Oxygen and CO2 levels

Occupants breathing and depleting oxygen is never a condition for setting the ventilation rate of housing. A person requires between 0.005 and 0.0075 litres of O2 per second. Because air consists of nearly 22 per cent oxygen, the required flow rate of O2 is adequate with a mere air exchange rate of 0.082 to 0.123 m3/h per person. This is a factor 300 to 400 smaller than the 20 to 40 m3/h of fresh air per person commonly recommended. This latter recommendation is derived from the so-called Pettenkofer limit (Max von Pettenkofer was a well-known researcher on human physiology at the University of Munich in about 1870), postulating that about 1000 ppm (volume) of CO2 as indoor air concentration is an upper tolerable limit for comfortable indoor air quality. The corresponding value of the overall air exchange rate depends on the outdoor air concentration of CO2, which is actually at about 360 ppm (volume). With an average indoor air volume per person between 75 m3 and 100 m3 (German dwellings), the above-mentioned volume rates of air exchange per person are equivalent to air change rates between about 0.25 h-1 and 0.50 h-1.

This same air exchange rate not only supplies (much more than required) O2 to the system and maintains its CO2 concentration within the desired range of values, but it also removes much of the water vapour that is produced inside dwellings.

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