Radon is a naturally radioactive noble gas with gas density 9.73 g/L and melting point −71 °C, discovered in 1899 as an intermediate in the uranium and radium decay chains, continuously exhaled from granite, soil and groundwater. It has no positive industrial use today; instead it is a strictly regulated indoor air pollutant. Because radon is a gas whose decay products are solid radioactive particles of polonium and lead that lodge in the lungs, their alpha irradiation over long periods makes indoor radon the second leading environmental cause of lung cancer after smoking; the WHO and national environmental agencies list indoor radon concentration as a mandatory residential metric, especially in basements, homes built over granite, or houses using too much groundwater. Historically, in the early days of radiotherapy, radon was sealed in tiny gold seeds and implanted directly into tumors for brachytherapy; with the advent of cobalt-60, linear accelerators and particle therapy, this method is long obsolete. Today radon only appears as an environmental monitoring target and a tracer in geological exploration, where measuring fault-gas radon aids seismic studies and geothermal prospecting; it enters no consumer product.
Radon is a noble gas with very low chemical reactivity; its known compounds are extremely few and can only be prepared in trace amounts at low temperature in the laboratory, with no large-scale application. Radon difluoride (RnF2) is the only comparatively well-studied compound, formed by reacting xenon fluorides with radon; it is unstable at room temperature and decomposes on heating, used only to verify that even heavy noble gases can form chemical bonds. Radon oxides and clathrates appear only in theory and low-temperature experiments. Since the longest-lived isotope, Rn-222, has a half-life of only about 3.8 days, every radon compound decays away within a few days with its parent gas, so none can be stored, shipped or fed into an industrial process. In practice, the radon people encounter is almost always the elemental gas: building radon detectors, groundwater monitoring and indoor-air-quality surveys. Its compounds have no presence in everyday life.
