There's no level of radon that has zero risk. (Note I didn't use the words "safe", which is a loaded word).
That sounds scary... we all want zero risk, right? Unfortunately, zero risk doesn't exist. Living is risk.
The official EPA number is 4 picocuries/liter. If you have a level above this, the EPA recommends you lower it below 4, and is called the "action level".
The "action level" numbers are different in different countries.
In the EU, this number is 8 picocuries/liter. In Canada the number is about 5.4 picocuries/liter. They used to be even higher in the EU and Canada.
The WHO has recommended an action level of 2.7.
That's crazy, right? Each country has the same data about the risks of Radon, has smart people making decisions, but the "recommended action level" varies by a factor of 2 or 3! Why?
Honestly, the numbers likely say more about each country and its culture than they do about the dangers of radon. Herein lies the problem with "no level of radon has zero risk". What does the official in each country say is "acceptable risk"? Zero is essentially impossible, since even outdoors you're being exposed to .4 picocuries/liter. So they pick a number based on whatever they arrive at as "safe".
So I can't tell you what "safe" is, since "safe" is relative (and obviously so since each country has decide different levels of safe). This is something only you can decide.
Just take into account that it's not like you're going to get cancer if you have a level of 7, and you're completely safe of you lower that number to 3.9. Neither are even remotely true. Canada, for instance USED to have a level of 22 picocuries/liter before 2007. I don't recall hearing about an epidemic of lung cancer in Canada in pre-2007.