Where there has been a time that the CO2 level is seen as an indication for ventilation, nowadays CO2 is seen as a pollutant itself.
CO2 (Carbon Dioxide) occurs naturally. Before the industrial area, the concentration level was about 280 Parts Per Million (PPM). Since then, the level has increased to 0.04% (412 PPM).
It is important that the CO2 concentration remains less than 800 PPM. If concentrations get higher, humans are beginning to suffer. And CO2 levels rise mostly unmarked, as people are concentrated on their work. CO2 levels rise fast, but also too gradually to register immediately. You may not even recognize you’re not feeling unwell, until you get outside or feel really uncomfortable already. Probably you have experienced being in a ‘sick building’ yourself. When 1 person enters a room of about 4 m2, the level of CO2 may increase from 500 PPM (healthy) to over 1000 PPM (affects your well-being) in just 45 minutes.