Radon is a radioactive gas. It is colorless, odorless, and tasteless. It enters your home the same way air and other soil gases enter your home: through cracks in the foundation floor or walls, hollow-brick walls, and openings around floor drains, pipes and sump pumps. Unless you test for it, there is no way of telling how much radon is present.

Radon is formed by the natural radioactive decay of uranium in rock, soil, and water. Low levels of uranium occur naturally in the ground and can be found in all 50 states (although the average radon level in Minnesota is more than three times higher than the national average!) Once produced, radon moves through the ground to the air above. Some remains below the surface and dissolves in the water table flowing underground. 

When radon undergoes radioactive decay, radiation is emitted in the form of alpha particles. It also produces decay products, often called progeny, some of which are also radioactive.

Unlike radon, the progeny are not gases and can easily attach to dust and other particles. Those radioactive particles can be transported by air and can also be inhaled.