What does Bq/m³ mean, and how much radon is too much in a home?

Where the unit comes from

The becquerel (Bq) is the unit of radioactivity: one becquerel is one nuclear disintegration per second, as Spain's Royal Decree 1029/2022 on protection against ionising radiation states. Divided by volume, it tells you how much radioactivity there is in the air of a room.

Radon is a natural radioactive gas that comes out of the ground and builds up indoors, especially on ground floors and in basements. That is why its concentration varies from one building to the next, and even from one floor to another.

The number that matters: 300 Bq/m³

Article 72 of Royal Decree 1029/2022 sets 300 Bq/m³ as the reference level for radon concentration in enclosed spaces, and Spain's Nuclear Safety Council (CSN) applies it to homes as an annual average. It is not a limit above which a home is uninhabitable: it is the reference above which it makes sense to act.

To know how much radon there really is, you have to measure it: the CSN says the detectors should be left in place for at least three months. The Spanish Building Code, through section HS 6 added by Royal Decree 732/2019, also classifies municipalities into zone I and zone II by radon potential and requires protective measures in new buildings there.

What the HOUSINGFAX report tells you

Check 22, «Radon gas», places the building on the CSN's radon potential map, with categories from under 100 to over 400 Bq/m³, and checks whether the municipality is in zone I or II of the Building Code. It tells you whether measuring is advisable; it does not measure the radon in your home.

Look through the sample report and check your home for free.

What the map does not tell you

Radon potential describes the area, not your home: a home in a high-potential area may measure low, and vice versa. Only a measurement over several months gives you the real figure. The report sums up what the official sources say and does not replace an on-site technical inspection of the building.