The Decay Chain of Radon: Radon originates from Uranium-238 which naturally occurs in most types of granite and soil in varying degrees. The following table for uranium-238 shows the various changes. As it undergoes radioactive decay, a chain of products is formed as a result of one by-product itself decaying to another element, which in turn decays further until finally reaching an element that is stable. In this case that stable element is Lead. The element we're interested in is Radon-222 (there is another form, Radon-218, but the half-life of this isotope is only a few hundredths of a second, so it is less of a problem). This is produced roughly halfway down this decay chain from Radium-226. Radon is a particular problem because it is a gas, and as such can leave the surrounding rock and enter buildings along with atmospheric air.
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