Radon emission - from materials or the ground?

The problem of radon emission in buildings first came to light at the end of the 1970s, when a report by the Swedish National Institute of Radiation Protection revealed high radon daughter concentrations in some houses. Temporary limits on permitted concentrations in different types of dwellings were imposed in Sweden. They were related to the age of dwellings, due to the known historical use of alum shale in lightweight concrete. Described here are the tests later carried out by the Swedish National Testing Institute and the assessment methods involved in co-operation with Lund University. They have absolved modern-day building materials from blame. Suggests the main source of the trouble is the ground on which the houses are built.