CleanZone Field Brief
Radon: The Postcode Lottery That Could Cost You Your Lungs
Radon is the second leading cause of lung cancer after smoking, and it is entirely invisible. It seeps from the ground, accumulates in buildings, and varies block by block depending on geology, soil permeability, and the age of the house beneath which it collects. No national dataset captures the indoor concentration of your exact address. Only a long-term test can.
At a glance
- Radon-222 is a radioactive noble gas from uranium decay in rock and soil; it is odourless, colourless, and enters buildings through sumps, slab cracks, and construction joints.
- The EU (Euratom Directive 2013/59) sets a 300 Bq/m³ reference level; the US EPA action level is 4 pCi/L (≈148 Bq/m³); WHO's guidance figure is 100 Bq/m³.
- Two houses on identical geology can differ tenfold depending on sub-floor ventilation and slab integrity — geology sets the potential, the building sets the outcome.
- Only a long-term (3-month+) alpha-track or electret test taken on the lowest occupied floor is reliable; two-day charcoal canister kits are for transaction speed, not diagnosis.
- Active sub-floor depressurisation typically cuts indoor radon by 80–99% (US EPA) for roughly €1,500–4,000.
Where radon comes from
Radon-222 is a radioactive noble gas produced by the decay of uranium-238, which is present in most rocks and soils at low concentrations. It migrates through fractured bedrock and permeable overburden until it reaches the surface. Because it is chemically inert, it does not bind to soil particles; it follows pressure gradients and enters buildings through sumps, cracks, construction joints, and cavity walls.
The concentration in outdoor air is negligible — typically 3 to 10 Bq/m³. Indoors, it accumulates because buildings are imperfectly sealed. The rate of accumulation depends on the radon flux from the ground (driven by uranium content, soil permeability, and the pressure differential between indoors and subsoil), the building ventilation rate, and the internal volume-to-surface ratio of the lowest occupied level. Two neighbouring houses on identical geology can differ by a factor of ten because one has a ventilated sub-floor void and the other has a tight concrete slab.
Radon's chemical inertness is what makes it dangerous rather than harmless. Because it never bonds with soil minerals, nothing in the ground filters it out — it simply follows the path of least resistance, which is almost always the small pressure difference between a warm house and the cooler soil beneath it. That is also why geology alone cannot predict an indoor reading: the building envelope decides how much of the available gas actually gets trapped.
The two action levels
Europe and the United States use different units and different reference points. In the EU, Directive 2013/59/Euratom sets a national reference level of 300 Bq/m³ for indoor radon concentration in homes. Member states may adopt lower values — several use 200 Bq/m³ — but 300 is the regulatory ceiling.
In the United States, the Environmental Protection Agency (EPA) uses a unit called picocuries per litre (pCi/L). The EPA action level is 4 pCi/L, which converts to approximately 148 Bq/m³. A large share of US homes in radon-prone geological zones regularly exceed this threshold, particularly in the northern Appalachian and upper Midwest regions where uranium-rich granite and shale underlie residential areas.
| Jurisdiction | Reference level | Equivalent in other units | What it means in practice |
|---|---|---|---|
| EU (Euratom Directive) | 300 Bq/m³ | ~8.1 pCi/L | Member states must ensure remediation programmes; media campaigns required in high-potential areas |
| US (EPA action level) | 4 pCi/L | ~148 Bq/m³ | EPA recommends active mitigation (sub-slab depressurisation) at or above this level |
| UK (radon Affected Areas) | 200 Bq/m³ (target) | ~5.4 pCi/L | Building regulations require radon-proof membranes in new-build in Affected Areas |
| WHO | 100 Bq/m³ | ~2.7 pCi/L | Not a regulatory limit; offered as guidance for "as low as reasonably achievable" |
Geology, soil, and building age
The European Commission's Joint Research Centre has published radon potential maps for most member states, classifying areas as low, medium, or high potential based on surficial geology, soil permeability, and indoor measurement data. In the United States, the USGS and EPA jointly publish a national radon zone map dividing counties into three zones by predicted indoor screening level. Both datasets correlate strongly with granitic, metamorphic, and black-shale lithologies.
Building age matters because construction standards have changed. Pre-1990 buildings in Europe generally lack radon-proof membranes and sub-floor ventilation. In the United States, post-1990 construction in Zone 1 counties is more likely to include passive radon-resistant features, but compliance varies by state and enforcement depth. The CleanZone grid flags this through the building age field, which, paired with radon and soil type, gives a first-order risk ranking.
Myth
New-build homes are automatically safe because building regulations require radon-proof membranes in Affected Areas.
Data
A membrane without a working sump or positive-pressure system is only one layer of a system; enforcement and installation quality vary by builder, and "Affected Area" boundaries are geological-unit approximations, not per-plot certifications. The JRC and USGS/EPA maps flag potential — they do not certify any individual building.
What to do with a high-potential postcode
A postcode classified as high radon potential is not a sentence. It is an instruction to test. The only measurement that counts is the one taken inside the specific building, on the lowest occupied floor, over at least three months using an accredited alpha-track detector or an electret ion chamber. Short-term (two-day) charcoal canister tests are widely sold but have poor precision; they are useful only for real-estate transactions where delay is impossible.
Radon concentration swings with weather, ventilation habits, and season — a two-day snapshot can land well above or below a home's true annual average. Treat a short-term charcoal result as a screening flag, not a verdict; a reading near or above the local action level should always be followed by a full long-term test before any decision on mitigation.
If the long-term test reads above the national action level, mitigation is straightforward and cost-effective. Active sub-floor depressurisation — a small fan drawing air from beneath the slab and venting it outside — routinely reduces indoor radon by 80 to 99 percent. Typical contractor quotes across European and North American markets run €1,500–€4,000, far below the lifetime cancer-risk reduction it yields.
How CleanZone models it
The CleanZone grid computes radon from the JRC and USGS national classifications, soil type from the European Soil Data Centre (ESDAC) and US NRCS STATSGO2, and bedrock geology from national geological surveys. These are coarse inputs — the geological map unit at survey mapping scale does not know about the fracture that runs under your basement. They are useful for comparative screening: "this cell is higher-risk than that one." The definitive answer remains the three-month detector kit on the ground floor.
National radon-potential maps from the JRC and the joint USGS/EPA classification are built at the resolution of geological survey units — typically kilometres wide, not individual parcels. A "high potential" county or grid cell can contain both the worst basement in the region and a perfectly ventilated new-build across the street. The map narrows the search; it does not replace the detector.
Radon is the ultimate postcode lottery because the public data stops at the geological unit and the building code era. The CleanZone grid (radon, soil type, bedrock geology) gets you to the right postcode. A long-term alpha-track test gets you to the right answer.