cpappy

I just got a CPAP machine and wasn't sure exactly why CO2 didnt build up until I reseached it and discovered masks have vents. Now I remember a discussion I had a long time ago with one CPAP user who had problems with his mask marking his nose because he could sleep with his mask while his face was in his pillow. Now I realize this must have blocked his mask vent. What prevents someone inadvertently sleeping with a blocked mask vent?