John, the originally identified area of concern for radon was the Reading Prong. In Pennsylvania.
We have a significant radon problem in our area, and it's very interesting to read that people in widely separated areas of the country do too.
For those who want to test, you may be able to get an inexpensive test from your local health department. There are several methods of testing, with longer duration tests probably more reliable than the shorter ones, but the shorter ones are reasonable screening tests. Personally, I would be cautious about relying on a mitigation company to do the test--they have an inherent bias in favor of finding, and fixing, a problem.
Curiously, this is a situation where an old, leaky house is an advantage. The more the air in your house turns over, the less the radon accumulates. Since maybe the late 1970s, houses have been built much tighter, to conserve energy, with the unfortunate consequence of trapping more air and increasing radon levels.
It's a little like dust collection and respiratory protection. It's a very sneaky problem and you don't know you've got it until someone gets cancer. Check your local health department or extension office to see if radon is an issue in your area and get a test. It's cheap and the potential harm is considerable.
PS It's not just the basement. The radon seeps up from the ground, but it gets all over the house.