Radon (Chemical Symbol/Element Number: Rn222) is a cancer-causing radioactive gas. You cannot see, smell or taste radon. When you breathe air containing radon, you increase your risk of getting lung cancer. The Surgeon General has warned that radon is the second leading cause of lung cancer in the United States today. If you smoke and your home has high radon levels, your risk of lung cancer is especially high. 

Scientifically Radon is a highly radioactive gaseous element. Radon was discovered in 1899 by English physicist Ernest Rutherford. Some also credit German physicist Friedrich Ernst Dorn in 1900.  By all appearances Rutherford discovered radon's alpha radiation and  Dorn found that radium gave off a gas.

Radon is a colorless, chemically unreactive inert gas, it is the densest gas known. The gas and its highly radioactive metallic daughter products emit alpha and beta particles and gamma rays. It has been used in the treatment of cancer by radiotherapy.

In homes and other buildings, in some areas of the world, radon produced by the radioactive decay of uranium-238 present in soil and rock can reach levels regarded as dangerous.