Carbon dioxide

The human body permanently emits a large number of organic substances such as acetone, alcohols and odourous substances to indoor air. These substances may be responsible for fatigue, irritated mucous membranes and other symptoms. As the entire range of substances is hard to capture analytically, usually only carbon dioxide (CO2) levels are measured as they can be determined easily.

Humans and animals exhale CO2. As CO2 levels increase, emission of all other substances also goes up. This is why CO2 serves as an indicator. The substance as such is completely harmless even at the usual indoor air levels. In airtight modern buildings CO2 concentration may quickly exceed levels of 1000 ppm, which is always an indicator of insufficient ventilation.  CO2-Schlafzimmer

Reference value 1000 ppm

The target is not to exceed CO2 levels of 1000 ppm. 1400 ppm is the upper limit for acceptable indoor air quality. To reach this level, active ventilation is usually required every 1 to 2 hours. Comfort ventilation systems automatically ensure that this level is not exceeded.


Ventilation engineering methods use CO2 to find out if rooms are sufficiently ventilated and to define quality criteria for indoor air (see Austrian reference values for CO2 and ÖNORM (Austrian Standard) EN 13779).

For mechanically ventilated rooms, the Austrian Ministry of Life requires a target value of less than 800 ppm and a maximum value of 1400 ppm.


Rising CO2 levels lead to higher risks of infection

Meanwhile, various investigations have confirmed another assumption by Pettenkofer: CO2 is an indicator of the overall indoor air quality in rooms. Where CO2 levels are high, the germ count is particularly high. In 2003, American scientists Rudnick and Milton, for instance, analysed the risk of catching influenza in a classroom. 30 individuals spent four hours in a classroom, one of them already suffering from the flu. The result: At 1000 ppm of CO2 five, at 2000 ppm twelve, and at 3000 ppm as many as 15 individuals caught the disease.

 CO2-Grenzwerte Tabelle