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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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.