It's obvious masks reduce oxygen intake by a significant amount

In this test simple test, the reduction from 20.6 to 17.4 means the amount of O2 you get has been reduced by 15.5%. There's a reason OSHA gives 19.5 as a minimum.

https://d.tube/#!/v/markdgold99/QmTMUUEmA7BCKLcvj9Vswp96gL2jAAPYJxLkpETt8Dv3co

https://twitter.com/InAMadWorld111/status/1302065401271144451?s=20

Dangers of oxygen-deficient atmospheres
[the air under a mask, at 17.6 oxygen, would qualify as a highly "oxygen-deficient atmosphere"]
https://www.airproducts.com/~/media/Files/PDF/company/safetygram-17.pdf

In Huxley's Brave New World, the lower the caste of the embryos, the less oxygen they get (h/t @helvetico2)
A mask may bring your oxygen intake down between 15-19% (maybe more with the N95) so you're getting close to some of those hypoxic Epsilon fetuses