Because you pass out at what like 5-10% oxygen considering 14,000 feet is around 12% oxygen. Earth’s atmosphere at sea level 21%. Doctors are working in an environment that is what I said 17% with the standard mask on with no exhaust vent. Wearing a mask puts the oxygen levels below the OSHA standard OSHA requires for a SAFE work environment of 19.5% oxygen as defined by them. learn about OSHA “law” micron size and you have to get your mask fitted and it has to be a silicone seal if you stand any chance at the mask being effective against the virus