We seem to know:
- Elevated CO2 in rooms impairs cognitive performance.
- Elevated CO2 in submarines, at levels far higher than you would see in a normal room does not appear to impact cognitive performance.
- Installing carbon filters (what this study actually looked at) might improve classroom performance.
- People don’t like stuffy rooms.
All this is consistent with multiple hypotheses. It could be that we just don’t know anything about it. Or maybe there is some gas or gasses emitted by people that isn’t CO2 that makes people mildly uncomfortable and have worse cognitive performance.
CO2 is certainly a good proxy for ventilation quality in a space where air is exchanged with outdoors but where the gasses in the air are not otherwise changed. Carbon-filtered classrooms and submarines are not examples of this.