In the summers of 1990 and 1991, outdoor 222Rn detectors were installed for 3 mo in 78 communities across Canada. The 1990 measurements showed large regional variations with the average outdoor levels for the provinces of Manitoba (59 Bq m−3) and Saskatchewan (61 Bq m−3) exceeding the average indoor levels of 55 Bq m−3 for the U.S. The variations in the 1990 222Rn levels showed little correlation with the uranium concentration of the ground and were attributed to changes in soil moisture content. All measurements above 30 Bq m−3 were found to be in dry climatic regions where the annual precipitation was less than 550 mm. In addition, most of the sites with high 222Rn levels were on glacial lake clays. A soil moisture modeling program showed that because of the low precipitation, these clays were drying out in the summer allowing fractures to develop and 222Rn to migrate easily through the ground. The much lower 1991 measurements for Manitoba (10 Bq m−3) and Saskatchewan (15 Bq m−3) were believed to be due to the increased precipitation in the prairie regions in the summer of 1991 compared to the unusually dry summer of 1990.