First of all there are really no safe radon levels since it's a radioactive gas that causes lung cancer. But with that said we need to recognize that we all live life with some risk that is unavoidable. For instance the average outdoor level is 0.4pCi/L thus it's impossible to completely eliminate our exposure to radon gas. But we do have options if our indoor radon results are elevated. So what's a high radon reading?
The EPA set 4pCi/L as the action level for the United States back in 1988. This is not health based level as they are quick to add. Homes with readings between 2pCi/L and 4pCi/L should consider mitigation also. As of 2009 the World Health Organization set their threshold to mitigate at 2.7pCi/L.
So what is the risk of having a level 4pCi/L in your home.
Risk at 2pCi/L
As your radon number doubles so does your risk!
So at 10pCi/L your risk is like smoking more than a pack a day!
How to interpret your radon results
Radon readings will vary throughout the year. Highest levels are typically recorded in mid winter with levels decreasing to there lowest concentrations in summer. So 4pCi/L in summer could be considerably higher in December. Thus an average of well above the EPA action level can be assumed.
Where as the the opposite may be true of a level of 4pCi/L in winter. Thus the only way to really know for sure is conduct a long term test to account for the fluctuations through out the seasons.
Why the variation in levels?
Also to consider is the time spent in the lowest level of the building. If your level is 2pCi/L in a basement used only for storage then it's likely that your exposure will be low. But if you have a forced air furnace and central AC system this could create the same level on all floors of the home when in operation. To be sure of your exposure a long term test kit will give the best picture of overall exposure to radon in your home.