My understanding is the CO2/O2 thing is almost completely a red herring/non-issue. Firstly of course any mask or filter is going to let through O2 and CO2 molecules completely indiscriminately since they’re far too small to be affected. And secondly you always breathe in some of the air you breathed out, since it’s still in your airways. In the worst case, adding a mask would increase this re-inhaled amount by the volume of the space between the mask and the face, which is pretty small. So breathing through a mask is like breathing through a tube with the same inner volume as the inside-mask space—a regular swimming snorkel results in much more re-breathing, and is also not a problem. It wouldn’t surprise me if some people are re-breathing more without a mask than others do with a mask, just because they have a longer neck or larger airways.
My uneducated concern is that masks that are not intended to seal may not allow air to flow sufficiently easily through their “filter” part (without it turning out to be a problem during “normal” use due the air easily flowing through the edges). Re volume argument, maybe we also need to consider the volume of the air we inhale each time (and whether that volume becomes smaller if something is partially blocking the air flow, and whether we notice).
My understanding is the CO2/O2 thing is almost completely a red herring/non-issue. Firstly of course any mask or filter is going to let through O2 and CO2 molecules completely indiscriminately since they’re far too small to be affected. And secondly you always breathe in some of the air you breathed out, since it’s still in your airways. In the worst case, adding a mask would increase this re-inhaled amount by the volume of the space between the mask and the face, which is pretty small. So breathing through a mask is like breathing through a tube with the same inner volume as the inside-mask space—a regular swimming snorkel results in much more re-breathing, and is also not a problem. It wouldn’t surprise me if some people are re-breathing more without a mask than others do with a mask, just because they have a longer neck or larger airways.
My uneducated concern is that masks that are not intended to seal may not allow air to flow sufficiently easily through their “filter” part (without it turning out to be a problem during “normal” use due the air easily flowing through the edges). Re volume argument, maybe we also need to consider the volume of the air we inhale each time (and whether that volume becomes smaller if something is partially blocking the air flow, and whether we notice).