Radon is a naturally occurring gas produced by the radioactive decay of the element radium. Radon itself is radioactive and it decays to form the element polonium. Polonium is also radioactive and it is this element, which is produced by radon in the air and in people's lungs, that can hurt lung tissue and cause lung cancer.
Radon is ubiquitous (usually in small amounts) in rock and soil and can be carried in water, air, and in natural gas. Some rock types have the potential to produce higher than average amounts of radon gas.
USGS research on radon has mostly been limited to its presence in groundwater. The Environmental Protection Agency (EPA) is the best starting point for information about air quality and radon in buildings.