Radon is a colorless, odorless, radioactive gas. According to the EPA:
Radon comes from the natural (radioactive) breakdown of uranium in soil, rock and water and gets into the air you breathe. Radon can be found all over the U.S. It can get into any type of building — homes, offices, and schools — and result in a high indoor radon level.
To protect the Savoy Passivhaus from radon, pipes have been sandwiched between two layers of heavy gauge polyethylene and buried below the slab. Any radon that comes up through the soil will be contained and vented out through these pipes.