The risks to human health posed by ionizing radiation are well known.

Radon gas is by far the most important source of ionizing radiation among those that are of natural origin.

Radon (222Rn) is a noble gas formed from radium (226Ra), which is a decay product of Uranium (238U).

Uranium and radium occur naturally in soils and rocks.
Other decay products of uranium include the isotopes thoron (220Rn)
and actinon (219Rn).

Radon gas, which has a half-life of 3.8 days, emanates from rocks and soils and tends to concentrate in enclosed spaces like underground mines or houses.

It is a major contributor to the ionizing radiation dose received by the
general population.

When radon gas is inhaled, densely ionizing alpha particles emitted by deposited short-lived decay products of radon (218Po and 214Po) can interact with biological tissue in the lungs leading to DNA damage.

Cancer is generally thought to require the occurrence of at least one mutation, and proliferation of intermediate cells that have sustained some degree of DNA damage can greatly increase the pool of cells available for the development of cancer.

Since even a single alpha particle can cause major genetic damage to a cell, it is possible that radon-related DNA damage can occur at any level of exposure.

Therefore, it is unlikely that there is a threshold concentration below which radon does not have the potential to cause lung cancer.

Health effects of radon, most notably lung cancer, have been investigated for several decades.

Initially, investigations focused on underground miners exposed to high concentrations of radon in their occupational environment. However, in the early 1980s, several surveys of radon concentrations
in homes and other buildings were carried out, and the results of these surveys, together with risk estimates based on the studies of mine workers, provided indirect evidence that radon may be an important cause of lung cancer in the general population.

Recently, efforts to directly investigate the association between indoor radon and lung cancer have provided convincing evidence of
increased lung cancer risk causally associated with radon, even at levels commonly found in buildings.

Risk assessment for radon both in mines and in residential settings have provided clear insights into the health risks due to radon.

Radon is now recognized as the second most important cause of lung cancer after smoking in the general population.

(Sources: WHO HANDBOOK ON INDOOR RADON)
 http://www.nrsb.org/pdf/WHO%20Radon%20Handbook.pdf