Radon is a radioactive gas that is found in the earth's rock and soil. It is formed by the natural breakdown of uranium,which decays into radon gas.

As radon decays, it forms radioactive by-products called either "progeny," "decay products" or"daughters" which, if inhaled, can damage lung tissue and cause lung cancer.

Invisible and odorless, radon is a health hazard when it accumulates inside homes or other structures. And it is deadly. Indoor radon exposure is estimated to be the second leading cause of lung cancer deaths each year in the United States. Cigarette smoking is responsible for the large majority (87 percent) of lung cancer deaths.