Radon is a radioactive gas. It is colorless, odorless, tasteless, and chemically inert. Unless you test for it, there is no way of telling how much is present.

Radon is formed by the natural radioactive decay of uranium in rock, soil, and water. Naturally existing, low levels of uranium occur widely in Earth's crust. It can be found in all 50 states. Once produced, radon moves through the ground to the air above. Some remains below the surface and dissolves in water that collects and flows under the ground's surface.

Most indoor radon comes into the building from the soil or rock beneath it. Radon and other gases rise through the soil and get trapped under the building. The trapped gases build up pressure. Air pressure inside homes is usually lower than the pressure in the soil. Therefore, the higher pressure under the building forces gases though floors and walls and into the building. Most of the gas moves through cracks and other openings. Once inside, the radon can become trapped and concentrated.

Nearly one out of every 15 homes in the United States is estimated to have an elevated radon level (4 pCi/L or more). The U.S. average radon-in-air level in single family homes is 1.3 pCi/L. Because most people spend as much as 90 percent of their time indoors, indoor exposure to radon is an important concern.

While radon problems may be more common in some geographic areas, any home may have an elevated radon level. New and old homes, well-sealed and drafty homes, and homes with or without basements can have a problem. Homes below the third floor of a multi-family building are particularly at risk.

The only way to determine the level is to test. the EPA and the Surgeon General recommend testing all homes below the third floor for radon.

A map of radon zones has been created to help national, state, and local organizations to target their resources and to implement radon-resistant building codes. However, the map is not intended to be used for determining if a home in a given zone should be tested for radon. Homes with elevated levels of radon have been found in all three zones.

In addition, indoor radon levels vary from building to building. Do not rely on radon test results taken in other buildings in the neighborhood, even ones next door, to estimate the radon level in your building.

Source: http://www.nsc.org