Radon (pronounced /ˈreɪdɒn/) is a chemical element with symbol Rn and atomic number 86. Radon is a colorless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health hazard. The most stable isotope, ²²²Rn, has a half-life of 3.8 days. While having been less studied by chemists due to its high radioactivity, there are a few known compounds of this generally unreactive element.

Radon is formed from the normal radioactive decay of uranium. Uranium has been around since the earth was formed and its most common isotope has a very long half-life (4.5 billion years), which is the amount of time required for one-half of uranium to break down. Uranium, radium, and thus radon, will continue to exist indefinitely at about the same levels as they do now.^[1]

Radon is responsible for the majority of the mean public exposure to ionizing radiations. It is often the single largest contributor to an individual's background radiation dose, and is certainly the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as basements. Radon can be found in some spring waters and hot springs.^[2]

Breathing high concentrations of radon can cause lung cancer. Thus, radon is considered a significant contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency, radon could be the second most frequent cause of lung cancer, after cigarette smoking; and radon-induced lung cancer the 6th leading cause of cancer death overall, causing 21,000 lung cancer deaths per year in the United States.^[3]