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Norwegian Radiation and Nuclear Safety Authority

What is radon?

Radon is an invisible and odourless radioactive gas which is formed continuously in the Earth’s crust. Outdoors, radon concentrations are normally low, and health hazards only arise when the gas seeps in and builds up in our indoor environment. 

Last updated: 19. mars 2021 11:05


  • Radon is a radioactive gas.

  • Elevated radon levels indoors pose a health risk.

  • The subsoil is the main source of elevated radon levels in a building.

  • Both our geology and our cold climate are major reasons why Norway is among the countries with the highest average radon levels.

How is radon formed?

The element uranium is naturally radioactive and is found in varying concentrations in bedrock and soil. When uranium decays or breaks down, it forms new radioactive substances, including radium and radon.
Radon is a noble gas and has limited propensity to bind to solids. This means that the gas can escape, be transported through the ground and released into the air.

Sources of radon

The subsoil is the main source of elevated radon concentrations in buildings in Norway. Radon in the bedrock seeps in through cracks and small gaps between the subsoil and the building.

Domestic water from drilled wells in solid rock can contain high concentrations of radon. When the water is used in showers, dishwashers and the like, radon will be released into the indoor air.

The use of stone as a building material can add radon to the indoor air, but in Norway this is rarely a major source. Manufactured aggregate and gravel in the ground can also create problems with radon in areas that are otherwise at low risk. 

Why are there high radon concentrations in Norway?

Major factors affecting radon concentrations in indoor air are the design of the building and the permeability of the foundations, the ventilation, geological conditions and climate. Our cold climate is a major reason for the high radon levels in Norway. Heating buildings in the winter season causes the warm air to rise and negative pressure to form on the lowest floors. Radon can enter a building from the ground through cracks and gaps and thus cause elevated concentrations in the indoor air. The other major cause of high radon levels in Norway is our geology. Areas of permeable soil and bedrock containing radium-rich rocks such as alum shale, granites and pegmatites can lead to very high radon levels indoors.

The unit of measure – the becquerel

Radon concentrations in air are expressed in becquerels per cubic metre of air (Bq/m³), and in water in becquerels per litre of water (Bq/l, or sometimes kBq/m³).

Radioactive substances are not stable and emit energy in the form of radiation while forming new substances. This process is called radioactive decay or disintegration and cannot be stopped or controlled. One becquerel (Bq) is defined as one decay per second.

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