Fort Smith area, NWT

Skyvan airplane

Some published reports suggest that there may be certain health risks associated with long-term exposure to very high levels of "natural background" radiation. Airborne gamma ray surveys (see photo of Skyvan airplane) provide a measure of natural background radiation coming from the earth's surface. Radiation exposure levels are expressed in units of microroentgens per hour (uR/h). The Roentgen, which has a precise definition in terms of ionizaton potential, was originally defined as the amount of radiation required to kill a mouse. It is a useful unit because it can be directly related to physical damage in living cells.

Airborne surveys by the Geological Survey of Canada indicate that there is an average radiation exposure level of 4.4 µR/h for the Canadian land surface (including bedrock outcrops and surficial deposits). United Nations reviews indicate that the average world level is about 5.2 µR/h. Areas of natural radioactivity three times greater than the world average are considered "elevated natural background areas", whereas areas with levels ten times greater than the world average are considered "high natural background areas".

The Radioactivity Map of Canada shows that large parts of the Northwest Territories have areas of elevated radioactivity whereas much of southern Canada is characterized by relatively low levels. An area of elevated radioactivity near Fort Smith (see inset maps) covers several thousand square kilometres, and is associated with Precambrian granitic bedrock. The granite, with an average radiation exposure level greater than15 µR/h, has radioelement concentrations of 6.5% potassium oxide, 10 ppm uranium, and 80 ppm thorium. Some areas of more than a hundred square kilometres have twice these concentrations of uranium and thorium. According to the above criteria, the natural background radioactivity of the granite near Fort Smith qualifies as "elevated", but not "high".

Radioactivity of the granite near Fort Smith

This material was extracted from Figure 1H: "Environmental applications of gamma ray spectrometry surveys", by B.W. Charbonneau, R.J. Hetu and J.M. Carson (Radiation Geophysics), in "Environmental geochemistry and geochemical hazards", compiled by R.D. Knight and R.A. Klassen, included in "A synthesis of geological hazards in Canada", Geological Survey of Canada Bulletin 548, edited by G.R. Brooks, 2001.