Dumb air is what happens when a room fills up with the CO2 you breathe out and nobody lets it escape. You can't see it. Your brain can.
Under 1000 is fine. Over 1000, the air is getting dumb. Over 2000, don't sign anything important.
Harvard researchers put people in rooms at different CO2 levels and ran cognitive tests. As the CO2 climbed, the scores dropped, across decision making, using information, and strategic thinking. At the levels found in normal, busy meeting rooms, some scores fell by close to half.
This isn't fringe. Ventilation guidance around the world treats CO2 as the tell-tale sign of a stuffy room: the UK's health and safety body flags anything consistently above 1500 ppm as poor ventilation, and stricter Nordic guidance treats 1000 ppm as the line. Your monitor is just reading you the same story in real time.
Think about the last time you pulled the blanket over your head and had to come up for air. That tight, foggy, get-me-out feeling? That's CO2. It's extreme and fast under the blanket, so you notice it. In a stuffy meeting room it's slower and quieter, so nobody says it out loud. Same gas. Same effect. Dumb air is just the blanket in slow motion.