Below we offer a thought experiment making the following four assumptions:
Assumption 1: Fresh outside air has the oxygen concentration of 20.9%.
Assumption 2. Typically, exhaled air has a lower oxygen concentration of about 16%.
Assumption 3: A human wearing a mask, at each breath inhales the air mixture containing 50% of fresh air coming through the mask and 50% of the
Assumption 4: The human wears the mask while at controlled activity (e.g., at rest) for a prolonged time allowing to reach a plateau in all
body’s physical parameters.
The table below shows the results of our calculations:
As the air properties stabilize with time, at the Breath Number 7 (Plateau), the oxygen concentration of the inhaled air under the mask approaches and stays at about 15%. Interestingly, this concentration corresponds to an altitude of about 8,500 feet, quite an elevation!
Accepting the above assumptions and the estimates using them, it seems plausible that a prolonged wear of the mask should lead to triggering in the human body of some adaptive measures offsetting the oxygen deprivation caused by the mask. The extent of the resources needed to reach this adaptation is a subject of a different discussion.
The effects of the altitude on humans may be instructive, even with the oxygen concentration under the mask predicted to be way below of the dangerous levels: