Radon, the lognormal distribution and deviation from it

It has been recognised for many years that the distributions of indoor radon concentrations in many countries conform more or less closely to a lognormal distribution. For homes situated over a single geological unit with a consistent source of radon in the ground, conformance with the lognormal distribution can be understood in terms of the multiplicative factors affecting the relationship between radium in the ground and radon in indoor air. The fact that national as well as local distributions of indoor radon also usually conform to the lognormal can be attributed to the fact that the mixture of a number of different lognormal distributions will often result in a lognormal distribution. In some cases, however, the national distributions of indoor radon deviate from a lognormal distribution: the reasons for this are examined using model normal distributions. A study of the deviation of the UK distribution from log-normality above 500 Bq m−3 provides a better estimate of the number of homes with very high radon concentrations.