Just for clarification, that's without a fan. If you build a home with the pipe that goes up to the ceiling and out the roof, and put in the fan, you're very likely to lower your concentration to negligible levels. If you just build the pipe and don't build the fan, you don't know what will happen.
The other thing is that because it's a passive system, you might help a bit or you might not. In some cases, you might even increase the level of radon in your house, especially if you open up the pipe and use it for your sump pump or a drain of some sort, as people do without knowing what it's for.