Without ventilation, a building’s occupants will first be troubled by odours and other possible contaminants and heat. Humidity will rise, thus enhancing moisture hazards (e.g. mould growth and condensation). Oxygen will not be missed until much later. The purpose of ventilation is to eliminate airborne contaminants, which are generated both by human activity and by the building itself. These are:
• bad odours, to which people entering the room are very sensitive;
• moisture, which increases the risk of mould growth;
• carbon dioxide gas, which may induce lethargy in high concentrations;
• dust, aerosols and toxic gases resulting from human activity, as well as from the materials of the building (in principle, ‘clean’ materials should be chosen for internal use, but this is not always possible);
• excessive heat.
If there are several contaminants, the calculation is performed for each contaminant. The airflow rate corresponds to the largest calculated value. Since, at a given airflow rate, the indoor pollutant concentration is proportional to pollutant source intensity, indoor air quality can be greatly improved at low cost by avoiding or reducing indoor air pollution sources. In well-designed buildings, during the heating season the occupants are the main source of contaminants (mostly odours and water vapour). The airflow rate should then be between 22 cubic metres per hour (m3/h) per person, which limits the carbon dioxide (CO2) concentration to about 1000 parts per million(ppm) above the outdoor concentration, and 54m3/h per person, which limits the CO2 concentration to about 400 ppm above the outdoor concentration – meaning that less than 10 per cent of people entering the room will be dissatisfied by the odour (prEN 13779, 2004). Airflow rates should be much larger in poorly insulated buildings (where there is a risk of mould growth and water vapour condensation) or in spaces where there is a particular source of contamination, including spaces where smoking is allowed.
During the summer, the minimum airflow rate may be much larger than the hygienic airflow rate in order to evacuate heat or provide cooling draughts. However, when the outdoor temperature exceeds the indoor temperatures, it may be wise to decrease the ventilation rate, only allowing high levels of ventilation at night when the outdoor temperature is low.