[M T]

He also told me wearing a mask too long makes you breathe in so much more C02 (don't know what kind he was referring to exactly), that your blood oxygen level goes down even if not sick, as a result of re-breathing so much exhaled air. He equated it to exercising at high altitude.

Hmm... 'Exercising at high altitude" doesn't sound right to me at all.  There are primarily 3 gases to think about in air: O2(21%), CO2 (< 0.1%), and nitrogen (78%).  Unless you're diving, we can ignore nitrogen.  Exhaled air consists of 78 percent nitrogen, 16 percent oxygen, 4 percent carbon dioxide

Masks have different amounts of residual air that stays inside the mask.  Some masks (most N95s) have a lot of volume inside, while others (disposable surgical masks) have little.   The larger the volume, the larger the surface area, so (for the material) easier to draw.   But the air in that volume will be exhaled air.  A little less oxygen, and a lot more CO2.

I would think a N95 mask contains maybe 5-10% of the volume of a breath.  At 10%, that becomes about 0.4% CO2 in your lungs.  That's well higher than normal air.  So your CO2 level in the blood leaving your lungs is higher.  The O2 level will be less, but I don't think that will be too significant (20.5% vs 21%), and could be considered some what related to being at slightly higher elevation (in terms of O2 molecules, not % O2).

As I understand it, your body more readily responds to CO2 levels in your blood.  Higher CO2 means faster breathing among other things.  But I imagine there ae chemical changes as well.

Now, if you breath faster, you might actually expose more O2 molecules to your blood.  But that's where my understanding breaks down.  I don't know whether the blood will preferably exchange CO2 for O2 in the faster breathing but 0.4% CO2 scenario.     (And I'm not sure what causes the dizziness that one gets from hyperventilating (remember its treatment may involve intentionally rebreathing your exhalation in a paper bag.)   I do know that at high pressure (ie, scuba diving), too much oxygen is a problem ("oxygen poisoning"), but I don't think that is a problem here.)

But I wouldn't automatically presume you'd have much less oxygen in your blood.  I do believe you will have more CO2, and that will likely be the important factor. (Possibly more important psychologically as well as physically.)  You may also be expending more energy just for the breathing.

I have decreased lung function.  When I go anaerobic due to exercise, I quickly devolve to a state where I feel I can't get enough air.  Psychologically that's very scary.

We do know that other cultures have been wearing masks a lot over the past 20 years.  I expect they've studied this, but I am not certain.

[M T]

He also told me wearing a mask too long makes you breathe in so much more C02 (don't know what kind he was referring to exactly), that your blood oxygen level goes down even if not sick, as a result of re-breathing so much exhaled air. He equated it to exercising at high altitude.

Hmm... 'Exercising at high altitude" doesn't sound right to me at all.  There are primarily 3 gases to think about in air: O2(21%), CO2 (< 0.1%), and nitrogen (78%).  Unless you're diving, we can ignore nitrogen.  Exhaled air consists of 78 percent nitrogen, 16 percent oxygen, 4 percent carbon dioxide

Masks have different amounts of residual air that stays inside the mask.  Some masks (most N95s) have a lot of volume inside, while others (disposable surgical masks) have little.   The larger the volume, the larger the surface area, so (for the material) easier to draw.   But the air in that volume will be exhaled air.  A little less oxygen, and a lot more CO2.

I would think a N95 mask contains maybe 5-10% of the volume of a breath.  At 10%, that becomes about 0.4% CO2 in your lungs.  That's well higher than normal air.  So your CO2 level in the blood leaving your lungs is higher.  The O2 level will be less, but I don't think that will be too significant (20.5% vs 21%), and could be considered some what related to being at slightly higher elevation (in terms of O2 molecules, not % O2).

As I understand it, your body more readily responds to CO2 levels in your blood.  Higher CO2 means faster breathing among other things.  But I imagine there ae chemical changes as well.

Now, if you breath faster, you might actually expose more O2 molecules to your blood.  But that's where my understanding breaks down.  I don't know whether the blood will preferably exchange CO2 for O2 in the faster breathing but 0.4% CO2 scenario.     (And I'm not sure what causes the dizziness that one gets from hyperventilating (remember its treatment may involve intentionally rebreathing your exhalation in a paper bag.)   I do know that at high pressure (ie, scuba diving), too much oxygen is a problem ("oxygen poisoning"), but I don't think that is a problem here.)

But I wouldn't automatically presume you'd have much less oxygen in your blood.  I do believe you will have more CO2, and that will likely be the important factor. (Possibly more important psychologically as well as physically.)  You may also be expending more energy just for the breathing.

I have decreased lung function.  When I go anaerobic due to exercise, I quickly devolve to a state where I feel I can't get enough air.  Psychologically that's very scary.

We do know that other cultures have been wearing masks a lot over the past 20 years.  I expect they've studied this, but I am not certain.