The fact that the dead space air doesn't meet OSHA standards for ambient air is a red herring. When you breathe out, you're filling up the mask with your exhal

The fact that the dead space air doesn't meet OSHA standards for ambient air is a red herring. When you breathe out, you're filling up the mask with your exhaled breath. When you breathe in, you'll breathe in most all of the dead space air, and then fresh, filtered air. They measured tidal volumes of about 900ml, and the average N95 mask dead space is 125ml. Therefore, about 15% of the inhaled air will be air that was previously exhaled. What they found was that the dead space CO2 was about 3000ppm. Suppose they're indoors and CO2 is 700ppm. The mask increases CO2 to 1045, which is still well within an acceptable range.