Radon is a radioactive gas that arises naturally in bedrock when radium decays. Long-term exposure to radon gas gives an elevated risk of developing lung cancer. The Swedish Radiation Safety Authority estimates that radon in dwellings causes around 500 cases of lung cancer per year in Sweden, most commonly among smokers.

Radon gas can enter buildings from the ground, building materials or household water. The general public’s main exposure to ionising radiation is from radon. Measurement is the only way to detect radon levels.