A natural radioactive gas
Editorial revision 2026
Radon exposure
Exposure to radiations emitted by radon and its descendants is expressed in becquerels per cubic meter (Bq/m3). The risk varies mainly with conditions of ventilation. Radon is a heavy gas that has the tendency to accumulate at the bottom of valleys, in holes and caves. Released by rocks containing uranium, radon escapes from granite and volcanic rocks. It can especially be found inside uranium mines.
© DR
Radon 222 is the main source of natural radioactivity to which humans are exposed.
This isotope of radon belongs to the radioactive lineage of uranium 238, and is its sixth generation descendant. It is formed by the decay of its precursor, radium-226. It decays within a few days into polonium 218 through the emission of an alpha particle. Its very short half-life (3.8 days) means that we would not be able to see it in our environment if it was not constantly being regenerated. The decay chain of uranium 238 is such that at any given moment as much radon is being created as is disappearing. This is the law of radioactive equilibrium.
Radon is the main source of exposure to natural radioactivity for humans because it is the only descendant of uranium to exist in a gaseous state. This allows it to leave the surface of a rock containing some uranium and enter the atmosphere. Its ability to escape from underground volcanic and granite-based rocks and enter the air that surrounds us has made radon exposure a ubiquitous feature of our terrestrial environment.
A few tens of grams of radon on the Earth surface are all that is needed to make it the most significant contributor to natural radioactivity levels. This astonishingly small quantity is the consequence of radon-222 half-life, which is very short by comparison to that of radium and even more when compared to that of uranium 238. According to the law of radioactive equilibrium, radioisotopes in a radioactive sery have an abundance inversely proportional to their half-lives: one atom for 154,000 radium atoms and one for 430 billion uranium 238 atoms.
Radonator
This curious apparatus, called radonator, was supposed in the years 1920-1930 to allow radon to circulate in bath water and provide the young woman with the supposed benefits of radiation. At the time of the photograph, the radium from which radon can be extracted was so rare and so expensive that fake powders and balms were often used instead. At the time, radon was highly valued for its radioactive properties.
Musée Curie
In a country such as France, radon accounts for 34% to 54% of the population’s total exposure to radioactivity, depending on the region. This exposure varies considerably from one place to another. Granite soils, which are richer in uranium, release more radon than sedimentary soils.
Radon is an inert gas and does not undergo chemical reactions. It does not combine to form molecules, but it can dissolve in water.
The risk it poses comes mainly from its radioactive decay products. When inhaled along with radon, these solid decay products (lead-214, bismuth-214, polonium-214 and lead-210) emit radiation that can irradiate sensitive cells, such as those lining the bronchi, where they may be deposited.
Good ventilation significantly reduces the possibility of radon accumulating to high concentrations. In recent years, regulations have been introduced, particularly for house construction, to limit the risks associated with radon exposure.
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