 Typical geometric standard deviations found in studies range between 2 and 3, meaning (given the 68–95–99.7 rule) that the radon concentration is expected to be more than a hundred times the mean concentration for 2 to 3% of the cases.Some of the highest radon hazard in the US is found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania.Iowa has the highest average radon concentrations in the US due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland.