Ventilation and CO2 Buildup: Why Stuffy Rooms Cause Headaches
A stuffy room gives you a headache, and carbon dioxide is probably not the direct cause — but it is the best single number you have for diagnosing the problem. Outdoor air sits around 420 parts per million. A well-ventilated occupied room stays below about 800. A closed bedroom with two sleepers and the door shut routinely reaches 2,000 to 3,000 by morning, and a packed meeting room with no fresh air supply can pass 3,000 within an hour. At those levels CO2 itself is far below any threshold for direct physiological harm, but it tells you that a large fraction of the air you are breathing is exhaled air, along with everything else that accumulates when a room is not ventilated.
That distinction between cause and indicator is the crux of the subject, and getting it right changes what you do about it. If you treat CO2 as a poison, you look for something to remove it. If you treat it as a gauge, you look for fresh air — and fresh air also removes the volatile organic compounds, odours, humidity, airborne particles and pathogens that were building up alongside it.
The practical upside is that this is one of the few headache triggers you can measure cheaply, verify quickly and often fix for nothing.