Carbon dioxide is one of just a very few air quality problems for which you have strong inborn biological detection systems. If you don't feel like it's suffocating you, it isn't.
Based on every article I found, above 1000 ppm clearly affects sleep quality, cognitive performance, it causes headaches, etc. so 3600ppm is generally considered really bad.
Even if CO2 would be harmless in itself, it is a good indicator for bad ventillation.
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.