The descendants of radon
An atom of radon successively decays into atoms of polonium 218, lead 214, bismuth 214 and polonium 214, all four of which can be inhaled. Owing to its very short half-life (16ms) polonium 214 does not deposit directly into the lungs, but the other three can all enter the body and continue the decay process until they reach lead 210. This isotope of lead is itself radioactive, but can be viewed as the final product of radon decay. It has a half-life of 22 years, but its concentration is very low and its activity negligible.