Unlike all other intermediate elements in the aforementioned decay chains, radon is, under standard conditions, gaseous and easily inhaled, and therefore a health hazard.
It is often the single largest contributor to an individual's background radiation dose, but due to local differences in geology,[3] the level of exposure to radon gas differs from place to place.
A common source is uranium-containing minerals in the ground. Due to its density, it can especially accumulate in subterranean areas such as basements. Radon can also occur in some ground water like spring waters and hot springs.
Radon typically moves up through the ground to the air above and into your home through cracks and other holes in the foundation. Radon can also enter your home through well water. Your home can trap radon inside.

There are generic and recognised tests for radon gas, these tests are methodically done in areas of known systematic hazards. Short-term radon test devices used for initial screening purposes are inexpensive.