Its been proven they dont reduce oxygen
Actually they’ve been proven to reduce oxygen but not to dangerous levels for healthy people:
“Preliminary report on surgical mask induced deoxygenation during major surgery” pubmed.ncbi.nlm.nih.gov/18500410/
Our study revealed a decrease in the oxygen saturation of arterial pulsations (SpO2) and a slight increase in pulse rates compared to preoperative values in all surgeon groups. The decrease was more prominent in the surgeons aged over 35.
“COVID-19 prompts Stanford engineers to rethink the humble face mask” news.stanford.edu/2020/04/14/stanford-researchers-reengineer-covid-19-face-masks/
Based on literature and confirmed by our measurements, N95 masks are estimated to reduce oxygen intake by anywhere from 5 percent to 20 percent. That’s significant, even for a healthy person. It can cause dizziness and lightheadedness. For healthy people, these side effects are temporary and usually not an issue. But if you are severely ill and are continuously wearing an N95 mask for several hours at a time, it can damage the lungs. For a patient in respiratory distress, it can even be life threatening.
Even where Stanford “debunks” the “myth” that masks reduce oxygen they are careful to state it doesn’t reduce oxygen for healthy people who are breathing normally.
“Myth: Face masks can reduce oxygen getting to the lungs and bloodstream:
digitalmedic.stanford.edu/myth/myth-face-masks-can-reduce-oxygen-getting-to-the-lungs-and-bloodstream/
“This is not true for healthy people wearing surgical or cloth masks....Masks should not be worn by children under the age of 2 years or by anyone with breathing problems. Remember, if you are healthy and breathing normally, a mask will not lower your blood oxygen levels.”
The mask not lowering oxygen levels only applies to the healthy who are breathing normally. It doesn’t apply to everyone, especially those with breathing problems.