Where I live in Central New Jersey, the Radon level has to be below 4.0 or you are required to install a remediation system if you want to sell your house.
When we bought our current home in 2010, the owners had to do the Radon test with the hockey puck looking thing in the basement.
They were told not to move it, or put a fan near it, and they were told not to open any doors or windows in the basement for 3 days during the test, and they were even told not to open the inside basement door to go down into the basement! Not even to wash their clothes! The Radon people said it would skew the test, and give a false low reading.
Anyway, the first test came back at 3.8. Even though that was below the 4.0 level that would require remediation, the Radon people wanted to do the test again, because they said the owners must have done something like open the doors or windows up. Yeah, right.
So, they did the test again, and again it came back at 3.8. The Radon people where not happy that it showed up as below acceptable levels, and they wanted to install a remediation system anyway, and charge the home owners $1500!!!
Since we were in the process of buying the house, we both (us as the buyers and them as the sellers) got our real estate lawyers involved, and they put a stop to the Radon people ripping off the seller of the house.
Even as long as a year after my wife and I moved into the house, we would get a few calls a month saying we should have the Radon system installed because it was too close to 4.0 to be safe. And every time the price got less and less. The last time they called us, the price was down to just over $500.
I think the whole thing is a bit of a scam too. If opening and closing just the basement door or one of the basement windows would skew the test and give a false low reading, according the Radon people who did the test, then how can the Radon be a problem when people are in and out of the basement multiple times a day? This alone would remediate the Radon to a point that it would never register anywhere near a 4.0 level.
I could see if you lived in your basement, and you were down there like 18 hours a day, with the doors and windows shut tight, that it could, potentially, be a problem. But for the average home owner, who goes in and out of their basement all the time, I just don't see it as anything more than a money-making machine for local and state governments.
Jim