I bought one of those because I have a couple radium clocks and a nice sample of uraninite, which is a tiny collection compared to the real enthusiasts but probably more radioactive stuff than the average person has. Radon is in the decay chain.

I've never seen above 0.7pC/L which is pretty good, although I don't know what proportion of the activity that is there is natural, or how much more it would be without the fan. I'm not sure what the natural radon levels even are in my area, I think the radon fan is just part of code here.