[en] Presence of radon in the living space is natural. Radon density depends on the radium/uranium density in the soil, but there are many other factors that determines the radon density. Most authors suppose that radon concentration is the result of many independent effects which must result in a so called lognormal distribution. All other statements are based on this hypothesis. The paper gives analysis of radon density data in 15000 flats in Hungary and proves the hypothesis. Furthermore, gives radon concentration dependence according to the flats' regional and topological location and its architectural features. (T-R.A.)