How to ensure that national radon survey results are useful for public health practice.

Henderson, Sarah B. ; Kosatsky, Tom ; Barn, Prabjit 等

Radon gas is a radioactive decay product of naturally occurring uranium. It is found in bedrock, and it enters soil and water under a variety of geological conditions. Radon penetrates indoors through building foundations, with the infiltration rate increasing as outdoor temperatures decrease (because the warmer indoor environment creates a pressure differential that draws soil gas into the building). Factors affecting the concentration of radon in a specific building include its construction (especially the design and material used for the foundation), heating and ventilation, and underlying geology and soil structure. However, there is no reliable way to assess the radon concentration in any given building without measuring it over several months. (1)

Radon has a half-life of 3.8 days, with each atom emitting three alpha particles (the most damaging type of radiation for cells) as it decays into non-radioactive lead. (2) Occupational and residential exposure to radon gas has been associated with lung cancer, (3,4) and estimates suggest that radon is a factor in 10-15% of all lung cancer cases in North America. (5) As with all environmental carcinogens, there is no threshold below which radon is considered to be safe. Current estimates suggest that the risk of lung cancer increases by 8-16% for each 100 Bq/[m.sup.3] increase in long-term concentration (6) (where one Bequerel indicates one radioactive decay per second).

In Canada, the Federal-Provincial-Territorial Radiation Protection Committee first established guidelines for exposure to residential radon in 1988. In 2007, Heath Canada revised the initial long-term concentration guideline of 800 Bq/[m.sup.3] downward to 200 Bq/[m.sup.3], with the recommendation that any dwelling over 600 Bq/[m.sup.3] should be remediated to the lowest practicable concentration within one year. (7) Furthermore, the new guidelines suggest that the most urgent action should be taken for buildings with the highest concentrations. In 2009, Health Canada began a nationwide survey of radon concentrations in 18,000 homes to better characterize the distribution of exposures in the Canadian population. Samples were taken from 121 administrative health regions across the country, each encompassing a large geographic area and multiple distinct communities. An interim report was released to stakeholders in December 2010 (8) wherein 6,474 measurements were summarized across the health regions using three concentration categories (0<200, 200<600 and 600+ Bq/[m.sup.3]). The document gives a good overview of residential radon concentrations throughout Canada, but its adherence to the spatial sampling framework and its broad categorization of measured concentrations suppress the community-scale information that would be most valuable to public health authorities.

To illustrate this loss of information, we take advantage of data from a 1991-1992 survey of BC residences conducted by the British Columbia Centre for Disease Control (BCCDC) and the University of British Columbia using Alpha Track passive samplers. Although measurement technology has been updated in the past 20 years, radon concentrations are stable over time. (9) Sampling included the main floors of 988 homes in southern BC, and was statistically designed to represent populations living in three categories of terrestrial radiation, with oversampling at the highest expected concentrations. Measured concentrations ranged from 0 to 1650 Bq/[m.sup.3], and 90% were lower than the current 200 Bq/[m.sup.3] guideline. In comparison, the Health Canada interim report included 433 samples for the same geographic area, with 91% of measurements lower than 200 Bq/[m.sup.3]. (8)

In the first step, we summarize the measurements by aggregating them to 11 of the 121 health regions sampled by Health Canada, and dividing them into the same three concentration categories (Table 1). In the second step, we take the data further by visualizing the values on a map of southern British Columbia (Figure 1). The spatial aggregation of measurements makes it impossible to identify which communities were sampled and, therefore, suppresses the specific locations of the high concentrations. In the third step, we address this by mapping the same concentration categories for all 22 communities included in the survey (Figure 2). Although the validity of the summary statistics has been decreased by disregarding the sampling framework, the practical value of the information has been increased because we can see which communities were sampled and where the high values were found. In the fourth step, we add another layer of information by expanding the summary to seven concentration categories (<100, 100<200, 200<400, 400<600, 600<800, 800<1000 and 1000+ Bq/[m.sup.3]) that cover a fuller range of measured values (Figure 3). It is now clear that concentrations were <100 Bq/[m.sup.3] throughout the coastal area, and that some parts of the interior had measurements spanning the entire distribution, with several homes greater than 1000 Bq/[m.sup.3].

There are two principal approaches to lowering radon exposures across Canada. The first is to lower the population exposure through construction practices that reduce indoor radon concentrations. This objective is already reflected in the 2010 revision of the National Building Code of Canada, which recommends measures that limit soil gas intrusion into new homes, schools and workplaces. However, each province and territory legislates its own building code, adopting specific sections of the national code (with the flexibility to specify regional exemptions) as deemed appropriate. Provincial and regional authorities need spatially explicit information about high radon concentrations to help identify areas where adoption of the new provisions should be prioritized, especially where they may be facing pressure from stakeholders to implement exemptions. Conversely, those regions where existing concentrations are well below the guideline values can also be identified. For example, Figure 3 indicates that coastal areas of southern British Columbia would remain a low priority even if Health Canada revised its guideline value to the World Health Organization minimum recommendation of 100 Bq/[m.sup.3]. (10) The second approach is to identify and remediate buildings with high radon concentrations, which requires long-term measurements in at-risk communities. Local health authorities have direct contact with their constituents and, therefore, the opportunity to actively encourage and pursue testing in homes, schools and workplaces. Although Health Canada recommends that all homes should be tested, practicality dictates that limited resources should be used to target areas where the highest concentrations are expected. Any spatially explicit information from existing surveys can inform such initiatives.

