Radon is a radioactive gas that is created by the decay of uranium. It is odorless, you can't see it or taste it. The only effective way of determining it's presence is through testing. In WI. 1 out of 5,"20%" of homes test over the EPA guide lines and corrective action should take place in the form of mitigation.
Radon has been proven to be the second leading cause of lung cancer in the U.S. Only smoking causes more lung cancer deaths in the U.S. The 2 combined significantly increase the risk of lung cancer.
The EPA protocols are dependent on the purpose of the test, whether it is for a Real Estate transaction or for safe guarding in the home you now have. Renters also have the same right to know about the Radon level where they live. Please refer to the EPA: Renters Right to Know. The vast majority of testing is done at the time of Real Estate transactions which the EPA protocol calls for the lowest level that "may be" occupied, which in most cases is the basement.
The advantages of approaching Radon during the new home construction phase are pointed out in this guide. Cost advantages, aesthetic appeal vs breakouts, power, just to name a few.