> And regardless: there's not NEARLY enough air space behind your mask to store a full (~1L) exhalation. What you claim isn't even physically possible. Accordi

> And regardless: there's not NEARLY enough air space behind your mask to store a full (~1L) exhalation. What you claim isn't even physically possible. According to Wikipedia, normal level exhalation (tidal) volume is ~500 ml and has ~5 % CO_2. If only 10 % of exhalation (50 ml) is breathed back with 90 % of fresh air, then one is breathing equivalent of ~5000 ppm CO_2. I am sure that improvized cloth facemasks have bigger air 'buffer' than this. Although measure it precisely would be hard.