Radon (Rn) is a colorless, radioactive, unreactive gaseous element. It is a member of the noble gas family. Radon is formed by the disintegration of radium, uranium, and thorium. Found in some radioactive minerals, Radon-222 (Rn222) has a half-life of 3.8 days, making it suitable as a radiation source in radiotherapy and to produce neutrons for research. Other natural and synthetic isotopes have shorter half-lives. Radon was discovered in 1899 by Ernest Rutherford. Radon (II) fluoride (RnF2) is the only radon compound known.


atomic number 86
relative atomic mass 222
melting point -71°C (-96°F)
boiling point -61.8°C (-79.2°F)
relative density (sol) 4
half-life 3.823 days


Natural radon production and risks to humans

Due to the very long half-lives of the radionuclides uranium-238, uranium-235 and thorium-232, the Earth's crust still contains contains considerable quantities of these substances four and a half billion years after its formation. These nuclides convert via a chain of radioactive intermediate products, with quite different half-lives, to stable lead as the final product. Among the intermediate products are three radon nuclides. Radon-222 (half-life 3.8 days) is generated as a decay product of radium-226, which results from the radioactive decay of uranium-238. In the decay chain of thorium-232, radon-220 (half-life 54 s) occurs, and in the decay chain of uranium-235, radon-219 (half-life 3.96 seconds).


Radon is released wherever uranium and thorium are present in the ground and enters the atmosphere or houses. The radium concentration of the ground and its permeability for this radioactive noble gas is decisive for the radon concentration in the air. Apart from regional variations, the radon concentration in the atmosphere close to the ground is also subject to seasonal and climatic variations. In buildings, the radon concentration depends essentially on structural circumstances. For example, in Germany, the annual average value of the radon concentration in the air close to the ground is 15 becquerels per cubic meter (Bq/m3) and in buildings 60 Bq/m3. Radon concentrations greater than 200 Bq/m3 in ground floor living rooms are not uncommon. Regarding the radiation exposure of people, it is not the radon itself that is important, but the short-lived decay products. These enter the respiratory tract with breathed-in air and may reach radiation-sensitive cells with their energy-rich alpha radiation. The short-lived decay products of radon, with 1.4 millisieverts per year, account for more than half the total effective dose by natural radiation sources.


If radon is present in high levels, a radon mitigation system can be installed to lower the concentration to acceptable levels.