Prof. Dr. Konrad Botzenhard, Hygiene-Institute of the Tübingen University
People must breath to extract oxygen from the air and exhale the carbondioxide produced in the body. For this gas-exchange, the human lung has an available capacity of approximately 80 m². During breathing the air in the lung-alveolus is only exchanged with fresh air to the extent of approximately 15 to 30 per cent, so that gas concentration during breathing only differs slightly here. The oxygen partial pressure in the alveolus air lies by 98 torr as opposed to 150 torr in the inhaled air.
The oxygen content in the arterial blood changes so little, up to 80 torr partial pressure in the alveolus air, that minor deviations in the oxygen content of the indoor air have only little effect on breathing. It is almost the same with the carbondioxide concentration. In the alveolus this lies at approximately 40 torr or approximately 5 per cent vol. so distinctly above the concentration of the outside air of approximately 0.03 per cent vol., minimal deviations in the outside air have no meaning in respect to breathing.
The Pettenkofer-Value of 0.1 per cent vol. seen as the margin of acceptable CO²-concentration is only of significance as an indicator for the pollution of the indoor air regarding human evaporates, but has no significance to breathing functions. During breathing, humidifying and warming of the air takes place in the upper respiratory tracts , where only a much smaller area is available. Especially in winter, the drying out of the mucous membranes is often complained about.
Due to this, it seems sensible to guarantee a minimal air humidity of 30 to 40 per cent indoors. Furthermore, particular harmful substances in the respiratory tracts should be held back and transported out, without them reaching the alveolus and hereby passing into the body. For this purpose, the bronchial pulmonary system including the mucous membrane of the nose has ciliary epithelia and gland cells for the production of mucous.
The overproduction in this system often leads to colds, it is therefore an important quality criteria that the air is free from dust. This especially applies to biological active dust, for example micro-organisms and allergens. The dust content of the air is dependent on the relative humidity. Chemical irritations of the respiratory tract as well as odours can have an effect on the innervation of the respiratory tracts through the autonomous nerve system. Generally this is indicated in a constriction of the respiratory tract and even involuntary holding of breath and an increased blockage of the respiratory tract.
The evaluation of the quality of odour of indoor air is the basis for the so-called Olf-Concept, which has since found its way into standardisation. It seems possible that through the increased use of plants indoors factors such as relative humidity, particle content and odour quality can be positively influenced.