Just small increases in the CO2 concentration can cause brain fog and headaches even if you won’t literally die from the room. Normally the feeling of a room being “stuffy” is the first sign. That’s a mere 600ppm where that begins to happen.

Here’s the basic levels of CO2 that are acceptable in a room. I know I get drowsy in rooms without good airflow when in meetings at the office.

https://www.engineeringtoolbox.com/amp/co2-comfort-level-d_1...