CO2 - the most important air quality indicator
Most people spend more than 20 hours a day in closed rooms. The CO2 content (carbon dioxide content) of the air is measured in % by vol. or in ppm (parts per million) and is the most important indicator for the quality of the air. The Joint Research Centre JCR of the EU was able 
to reveal in a study that interior rooms are subject to a higher level of pollution than the outside air. Concentrations which, in the long term, are harmful to health are not seldom.
A study according to Pettenkofer proved that people feel comfortable in rooms with a CO2 concentration below 0.1% (1,000 ppm), but feel clearly uncomfortable at levels above 0.2% (2,000 ppm).
The limit value for living areas in Germany is 0.15% (1,500 ppm) CO2. In comparison, often concentrations of up to three times as high (up to 5,000 ppm) can be measured in unaired bedrooms and also in fully occupied classrooms!
To limit the CO2 content to a maximum of 0.15% (1.500 ppm), an average of 25 m³/h of fresh air must be supplied per person, or even more if sport is exercised. Although we actually require only a tenth of this amount for the oxygen supply and for our metabolism, relatively high fresh air flow rates are necessary in order to discharge the CO2-loaded and polluted air. The easiest and most energetically effective way (with heat recovery from the exhaust air) of implementing this is with a convenience ventilation system.