Radon is interesting because not a lot of people think about it, yet it's very dangerous. It's almost everywhere in France, Germany and Scandinavia. If you don't have anti-radon measures in the mentioned countries, high chances are levels are exceeding the limit.

You can get activated charcoal filters to help. But just having positive pressure (ideally with the intake vent a decent distance from the ground, since radon is quite a lot heavier than air) should help, because radon usually seeps in through the foundation due to pressure in the ground, and the fact that lower levels of houses are often natural at a slight negative pressure.