There is great concern, particularly in the United States regarding the concentration of radon inside homes. In the United States radon is believed to cause a significant number of deaths from lung cancer. However, very few measurements have been made of the radon concentration in outdoor air.
In the summer of 1990, with the co-operation of provincial and local health authorities, 150 outdoor radon detectors were installed in 31 communities across Canada for a three month period from June through August, 1990. The summer season was selected to minimize differences in environmental conditions such as temperature, snow-cover etc. which would affect outdoor radon levels. A long measurement period was chosen because of the low level of radon in outdoor air and diurnal fluctuations.
Figure 1 shows the 3 month summer outdoor radon concentrations for the 31 communities. The average radon concentrations for the provinces of Manitoba (59 Bq/m3) and Saskatchewan (61 Bq/m3) were found to exceed the average annual indoor level for the United States of 55 Bq/m3(9) and are roughly 6 times the estimated United States outdoor level of 10 Bq/m3.
One possible explanation for the regional variations in outdoor radon is the variation in the uranium concentration of the ground. However, very little correlation was found between radon and uranium concentration. The major factor that influenced the outdoor radon data was found to be the soil moisture content of the ground.
Figure 2 shows that the radon levels are all relatively constant and below 40 Bq/m3 when the annual precipitation exceeds about 550 mm. However, when the annual precipitation is less than 550 mm, the radon levels all exceed 30 Bq/m3 but also show large variations. When the soil pores are filled with water, radon cannot move through the soil, since it is a gas. Furthermore, it has a half-life of only 3.8 days.
Analysis of the precipitation data also showed that the summer of 1990 was particularly dry on the prairies. The city of Winnipeg had its driest July to October for 117 years. Therefore the high radon levels found in the prairies during the summer of 1990 may not be typical. Subsequent measurements taken over a 3 week period in July 1991 showed that the outdoor radon levels in Manitoba were reduced by a factor of approximately 4 from their 1990 levels. This was attributed to the much greater precipitation which occurred during and shortly before the 1991 measurement period.
Many people in the U.S. are advocating the reduction of indoor radon guidelines to below the level of the outside air. The United Kingdom have recommended that the indoor levels for new houses should be reduced to levels which are less than the outdoor levels for several places in Saskatchewan and Manitoba. Does this mean people in these high radon communities should leave? In reducing the indoor radon levels and setting guidelines for these levels, it is almost certain that in the future the outdoor radon levels will have to be considered.