Radon (Rn²²²) is a colourless, odourless radioactive gas that occurs naturally in our environment. It is a product of the natural radioactive decay of uranium (U²³⁸) found as a trace element in most rocks, soil and water. Health Canada's fact page on radon provides a health-oriented perspective of radon gas.

For radon gas in the home, Health Canada considers 800 Bq/m³ to be the limit above which remedial action is recommended. In the US, the E.P.A. specifies an upper limit of 150 Bq/m³.

Radon risk evaluation includes consideration of the following issues:

• source (material capable of producing radon)
• transportation (means by which radon can travel from source into a dwelling)
• trap (restricted ventilation, which allows radon accumulation)

Factors that control the distribution and movement of radon and typical entry routes into a home for the radon. larger image [JPEG, 64.2 kb, 800 X 709, notice]

Health Canada & CMHC publishes Radon: A guide for Canadian homeowners that describes the health effects of radon, how to detect it and ways to reduce exposure to radon in the home.

Airborne gamma-ray spectrometry can be used to evaluate the first factor (source - material capable of producing radon), by identifying uranium-bearing materials at the earth's surface, which may generate radon. The technique provides a positive test, whereby high U values may indicate an increased risk of overexposure to radon. Rocks and soils, such as certain types of clays, with low to moderate U values may still produce high residential radon if factors controlling the transport or collection of radon are dominant (Grasty, 1994). The airborne data is best applied to groups of homes, or areas, rather than to individual sites. Although home-specific information is not provided by an airborne survey, it offers relatively inexpensive, systematic, and fast determination of radon source potential over a large area, even where no development has occurred.

Since 1967, through its National Gamma-Ray Spectrometry Program (NATGAM), the Geological Survey of Canada (GSC) has been mapping natural and anthropogenic radioactivity, using aerial and field-portable gamma-ray spectrometers. Data from this program accurately depict variations in concentrations of potassium, uranium, thorium and other radioactive elements in the ground, as an aid to geological mapping, mineral exploration, environmental radiation monitoring and land-use planning.

Studies in Canada (Letourneau et al. 1984; Cocksedge et al. 1993; Jackson 1992; Doyle et al. 1990), the United States (Otton et al. 1995), Sweden (Akerblom 1995) and Great Britain (Ball et al. 1995) have shown that the ground concentration of uranium determined using an airborne gamma-ray spectrometer provides a qualitative, first-order approximation of regional variation in indoor radon levels and can be used to identify and outline high risk areas.

While radon measurements in individual homes is the most direct method to determine the risk of overexposure to radon there is little predictive ability especially for areas where there is no development. Through quantitative determination of radioactive elements in geological materials at the earth's surface, airborne gamma-ray spectrometry in conjunction with other geoscientific information, offers an efficient technique for detection and delineation of high radon risk potential over large areas. In exceptional cases, such as Oka, Quebec, where airborne eU and in-home radon values show a strong association, the method provides a direct indication of radon risk and could be used as a stand-alone technique.

Several studies of radon have been undertaken in Canada, at the national, regional and local scale.

Native communities

In Canada, airborne gamma-ray spectrometry data from the GSC's NATGAM archives have been used to evaluate risk of overexposure to radon at national, regional and local scales. Cocksedge et al (1993) reported results of a Canada-wide study conducted for Health and Welfare Canada, to assess the potential risks from radon in native homes. To test the usefulness of regional geoscience data, the Geological Survey of Canada used airborne gamma-ray spectrometry, bedrock and surficial geology and geochemistry to evaluate radon potential. This resulted in the selection of 41 native communities that were likely to have high radon potential and 16 communities with low potential. Subsequent testing revealed that no homes in communities with low radon potential exceeded 400 Bq/m3 (half the Canadian guideline of 800 Bq/m3) whereas 42 out of 740 homes (5.6%) in high potential communities exceeded this level. Follow-up was initiated to measure all homes in communities where the probability of finding a home with radon levels above 800 Bq/m3 was estimated to be more than one percent.

Nova Scotia

In the Province of Nova Scotia, Jackson (1992) demonstrated good correlation between the percentage of homes in selected communities with average radon concentrations above 74, 148 and 370 Bq/m³ (2, 4 and 10 pCi/L) and corresponding average equivalent uranium (eU) concentrations determined by airborne gamma-ray spectrometry. Jackson concluded that the airborne gamma-ray spectrometry data would be of value to both government agencies and land developers.

Quebec

Doyle et al. (1990) selected two communities in Quebec (Ste. Agathe and Maniwaki) with different eU characteristics measured by airborne gamma-ray spectrometry for comparison of their indoor radon concentrations. They concluded that indoor radon variations essentially paralleled the eU patterns such that the lowest mean radon concentrations were associated with the lowest mean eU values, and highest mean radon concentrations were associated with the highest mean eU values.

