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Guides / Risks

Radon in Portuguese Homes: What Buyers Should Actually Worry About

Radon is a naturally occurring radioactive gas released by the decay of uranium in bedrock. It seeps up through soil and enters buildings through foundations, and long-term exposure at high concentrations is a recognised lung-cancer risk. Portugal has some of the more radon-prone geology in Europe — but the risk is very unevenly distributed, both geographically and within a single building.

Where radon matters in Portugal

Elevated radon potential follows granite. The affected band runs across the north and centre-interior: districts such as Porto, Braga, Vila Real, Viseu, Guarda and Castelo Branco sit on granitic bedrock with materially higher radon potential than the sedimentary basins of Lisbon or the Algarve littoral. Portuguese law (following the EU directive) uses 300 Bq/m³ as the reference level for indoor radon.

The part most listings ignore: floor level

Radon is a ground-contact hazard. It accumulates in basements, ground floors, and homes with poor under-slab ventilation. Concentration falls off sharply with height: an apartment on the 3rd floor of a granite-region building faces a small fraction of the exposure of the ground-floor unit in the same building. If you are buying an upper-floor flat, radon should almost never drive your decision — which is exactly how SirJohn treats it: our audits flag radon susceptibility for the location but waive the mitigation cost for verified upper-floor units.

If you're buying ground-contact space

  • Test before you commit if you can — passive detectors cost tens of euros; a short screening test gives a first signal in days.
  • Mitigation is a solved problem: sealing entry points, improved ventilation, or sub-slab depressurisation typically runs from a few hundred to a few thousand euros — a negotiation item, not a deal-breaker.
  • New builds fare better: post-2018 rules push radon-aware design in high-potential municipalities.

Check your area

Every SirJohn audit includes the radon susceptibility of the exact address, weighed by floor level. You can also browse the typical risk profile per neighbourhood on our area pages, or run a free audit on any listing.