Radon is a radioactive gas that comes from the breakdown of Uranium deposits in the earth. It is an invisible, odorless gas that is considered by the EPA as a class A carcinogen(a pollutant with adequate human data indicating the chemical causes cancer in people). As Uranium breaks down it loses protons, which changes it to Thorium. As the breakdown continues, it loses more protons, which changes it to Radium and then to Radon. Why do we test for radon and not the other forms? Because radioactive elements have a half-life, which details how long it takes for it to put off its radioactive material. Uranium has a half life of hundreds of million years. Radon has a half life of 3.823 days, which means that approximately every 4 days it puts off radioactive material.