[thaumasiotes]

> Based on every article I found, above 1000 ppm clearly affects sleep quality, cognitive performance, it causes headaches, etc.

Sleep quality and cognitive performance are often difficult to perceive in yourself.

Headaches aren't like that at all. If every article you found states that CO2 above 1000 ppm causes headaches, that should reflect poorly on everything else in all of those articles.

> Even if CO2 would be harmless in itself, it is a good indicator for bad ventilation.

This is perfectly true.