Radon A radioactive element, chemical symbol Rn, atomic number 86, and atomic weight 222 (Radon–222). Radon is a colorless, tasteless, odorless, naturally-occurring inert gas derived from the natural breakdown (i.e., radioactive decay) of three radioactive isotopes: uranium–238, uranium–235, and thorium–232. These isotopes are typically found in igneous and metamorphic rocks, such as granite and gneiss, and in sedimentary rocks such as organic-rich black shale, phosphatic rock, and coal. Uranium–238 is the most common parent of radon gas because it comprises more than 99 percent of uranium and thorium isotopes found on earth. Radon–222, one of the most common radioactive daughter elements of uranium–238 decay has a relatively short half-life of only 3.8 days. Consequently, radon–222 is relatively unstable and more likely to decay and emit radiation at any particular moment. Daughter isotopes of radon–222 have an even shorter half-life, resulting in a cascade or “burst” of radiation from radon and its daughter products. Radon occurs in groundwater, but not in surface water due to its high volatility. In groundwater, radon will stay in solution until the pressure on the groundwater is decreased. Due to its short half-life and the slow rate of natural groundwater flow, radon in groundwater typically cannot migrate far from its source. Radon is relatively easy to remove from water, several effective options for the treatment of radon in water include storage, adsorption on Granular Activated Carbon (GAC), and aeration. It has been proposed that radon provides about one-half of the radiation to which the average American is exposed. The chemically inert gas enters homes through soil, water, and building materials. The threat is not uniformly distributed across the United States. An important source of personal exposure to radon appears to be drinking water obtained from wells. The threat comes from the inhalation of the gas released from water during showering, bathing, cooking, and other water uses. Ingestion of water does not appear to present a threat.