I just had radiation safety training last week (to use our portable XRF equipment) and we talked about Radon. Apparently, most of the rest of the world uses 20ug as the safety threshold. The EPA uses 4ug. So even if you are over that level of 4, you are generally safe. Also, that level assumes that you are living in that space for significant periods of time. So generally you wouldn't live in an unfinished basement long enough for it to be a problem.
Radon is a huge problem in Denver because we have a lot of roll-front Uranium deposits.
I had the sellers put in a mitigation system in 2009. They installed the pipe below the level that the sump pump would kick on, so if we have a high precip day, you can hear the ventilation system chugging on water. It's right under the master bedroom. It sucks. So I unplug it most of the time.