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The latter is especially important because building owners bear the cost of radon remediation in Canada, meaning that we must ultimately rely on them to achieve national reduction objectives. It is easier for public health authorities to encourage testing (and subsequent remediation) if building owners believe that their property and its inhabitants are at elevated risk. A 2001 study in Winnipeg reported that homeowners were unlikely to act on radon concentrations less than 1100 Bq/[m.sup.3]. (11) However, the same group was willing to pay an average of $221 per each 100 Bq/[m.sup.3] reduction of concentrations greater than 702 Bq/[m.sup.3] (the guideline value was 800 Bq/[m.sup.3] at the time) after receiving information about the health risks. Although no similar study has been conducted since the implementation of the 200 Bq/[m.sup.3] guideline, we should assume that building owners will continue to use relevant risk information when making decisions about radon testing and remediation.

We have demonstrated how the spatial aggregation and broad categorization of household radon measurements suppress the community-level information that public health authorities need to help lower radon exposures in Canada. We strongly encourage Health Canada to release more spatially explicit maps (similar to Figure 3) of the national radon survey results. Although the validity of the summary statistics will be somewhat decreased, the practical value of information from this rich dataset will be markedly increased.

Acknowledgements: The data used to illustrate this commentary were collected by David Morley (BCCDC) and Chris Van Netten (UBC). We feel fortunate to have this rich dataset available to address questions related to residential radon in British Columbia.

Conflict of Interest: None to declare.

Received: August 11, 2011

Accepted: December 18, 2011

REFERENCES

(1.) Health Canada. Guide for Radon Measurements in Residential Dwelllings (Homes). Ottawa, ON: Health Canada, 2008;15.

(2.) Agency for Toxic Substances and Disease Registry. Draft Toxicological Profile for Radon. Atlanta, GA: US Department of Health and Human Services, 2008;230.

(3.) Samet JM. Radon and lung cancer. J National Cancer Institute 1989;81(10):745-58.

(4.) Al-Zoughool M, Krewski D. Health effects of radon: A review of the literature. Int J Radiation Biol 2009;85(1):57-69.

(5.) National Research Council Committee on Health Risks of Exposure to Radon (BEIR VI). Health Effects of Exposure to Radon. Committee on the Biological Effects of Ionizing Radiations, Board of Radiation Effects Research, Committee on Life Sciences, National Research Council. Washington, DC: National Academy Press, 1999.

(6.) Darby S, Hill D, Deo H, Auvinen A, Barros-Dios J, Baysson H, et al. Residential radon and lung cancer--detailed results of a collaborative analysis of individual data on 7148 persons with lung cancer and 14,208 persons without lung cancer from 13 epidemiologic studies in Europe. Scand J Work Environ Health 2006;32(S1):1-83.

(7.) Health Canada. Government of Canada Radon Guideline. 2009. Available at: http://www.hc-sc.gc.ca/ewh-semt/radiation/ radon/guidelines_lignes_directrice-eng.php (Accessed August 10, 2011).

(8.) Health Canada. Cross-Canada Survey of Radon Concentrations in Homes: Year 1 Interim Report. Ottawa: Health Canada, 2010;12.

(9.) Miles J. Temporal variation of radon levels in houses and implications for radon measurement strategies. Radiation Protection Dosimetry 2001;93(4):369 75.

(10.) World Health Organization. WHO Handbook on Indoor Radon: A Public Health Perspective. Geneva, Switzerland: WHO, 2009;94.

(11.) Spiegel JM, Krewski D. Using willingness to pay to evaluate the implementation of Canada's residential radon exposure guideline. Can J Public Health 2002;93(3):223-28.

Authors' Affiliation

Environmental Health Services, British Columbia Centre for Disease Control, Vancouver, BC

Correspondence: Sarah Henderson, BCCDC, 655 West 12th Avenue, Vancouver, BC V5Z 4R4, E-mail: sarah.henderson@bccdc.ca

Table 1. Summary of Main Floor Radon Measurements From 988 Homes in
the British Columbia Centre for Disease Control /University of British
Columbia Survey

Health Region                   Number of Homes   % Below 200
                                                  Bq/[m.sup.3]

South Vancouver Island                62              100
Fraser South                          22              100
Vancouver                             19              100
Richmond                              24              100
North Shore / Coast Garabaldi         94              100
Fraser North / Simon Fraser           36              100
Fraser East / Fraser Valley            3              100
Thompson / Cariboo                   176               79.5
Okanagan                             197               93.4
Kootenay-Boundary                    207               81.7
East Kootenay                        148               92.6

Health Region                   % 200 to 600   % Above 600
                                Bq/[m.sup.3]   Bq/[m.sup.3]

South Vancouver Island               0              0
Fraser South                         0              0
Vancouver                            0              0
Richmond                             0              0
North Shore / Coast Garabaldi        0              0
Fraser North / Simon Fraser          0              0
Fraser East / Fraser Valley          0              0
Thompson / Cariboo                  14.8            5.7
Okanagan                             5.6            1.0
Kootenay-Boundary                   13.0            5.3
East Kootenay                        6.8            0.6

Note: The results have been summarized in the same format used by
Health Canada for 11 of the same health regions.