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masks only seal well on inhaling, when you exhale the mask pushes out some, and so exhaling only stops projectile droplets, inhaling filters air
since exhaling lets gases travel more freely there is far less constriction and so the fact that people have been wearing masks for a hundred years now on regular basis and not getting co2 poisoning should indicate that its not a problem
moreover the pain associated with holding your breath is actually caused by the co2 buildup (and not lack of oxygen), thus if there was a co2 buildup occurring it would be detected by the mask wearer as a painful burning sensation in the lungs.
This factor means that even if there was some small chance of co2 buildup occuring from mask wearing in some situations the mask wearer would realize something was wrong and be compelled by countless generations of evolution to seek fresh air reflexively.
