The role of radon
Continuation of the uranium decay chain. Radon, which, being a gas, escapes from its underground prison, decays comparatively quickly into lead-210: a radioactive substance which survives for decades. The chain finally ends with the stable isotope lead-206. If the radon source is deep underground, then radon may not have the time to reach the surface; in which case, the radioactivity stays in the soil. If the radon does manage to break through, however, some of its decay products can be harmful to human lungs.