Radon is an odorless, colorless radioactive gas that comes from the soil. It is formed from the decay of uranium in soil, rocks and even building materials.
And it is a Class A carcinogen.
The EPA has placed a rating for indoor radon screenings measured in pico curies per liter. Homes with an average indoor radon screening level greater than 4 pCi/L are considered the highest potential for radon-induced lung cancer.
Radon is harmlessly dispersed in outdoor air, but when trapped in buildings can increase the risk of lung cancer, especially at elevated levels. It typically enters a home the same way air and other soil gases enter the home, through cracks in the foundation, floor or walls, hollow-block walls, and openings around pipes, sump pumps, and floor drains. It can also be present in some construction materials and in water from underground sources including private wells. Any home, regardless of age, energy-efficiency, or foundation type, could have a radon problem. The only way to know whether or not a particular home has a problem is to test THAT home.
Slight differences between indoor and outdoor pressure can affect the rate at which radon enters the home. Reduced indoor pressure draws the gas through any cracks and openings. This lower indoor pressure may be caused by open windows on the downwind side of the house, operation of kitchen or exhaust fans, and the use of air by furnaces and other large appliances. The fact that air in a house is often warmer than the surrounding air and tends to rise can also cause reduced indoor pressure. Another means of entrance for the gas is water supplies, particularly underground wells.