http://wiki.answers.com/Q/What_does_radon_decay_into What does radon decay into?
222Rn is the only natural isotope of radon. (Several other artificial isotopes are known, but about the only place they exist is in the physics lab. Let's work with the natural one.) This isotope is itself the daughter of 226Radium, by the way. (226Ra had to decay to create 222Rn.) The decay scheme for radon is as follows:
222Rn will alpha decay (half life of 3.8 days) into 218Po
218Po will alpha decay (half life of 3.1 minutes) into 214Pb
214Pb will beta decay (half life of 27 minutes) into 214Bi
214Bi will beta decay (half life of 20 minutes) into 214Po
214Po will alpha decay (half life of 160 microseconds) into 210Pb
210Pb will beta decay (half life of 22 years) into 210Bi
210Bi will beta decay (half life of 5 days) into 210Po
210Po will alpha decay (half life of 138 days) into 206Pb (stable)
In case it isn't obvious, radon and its daughters are all radioactive and pose a hazard. (Save lead, 206Pb, the final daughter.) And because radon is a gas and is inert, it travels around in the air and can be inhaled. And an airborne radionuclide is harder to defend against and contain than a liquid or solid one. Radon is suspected of accounting for a high percentage of lung cancer deaths since exposure to radiation can cause cancer.
What really sucks is that if you breathe in a radon atom and it decays in your lungs, it changes into a polonium atom while irratiating you. You probably can't get rid of the polonium atom (it's a metallic solid), and it is also radioactive. An atom of radon must undergo 8 radioactive decay events to get to a stable isotope of lead. That means if a radon atom you inhale decays, you get that shot of radiation, and you will probably get 7 more shots of radiation - in the same general location - before things are over. Lots of biological damage can occur. And these decay events involve some very damaging particulate radiation (alpha and beta radiation). It's about the worst of the worst.