Breathing with a snorkel mask
With a snorkel mask, the mouth and nose part is closed and you have separate areas for breathing in and out. If you exhale, you do not exhale the air through the valve, but via the side channels through a pipe. This prevents you from breathing in your own air again. The air goes out through the glass. An additional advantage of this is that you do not get fogged glass.
The silicone frame encloses the entire face, from the chin to the forehead. This ensures a watertight seal. The snorkel is equipped with a waterstop. At the moment your snorkel goes underwater, the waterstop ensures that it is sealed airtight. This means that no water can enter the mask through the snorkel. Should there be some water behind the mask while snorkeling, then you can use the blowout valve. By blowing out loud through your mouth, you push the water out through the valve.