Radon is a radioactive gas;
it is not visible, not to be scented or tasted.
More than 300 people get lung cancer because of radon gas (ref. Radiation and Nuclear Safety Authority, Finland).
Outdoors radon dilutes quickly, so the danger is radon gas gravitating indoors. Most of exposure to radon gas happens through breathing or drinking water from drilled wells. Ground under buildings is the biggest source for radon gas getting indoors. Risk for lung cancer increases from mutations in lung cells caused by radon gas.
Radon gas levels should be under 100Bg/m3 in apartments. This is recommendation from WHO (World Health Organization).
In Finland about 10 000 places is measured annually (STUK), this is only 5% from yearly measurements in Sweden.
In Sweden approximately 200 000 measurements is done yearly. In Finland only 115 000 measurements is done in 30 years.
1986 Tsernobyl nuclear accident causes the same amount of radiation in 50 years that we get from nature yearly in Finland.