EPA RECOMMENDS: If you are buying or selling a home, have it tested for radon. For a new home, ask if radon-resistant construction features were used and if the home has been tested. Fix the home if the radon level is 4 picocuries per liter (pCi/L) or higher. Radon levels less than 4 pCi/L still pose a risk and, in many cases, may be reduced.

EPA estimates that radon causes thousands of cancer deaths in the U.S. each year. Radon is estimated to cause about 21,000 lung cancer deaths per year, according to EPA's 2003 Assessment of Risks from Radon in Homes.

Common Radon Entry Points

There are four main factors that permit radon to seep into homes. All homes have some type of radon-entry pathway:

Uranium is present in the soil nearly everywhere in the United States.
The soil is permeable enough to allow radon to migrate into a home through the slab, basement or crawlspace.
There are pathways for radon to enter the basement, such as small holes, cracks, plumbing penetrations and sump pumps.
A difference in air pressure between the basement or crawlspace and the surrounding soil draws radon into the home.

Radon enters through:

Cracks in otherwise solid floors
Gaps in suspended floors
Cracks in walls
Cavities inside walls
Gaps around service pipes
Construction joints
The water supply

Every house has radon. The EPA does not approve any amount of radon to be safe. The EPA recommends mitigation in homes with 2-4 pCi/L. Do you know what your home's radon level is? Crawlspace, slab, basement and any combination of foundation can have radon. Radon is the leading cause of lung cancer aside from smoking.

Read the complete radon guide from the EPA