How the UK Radon Risk Map Works
Radon (*Rn-222*) is a naturally occurring radioactive gas formed by the radioactive decay of minute amounts of uranium present in UK rocks and soils.
The UK Health Security Agency (UKHSA) in partnership with the British Geological Survey (BGS) maintains the official *UK Indicative Atlas of Radon*. This map combines millions of indoor radon measurements with underlying geological bedrock maps.
The map assigns every 1km square across England, Wales, Scotland, and Northern Ireland into one of six statistical risk categories based on the percentage of homes predicted to exceed the UK Action Level:
| Radon Probability Band | Estimated Homes Above Action Level | Designated Status | Recommended Diligence |
|---|---|---|---|
| Band 1 (<1%) | Fewer than 1 in 100 homes | Low Risk Area | No action needed; routine building regs |
| Band 2 (1–3%) | 1 to 3 in 100 homes | Radon Affected Area | Testing optional; basic protection for new builds |
| Band 3 (3–5%) | 3 to 5 in 100 homes | Radon Affected Area | Recommended to test during house purchase |
| Band 4 (5–10%) | 5 to 10 in 100 homes | Moderate Risk Area | Testing strongly advised; Radon Bond in conveyancing |
| Band 5 (10–30%) | 10 to 30 in 100 homes | High Risk Area | Testing mandatory for peace of mind; mitigation common |
| Band 6 (>30%) | More than 3 in 10 homes | Very High Risk Area | Full radon sump or positive input ventilation required |
Why Postcode Maps Are Only Indicative
It is vital to understand that a postcode check provides an indicative probability, not a definitive measurement for an individual building.
Because radon enters buildings through micro-cracks in solid ground floors, pipe penetrations, and suspended timber floor gaps, two identical houses next door to each other can have radically different indoor radon concentrations depending on:
• Sub-floor ventilation: Poorly vented underfloor voids trap gas inside living spaces.
• Double glazing and draught-proofing: Highly insulated homes with low air exchange can trap radon.
• Local underground fissures: A granite fault line running beneath one house may bypass the neighbour entirely.
If your postcode is in a Band with >1% probability, order a validated physical test kit (£50–£60) containing two passive alpha-track detectors (one for the living room, one for the bedroom) left in place for 90 days.