My GF sent me a couple of videos of doctors "proving" that masks do not affect blood oxygen levels. In one of the vids the doc wore six masks with no change in blood O2 and that looked impressive but I suspect there's plenty of air leaking around the mask. In both vids subjects were sitting in an air-conditioned environment.
A couple of weeks ago I helped a friend move some furniture and other stuff into a new house. I was wearing a cloth mask and it became drenched with sweat. At one point I was almost gasping for breath. I had to stop and take the mask off to recover.
I think I was waterboarding myself with the sweaty mask. I'd like to see the docs repeat those videos outdoors on a warm day while doing moderate exercise and see if there's any difference in the O2 readings.