How much radon is too much?

There is no global consensus on what constitutes an elevated or dangerous concentration of radon.

The Environmental Protection Agency (EPA) considers 148 Bq/m3 as the maximum safe concentration level of radon.

The European Union places the limit higher, at 200 Bq/m3.

In Canada, health officials state that 800 Bq/m3 is the highest acceptable concentration but a lower limit is currently being researched.

Erring on the side of the lowest, American, number would definitely be safest. Radon doesn't care where you're from. It's not an equal-opportunity health hazard.