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Radon is a radioactive gas that seeps into houses from the ground. It's very easy when you're building to install a passive vent to exhaust gases from beneath the house through a vent in the roof. We ran a 4" PVC pipe from the gravel beneath the basement slab through the roof and also installed a electrical junction box near the pipe so that we can later install a fan if radon levels prove to be too high.
To vent the radon, a 4" PVC tee is buried in the gravel beneath the slab near the middle of the house. I drilled a lot of holes in the tee so that air from the surrounding area can enter the pipe. As air moves into the pipe, it leaves behind a negative pressure zone around the tee, which causes air in the gravel to constantly move toward the tee and then be vented through the roof. It's pretty simple and cheap protection when you're first building a house. It's obviously more difficult and expensive if you have to retrofit it.