Interpreting Map Legend Colours & Risk Shading
The UKHSA / BGS digital atlas uses a standardized color-coding system across England, Wales, Scotland, and Northern Ireland:
• Pale Yellow / White: Band 1 (<1% probability). The vast majority of UK homes. No radon precautions required.
• Pale Orange / Amber: Band 2 (1% to 3% probability). Lowest tier of Radon Affected Area. Basic precautions for new builds.
• Medium Orange: Band 3 (3% to 5% probability). Radon Affected Area. Testing advised when purchasing.
• Crimson / Magenta: Band 4 (5% to 10% probability). Significant radon concentration. Radon Bond common in conveyancing.
• Deep Violet: Band 5 (10% to 30% probability). High radon area. Full Building Regulations Part C protection required.
• Dark Purple / Navy: Band 6 (>30% probability). Maximum UK radon risk zone. Found in granite areas of Dartmoor, Cornwall, and Peak District limestone.
| Atlas Color Shade | Probability Band | Fraction of Affected Homes | Statutory Status |
|---|---|---|---|
| Pale Yellow | Band 1 (<1%) | Less than 1 in 100 homes | Low Risk Zone |
| Amber | Band 2 (1–3%) | 1 in 33 to 1 in 100 homes | Radon Affected Area |
| Medium Orange | Band 3 (3–5%) | 1 in 20 to 1 in 33 homes | Radon Affected Area |
| Crimson | Band 4 (5–10%) | 1 in 10 to 1 in 20 homes | Radon Affected Area |
| Deep Violet | Band 5 (10–30%) | 1 in 3 to 1 in 10 homes | High Risk / Full Protection |
| Dark Purple | Band 6 (>30%) | More than 1 in 3 homes | Extreme Hotspot / Full Protection |
Why Map Boundaries Follow Geology Rather Than Postcodes
Users are often surprised that two homes sharing the same postal outward code (e.g., TR13) have completely different colors on the map.
Postal boundaries are arbitrary administrative lines created by Royal Mail for sorting letters. The BGS Radon Atlas is governed by deep subterranean rock formations.
Where a granite outcrop terminates and transitions into impermeable Devonian slate or marine clay, the radon risk can plummet from Band 6 (>30%) to Band 1 (<1%) within a few hundred yards.