusion coefficients of vapourbarrier membranes used in Canadian building construction. Radiat. Environ. Biophys. 48, 153–158 (2009). 6. National Reseach Council of Canada. National Building Code of Canada, 13th edn. (2010). 7. Chem, J., Rahman, N. M. and Atiya, I. A. Radon exhalation from building materials for decorative use. J. Environ. Radioact. 101, 317–322 (2010). 8. Health Canada. Guide for radon measurements in residential dwellings (homes) (2008). Available on http:// www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/
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estimated that roughly 16 % of lung cancers in Canada are related to radon exposure. The annual average dose from indoor radon and thoron exposure in Canada is estimated to be 2.5 mSv, more than double the worldwide average dose of 1.2 mSv estimated by the UNSCEAR(14). It is clear that continued efforts are needed to further reduce the exposure and effectively reduce the number of lung cancers caused by radon. Current activities of radon research are, therefore, focused on radon mitigation techniques, strategies and standards. Information on radon control was developed for homeowners(20) and radon professionals(21). A follow-up study has been initiated in order to gauge the extent to which Canadians have taken action to reduce the number of homes with radon levels above the new 200 Bq m23 radon guideline. The success of any NRP will rely on the necessary actions taken by the population.
