The carbon dioxide is only replaced in the volume behind the mask. If that is not a big volume compared to a full breath (and it isn't), then the carbon dioxide you're breathing in isn't 10%. It's (v_mask * 0.21 * 0.5)/V, where v_mask is the volume behind the mask, and V is the full volume of a breath.
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The model only tests how oxygen and carbon dioxide replace each other, not nitrogen. So 50% of the 21% is oxygen, the other 50% is carbon dioxide.
Of course, it could be even worse, that carbon dioxide starts replacing the nitrogen behind the mask. More testing needed.
10% CO2 (> 40000ppm) is lethal. Have I been brainwashed to ignore the people dropping dead on the streets?
The carbon dioxide is only replaced in the volume behind the mask. If that is not a big volume compared to a full breath (and it isn't), then the carbon dioxide you're breathing in isn't 10%. It's (v_mask * 0.21 * 0.5)/V, where v_mask is the volume behind the mask, and V is the full volume of a breath.