Radon: A radioactive element that is formed, as a gas, during the breakdown of radium. Radon exposure is the second leading cause of lung cancer deaths in the United States, after smoking. Radon gas continuously seeps into the air from uranium- and radium-bearing soil and rock. Well water can be contaminated with radon and may carry radon into a house through the water pipes. There are several radon testing devices on the market.