Is there any level of radon in the home that is *immediately* dangerous?
According to this Radon Dose Calculator, if you are exposed to 30 pCi/L the dose rate is 6.275 µSv/h.  Multiply by 30 days and 24 hrs/day to get 4500 µSv, or 4.5 mSv dose over 30 days.  

Wikipedia suggests that is similar a CT scan (which is a significant dose).  You are probably not in the house 24 hours a day so you could adjust the numbers downward.  

Here's another nice visualization of different doses.