What are the health symptoms of to much radon gas in a residence?

First, radon is an inert gas that naturally occurs from uranium decay in the soil and can eventually build up in houses, in the air, and settle into water and food supplies. It is important to first note that there is no proven studies that show that household exposure to radon is linked to lung cancer. However, it has been well-established that unusually high levels of radon exposure are linked to lung cancer. The question is whether ambient household levels of radon pose a health risk to people in general.

The body has some defenses to get rid of radon, but over time radon can accumulate within the body. Once in the body, any kind of radioactive substance gives off radiation that directly harms surrounding tissue. Since radon most often enters the body through the lungs, lung cancer is often the first manifestation of radon gas ingestion. Other cancers can occur due to radon including cancers of the internal organs, such as stomach cancer, most likely depending on the source of radon.

Lung Cancer, but you don't want to wait for that symptom to find out.
Here is a link to the EPA web site. It should answer any further questions:
http://www.epa.gov/iaq/radon/pubs/citgui...