An Anaesthetist explaining how stuff works: a thread.....
There has been a lot on Twitter about whether wearing masks reduce your oxygen levels. Some of it, depressingly, from people who should know better: (1)

(2) so let me, with 20 years of living, and managing respiratory physiology, try to explain......
We breath in around 500-600ml of air with an average resting breath. Of this, 21% (roughly 100ml) is oxygen.

(3) Take a restful 12 breaths a minute, and you are breathing in 1200ml of oxygen a minute (that’s 100ml x 12 breaths) . However, all that you breathe isn’t useful. The last 100ml you breathe never gets to the lungs. It’s in your throat and windpipe.

(4) This “dead space” is gas that is inhaled but never gets to the bits of the lung where it can enter the bloodstream. That means of the 500ml you inhale only 400ml gets into the lungs. And 21% of 400ml is roughly 80ml of oxygen. So 12 breaths still delivers 80x12= 960ml oxy/min

(5) with me so far? We breathe in 960ml of usable oxygen a minute. Now, a very average PhD person like Zoe sitting at her desk only needs about 300ml of oxygen a minute to oxygenate her blood. So there is three times more oxygen in her lungs than she needs at rest. So the mask?

(6) The mask allows free flow of gas through it. It is not a plastic bag. Otherwise Zoe would suffocate. But it’s made of paper. Paper which does not magically filter out the oxygen, but let the nitrogen through.

(7) so as long as Zoe takes her normal breaths, she will get exactly the same amount of oxygen into her lungs as anyone else. And if she asks “Nick, what about the recirculating of gas I breathe out”? I would point to the diagram and say.....

(8) That the extra 20ml of air trapped in the mask would add as much dead space as the difference between the windpipes of 5’2 Zoe and 6’2 Nick. So no, it would not reduce your oxygen or increase your carbon dioxide which is why .....

(9) people that work in operating theatres have been wearing masks for 5,6 even 7 hours at a time able to perform complex tasks with no measurable drop in their oxygen saturation. So please Zoe, don’t use your blue tick and PhD to lie to us all.

So please, wear a mask if you are worried about spreading COVID but do not worry about not getting enough oxygen. Even with lung disease, asthma, COPD, a paper mask will not make a difference to how much oxygen you inhale. COVID will. The end