I had a little accident with my Chef knife and went to emergency room to stop bleeding.

Of course thy were taking my vitals and my nurse instructor wife noticed that when I had the mask off, my O2 level was 98% but 10minutes with the mask on, the O2 level dropped to 94%.

Since we were there over an hour, we tested it several times and it continued to rise and fall with mask off and on.

And I'd get short breathed after the mask was on a half hour.