Cheers, Ian. Oxygen depletion is guaranteed - our airways are open to the elements for a perfectly logical reason - to facilitate the free and unrestricted intake of essential oxygen into the body. You don't need to be any kind of specialist to work that out - - it doesn't take too much research to understand that which happens when the body is denied a natural and unrestricted intake of oxygen. It's as stupid as blindfolding people and expecting them to see with their usual clarity.
There are notionally exemptions - but there's masses of filmed evidence of 'mask exempt' people being abused and violently assaulted by police and masky punters alike - you abstain from mask use at your own peril - and imperil yourself if you comply - it's lose/lose. But my contention is that whilst there are people with conditions that are more readily triggered by oxygen depletion, the fact is masks harm the health of everybody. It's another one of those unsatisfactory trade-offs - wreck your own health in this manner in order to prevent wreckage of health in a different manner.