Plot radon index

Under the Czech Atomic Act, a plot's radon index is an indicator of the risk of radon migrating from the bedrock; it can be low, medium or high. It is determined by measurement directly on the plot, so unlike the radon risk map it describes the place of a future building, not a whole area. Radon in existing houses and its limits are covered under radon from the subsoil.

When measurement is required

Anyone applying for a permit for a new building or an extension with habitable rooms must have the plot's radon index determined (Section 98(1) of the Atomic Act). This is not needed if all external structures of the building will be separated from the ground by a freely ventilated air gap, or if the design includes venting radon from the subsoil away from the building (Section 98(3)).

When an existing building is altered or its use changes so that new habitable rooms are created, radon is measured inside the existing building instead (Section 98(2)).

How the measurement works

• Soil air is drawn through probes driven about 0.8 m deep within the footprint of the future building, and its radon activity concentration is measured.
• The gas permeability of the foundation soil is assessed, by measurement or expert judgement.
• The results are evaluated together and the plot is assigned a category (Section 96 of Decree No. 422/2016 Coll.).
• Determining a plot's radon index is a service significant for radiation protection; the company carrying it out needs a licence from the State Office for Nuclear Safety (SÚJB) under Section 9(2) of the Atomic Act.

Categories under the decree

The category depends on the combination of radon activity in soil air and soil permeability (Section 96(4) to (6) of Decree No. 422/2016 Coll.):

• low index: below 10 kBq/m³ for highly permeable, below 20 kBq/m³ for medium and below 30 kBq/m³ for low-permeability soils,
• medium index: 10 to 30, 20 to 70 or 30 to 100 kBq/m³ depending on permeability,
• high index: above 30, 70 or 100 kBq/m³ depending on permeability.

Based on the result, the designer specifies the building's radon protection under the Czech standard ČSN 73 0601, from insulating the substructure to venting the subsoil.

Frequently asked questions

Is the radon map enough instead of a measurement?

No. The bedrock radon risk map of the Czech Geological Survey distinguishes low, transitional, medium and high index only by area, based on rock types. It is a useful rough guide when choosing a plot, but a new building with habitable rooms needs a measurement on the plot.

Does the measurement make sense for an older house?

The plot radon index belongs to the preparation of a building. For an existing house, only an indoor radon measurement shows the actual situation. The reference level for habitable rooms is 300 Bq/m³ (Section 97 of Decree No. 422/2016 Coll.).