Why do you change your HVAC air filters?
Is the air filter on your home's HVAC system permanent, once in a lifetime, set-it-and-forget-it?
Why not?
What makes you think your facemask is different?
Putting anything between you and the oxygen you need to inhale reduces the flow rate of oxygen. As detritus of all sorts is trapped by the mask material, flow rate decreases further, and further.
Similarly, exhaling carbon dioxide pushes detritus of all sorts the opposite way through this filter medium. As flow rate decreases, more carbon dioxide remains between you and the mask.
Every subsequent inhale pulls less oxygen and pulls more carbon dioxide into your lungs. Research hypoxia and hypercapnia.
Normal fresh air contains 21% oxygen, and 400 parts per million (ppm) carbon dioxide. OSHA states that oxygen levels below 19% and carbon dioxide levels above 5000 ppm are detrimental to you health, and can cause serious problems.
I have personally measured oxygen levels below 18% and carbon dioxide levels above 10,000 ppm while wearing surgical masks and bandanas.
Why do you change your HVAC air filters?