Oka, Quebec

In 1995-96 the Direction régionale de la santé publique des Laurentides (DRSP) conducted a radon measurement program near Oka, Quebec. The DRSP used previously known radon distributions and geological information to define three zones of decreasing risk based on proximity to the known extent of a carbonatite intrusion. Results showed that a higher than expected proportion of houses exceeded 800 Bq/m3 but failed to adequately define the boundaries of all the areas at risk. In 1996 the Geological Survey of Canada (GSC) and the DRSP supported a detailed, 200m line spaced airborne gamma-ray spectrometry survey to further assess the risk of overexposure to residential radon and to accurately define the geographic extent of high-risk areas. An analysis of the airborne gamma-ray spectrometry data and the radon measurements revealed some exceptional associations between increasing eU concentrations and the percentage of homes with increasing radon concentrations.

Equivalent uranium patterns from a 200m line spaced airborne gamma-ray spectrometry survey of the Oka, Quebec area. Scale provided by 1 km UTM grid. larger image [GIF, 354.0 kb, 610 X 700, notice]

The clear association between high eU concentrations and high radon in homes supported a number of land-use and public health initiatives. These included a recommendation by public health officials that the municipality apply, in its construction by-laws, the Canadian Building Code's Addendum regarding radon gas for its territory, and withhold building permits for certain particularly affected areas. Health authorities offered free radon sampling to homeowners in the affected areas not covered in the initial testing.

St-André-d'Argenteuil, Quebec

A subsequent study was initiated over a similar alkaline intrusion in the St-André-d'Argenteuil area 20km west of Oka, where a second airborne gamma-ray spectrometry survey was also completed. A program to test well waters for the presence of uranium in Oka and St-André-d'Argenteuil was also initiated. Given the fact that the situation in Oka was extraordinary and unique, a program of financial assistance from the Quebec government was recently approved to help homeowners in the affected areas to apply mitigation techniques to their homes. This program includes additional radon measurements, after the application of mitigation techniques, to confirm their effectiveness.

Akerblom, G. 1995.
The Use of Airborne Radiometric and Exploration Survey Data and Techniques in Radon Risk Mapping in Sweden. Application of Uranium Exploration Data and Techniques in Environmental Studies, International Atomic Energy Agency Technical Document 827, pp. 159-180.

Ball, T.K., Cameron, D.G., Colman, T.B. and Roberts, P.D. 1995.
The Use of Uranium Exploration Data for Mapping Radon Potential in the UK - Advantages and Pitfalls. Application of Uranium Exploration Data and Techniques in Environmental Studies, International Atomic Energy Agency Technical Document 827, pp. 139-149.

Cocksedge, W., Rankin, W., Tostowaryk, K., Charbonneau, B.W. and Grasty, R.L. 1993.
National Native Home Radon Survey - Maximizing Resources through Radon Potential Assessment. Proceedings of the 26^th Midyear Topical Meeting of the Health Physics Society, Environmental Health Physics, January 24^th-28^th, 1993, Lake Coeur d'Alene, Idaho, pp. 391-402.

Doyle, P.J., Grasty, R.L. and Charbonneau, B.W. 1990.
Predicting geographic variations in indoor radon using airborne gamma-ray spectrometry. Current Research, Part A, Geological Survey of Canada, Paper 90-1A, pp. 27-32.

Ford, K.L., Savard, M., Dessau, J.-C. and Pellerin, E., 2001.
The role of gamma-ray spectrometry in radon risk evaluation: A case history from Oka, Quebec. Geoscience Canada, Volume 28, Number 2.

Geological Survey of Canada, 1996.
Airborne Geophysical Survey, Oka, Québec. Open File 3417.

Gold, D.F., Vallée, M. and Charette, P. 1967.
Economic geology and geophysics of the Oka alkaline complex, Quebec, in Canadian Mining and Metallurgical Bulletin, Vol. LXX, pp. 245-258.

Grasty, R.L. 1994.
Summer outdoor radon variations in Canada and their relation to soil moisture. Health Physics, Vol. 66, No. 2, pp. 185-193.

Jackson, S.A. 1992.
Estimating Radon Potential from an Aerial Radiometric Survey. Health Physics, The Radiation Protection Journal, Vol. 62, No. 5, pp. 450-452.

Letourneau, E.G., McGregor, R.G and Walker, W.B. 1984.
Design and Interpretation of Large Surveys for Indoor Exposure to Radon Daughters. Radiation Protection Dosimetry, Vol. 7, No. 1-4, pp. 303-308.

Otton, J.K., Gundersen, L.C.S., Schumann, R.R., Reimer, G.M. and Duval, J.S. 1995.
Uranium Resource Assessment and Exploration Data for Geologic Radon Assessments in the United States. Application of Uranium Exploration Data and Techniques in Environmental Studies, International Atomic Energy Agency Technical Document 827, pp. 135-137.

Savard, M., Dessau, J.-C., Pellerin, E. 1998.
Le Radon à Oka - Rapport d'intervention de santé publique. Direction régionale de la santé publique des Laurentides, 134p., ISBN 2-921581-83-3.

• Indoor radon (Geoscape Ottawa-Gatineau)
• Radon gas: Beware of the stuffy basement (Geoscape Whitehorse)
• Radon in Earth, Air and Water (United States Geological Survey)
• The Geology of Radon (United States Geological Survey)
• High-Radon Project (E.O. Lawrence Berkeley National Laboratory)
• Radon links (Idaho State University Physics Department)
• Indoor Air - Radon (Rn) (US Environmental Protection Agency)