Radon is the second leading cause of lung cancer after smoking. It is a colorless, odorless, inert radioactive gas so unfortunately you cannot see it, smell it, taste it or feel it. Because of this the EPA recommends testing all homes and buildings for safe levels of Radon in the air.
Radon comes from the natural radioactive decay of radium and uranium found in the soil. The amount of radon in the soil depends on the composition of the soil. This is why it varies from location to location.
If your home has a level of 4.00 pCi/L of radon or lower then you are below the federal action level. If your home exceeds 4.00 pCi/L then it is generally expected that you should mitigate the level in the home to a level below 4.00 pCi/L for your health.