Radon: exploring the log-normal mystery

The term “log-normal mysticism” has been coined by Tóth et al. (2006) for the so far unexplained observation that indoor radon concentration data from geographical surveys very often appear to be quite accurately log-normally distributed. In this contribution, I try to verify the statement with the dataset of the Austrian indoor radon survey and data from the ongoing European indoor radon mapping project. It appears that, with some limitations, an approximate log-normality is present for a large scale of spatial ranges. Investigation of the frequency of outliers points to the systematic presence of a “fat tail” in Rn frequency distribution, i.e. of extremes which disturb log-normality. “Local log-normality”, i.e. within a neighbourhood, possibly restricted to geological units, is an important assumption in certain radon mapping approaches.