I’ve said this before about masks. When I was at work (I’m an RN) and had to wear a mask I used an oximeter and measured my oxygen saturation. It normally is 98 but is constantly with a mask on 86 and lower when walking. Medicare puts one on oxygen when its below 88. It’s because below 88 you are not getting enough oxygen to your vital organs…brain, heart, lungs, kidneys…
I do the same thing out of curiosity. I’ve seen it go into the 70’s after wearing a mask for a short period of time.