Respiratory therapist debunks mask danger disinformation

The page also offers these numbers to support the points it makes:

In another video currently circulating, it shows a gentleman measuring oxygen in the atmosphere (20.5%) and a measurement of 17% behind the mask. Im assuming the concern is breathing 17% O2 during inspiration. A normal breath is around 500cc. The volume behind the mask is about 10% or 50cc. When you take a breath, you breath in the 50cc of 17% O2 and 450cc of air at 21% O2. The net result is minimal and that's why it doesn't show up on a pulse oximeter. 2 minutes or 8 hours, it doesn't make a difference with MOST people. The EXCEPTION is people with preexisting heath issues.