[Radon's properties, Risks of radon, Radon in habitations]
Radon 222 is the mais 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.
The risk radon poses is mainly due to its highly radioactive descendants. Inhaled alongside radon, these solid radioisotopes emit radiation which can severely affect sensitive cells, such as those in the lungs.
A good ventilation system significantly reduces the lprobability of high concentrations of radon. Over the last few years, regulations about building construction work have been put in place to limit the possible risks of exposure.
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