The mean concentration ranges from less than 10 Bq/m3 to over 100 Bq/m3 in some European countries.[20] 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.