Radon can make you very sick and has been linked to more cases of lung cancer than tobacco. Radon is a radioactive gas that is produced when uranium degrades. Uranium has a tendency to geologically ‘hang,’ with granite. This is significant due to the fact that the geology of granite and other minerals can be mapped.
The EPA has a fantastic resource for general knowledge on all kinds of different environmental issues. They have a radon exposure map that is based on a geologic map of the United States. Basically it categorizes areas of the US depending on what type of rock is found in that area.
Unfortunately this map is only a good guess. It cannot tell your home or any other home has a radon issue. The only way to determine if your home has a radon issue is to (shameless plug) get it tested. If you live in Salem, Oregon I just happen to know of a fantastic home inspector that can help….