Radon
is a chemically inert, but radioactive gas released as a radioactive decay product of radium, itself a product of thorium decay, which is in turn a product of the radioactive decay of uranium. Uranium and its decay products are found in small, but significant, quantities in granite.

Radon has a short half life of about 4 days, and decays by releasing alpha particles. It's dangerous because it's a gas, and it therefore can release its alpha particles right into the lungs. Alpha radiation is normally not that dangerous because of its very short range. But in the lungs, it can cause all manner of problems - primarily lung cancer.

For this reason, basements of granite houses should be well ventilated to avoid the build up of radon gas.

Here endeth today's science lesson.
( K2k6 has a proper job these days, Wed 15 Oct 2008, 14:32, Reply)