Radon index of a local administrative unit.

The determination of radon-prone areas is usually based on indoor radon data and on the prognosis of the occurrence of houses exceeding the action level. However, the sample of houses in the survey must be representative and large enough, which is not easy to fulfill. Despite this, the determination of localities with high radon risk is useful not only for planning of indoor radon surveys, but also mainly for predicting the risk in newly built houses.