Radon is the natural radioactive element uranium and is present everywhere in rocks and soil. The radio active decay of uranium produces radium, which in turn decays to Radon, a radiocative colourless and odourless inert gas. As it is a gas, it can move easily through bedrock and soil and escape into the outdoor air or seep into a home or building. All soil contains uranium, so Radon is present in all types of soils. Radon that moves from the ground to the outdoor is rapidly diluted to low concentrations and is not a health concern.

  The air pressure inside a building is lower than in the soil surrounding the foundation. This draws in the gases, including Radon , through openings in the foundation where it is in contact with the ground. This includes construction joints, gaps, around srvice pipes and support posts, floor drains and sums, cracks in foundationwalls, and in floor slabsa, and in openings in concrete block walls. Once inside the building, Radon can accumulate to high levels and becomes a long term health concern.

Although High Radon concentrations are associated with some geological formations, types of soil, housing tye, and foundation construction vary so much from place to place that "Radon Potential maps" are poor indicators of the Radon concentration in an individual home. Even similar houses next to each other can vary different average Radon concentrations. 

The only way to know if a home has a high Radon concentration is to measure the Radon Concentration.