Radon risk in Spanish homes: what buyers should know

There is a gas you cannot see, cannot smell and cannot feel... yet it could be building up in your home right now. It is called radon, and the World Health Organization classifies it as a Group 1 carcinogen: the highest-risk category for human health.

This is not alarmism. It is a real, measurable risk, and it depends to a large extent on where and how your home is built. So much so that since 2025 it has already been mandatory to measure it in thousands of Spanish workplaces. At Hogarfax, we include it in the analysis of every property.

What is radon?

Radon (Rn-222) is a naturally occurring radioactive gas. It is produced by the decay of uranium present in soil and certain rocks, especially granite. Outdoors it does not usually reach high levels, but it tends to accumulate inside homes, particularly in areas with highly permeable soils or high radio-226 content.

Because it is heavier than air, it tends to concentrate in the lowest parts of the building. The problem is not occasional presence - the average level in Spain is around 40-50 Bq/m3, which is considered normal - but prolonged accumulation in enclosed, poorly ventilated spaces.

Why is it dangerous?

When inhaled at elevated concentrations over many years, radon emits alpha particles that damage lung tissue. It is the second leading cause of lung cancer in Spain, after tobacco, and the leading cause among non-smokers.

What level is dangerous?

Royal Decree 1029/2022 sets a reference level of 300 Bq/m3 as the annual average in homes, in line with European Directive 2013/59/Euratom.

Where is radon more common in Spain?

Radon is especially prevalent in areas with granite soils, such as Galicia and Extremadura. Galicia tops the map, with 91% of its municipalities classified as priority areas, followed by Extremadura with 70%, and the Community of Madrid with close to 50%.

In Madrid, the CSN considers 86 municipalities to require priority action and a further 59 to present moderate risk - together, 81% of the municipalities in the region. The areas with the highest incidence are in the Sierra Norte, the west of the region and the Guadarrama area, where permeable terrain and granite soils are predominant.

If you want to see this risk more visually, we have prepared a national radon map of Spain showing municipalities coloured according to the official zoning. It gives you a quick way to understand where the greatest risk potential lies before drilling down to the level of a specific property.

The 2025 change: measuring it is already mandatory in workplaces

Until now, measuring radon was a recommendation for private individuals. That has changed in the workplace context.

In May 2025, Instruction IS-47, published by the Consejo de Seguridad Nuclear, entered into force. It requires businesses and public bodies located in 20% of Spanish municipalities to carry out measurements in all their workplaces. Since 1 October 2025, companies in priority municipalities must measure for at least three months, preferably between October and May.

How can you tell whether your home has radon?

Radon potential maps provide general guidance, but the only way to determine whether a home is exposed is through a property-specific measurement. The CSN recommends contacting a laboratory accredited under ISO 17025.

Typically, the laboratory sends the detectors by post with instructions, and after the exposure period they are returned for analysis. If you have a single-storey home, place one detector in the living room and another in a bedroom. If you have a basement, prioritise that area.

What can you do if levels are high?

For concentrations around a few hundred Bq/m3, natural ventilation for around 15 minutes each morning can be effective. At higher levels, construction-based solutions may be necessary.

• Waterproof barriers in the floor
• Sub-slab air extraction systems
• Mechanical ventilation with heat recovery