NeverLoseTruth channel:
Ohio State Senator Nino Vitale Demonstrates Oxygen Levels Dropping Inside A Mask.
The GX 2009 made by RKI “instant oxygen sensor” beeps when oxygen levels drop below the 19.5% oxygen level recommended by OSHA (Occupational Safety & Health Administration) for healthy air.

The sensor starts out at 20.9 in the open air surrounding Senator Vitale. Next he brings in a series of volunteers wearing masks, to see what oxygen levels are inside the masks.
For comparison he tests the exhaust pipe of an automobile and tests holding the detector up to his face without a mask (to prove it’s not just proximity to a face that made the sensor beep.