Radon from the subsoil (radon index)

Radon is a natural radioactive gas formed in rocks that enters buildings through gaps in floors and foundations. According to the Czech Radon Programme it is the second most common risk factor for lung cancer after smoking. The Czech Atomic Act defines a plot's radon index as an indicator of the risk of radon migrating from the bedrock; it can be low, medium or high.

Plot radon index and the radon map

A plot's radon index is determined by measuring radon in soil air and assessing the permeability of the foundation soil (Section 96 of Decree No. 422/2016 Coll.). Anyone applying for a building permit for a new building or extension with habitable rooms must have it determined (Section 98 of the Atomic Act). This is not needed if the design vents radon from the subsoil away from the house or if the building is separated from the ground by a ventilated air gap.

The radon risk map of the bedrock by the Czech Geological Survey (often searched for as the “radon map”) is only an area-wide estimate based on rock types and distinguishes low, transitional, medium and high index. It proves nothing about a particular house: the indoor concentration depends on the insulation of the substructure, the floors, the cellar and ventilation.

Indoor measurement and limits

• The reference level for habitable rooms is 300 Bq/m³ as an average under normal ventilation (Section 97 of Decree No. 422/2016 Coll.).
• If the annual average exceeds 3,000 Bq/m³, the building owner must take measures to reduce exposure.
• The Radon Programme recommends asking for a measurement report when buying a family house, ideally no older than 3 years.
• The State Office for Nuclear Safety (SÚJB) offers free measurement if conditions are met; the application is filed online via the National Radon Database.

What to watch out for

Insulation or new windows without also limiting radon entry from the ground can raise the indoor concentration because air exchange drops. If the reference level is exceeded, options include adjusting ventilation, for example mechanical ventilation, or structural remedial measures.