[iofiiiiiiiii]

I have the unfortunate pleasure of being very sensitive to CO2 levels. At around 800-900 ppm I get a general feeling that something is abnormal. If it goes to 1000 ppm then I can usually tell that I am not thinking well anymore. It is a similar feeling to being slightly drunk - for example, I start a thought but find it difficult to carry it through to the conclusion.

Before I got a CO2 meter, I could always tell within hours when my building's flaky central ventilation fan had failed because I simply felt something was off.

Unless you are similarly cursed/blessed, just get a CO2 meter. I could not imagine learning this, it is like learning to feel temperature.