Unfortunately, we're not sure. On one hand, since radon doesn't have any health benefits, the ideal safe radon level is zero: no radon exposure. On the other hand, there are levels under which exposure to radon, while not good, is rather negligible.
What is this boundary between negligible and harmful or lung cancer causing?
That depends on which experts you ask!
In the United States, for example, the Environmental Protection Agency (EPA) deems the limit, expressed as maximum exposure per year, to be 148 Bq/m^3.
In Britain, and across the European Union, the limit is slightly higher: 200 Bq/m^3.
While, in Canada, the dividing line between safe and harmful is much higher. Canadians, according to Health Canada, are safe unless they're exposed to more than 800 Bq/m^3!
In other words, we don't quite know what the magic number is. Or if there even is one.
But, erring on the side of caution—in this case the US number—seems like the sensible thing to do, whether you're in the US, Britain or Canada. It's your health, after all.
What are safe radon levels?