[im3w1l]

Just thinking about the numbers. Exhaled air has 4% CO2, while fresh air has 0.04%. But if you inhale air with 3% CO2 that should add to those 4% yielding 7% exhale.

Now, in order to get 3% CO2 in your inhale this would mean that you would have to breathe in 42% stale air (with 7% CO2) mixed with 58% fresh air (~0 CO2).

I wonder if this 42-58 mix is plausible.

[kadoban]

That would mean that the volume of gas trapped in a mask is 42% of lung capacity? Seems extremely implausible to me.

[im3w1l]

A typical breath is much less than full lung capacity, but even then I don't think it adds up.