The air that moves into and out of the school building can be referred to as natural ventilation. The amount of natural ventilation is affected by temperature, wind speed and leakage area, thus it is always changing. The natural ventilation rate is far higher on a cold and windy day than on a calm, warm day. Natural ventilation is less advisable if the windows open onto the street, as particles from traffic emission and other airborne contaminants that are harmful to health can enter the school building.
If there is no mechanical ventilation system installed, natural ventilation (i.e. open windows) should be the first option for introducing fresh air. Windows should be opened systematically before classes and during lesson breaks.
If it is not possible, or permitted, to open the windows because of the weather, the levels of outdoor air pollution, noise, or safety reasons, mechanical ventilation (with a controlled air supply) should be considered. Ventilation rates should be calculated according to the occupation density and the size and layout of the classroom.
Classrooms should be equipped with monitors that indicate when the level of carbon dioxide becomes too high (e.g. over 700 parts per million) and thus when ventilation is required. Ventilation should be regularly adjusted in order to maintain stable and comfortable temperature and humidity levels.
In certain weather conditions, ventilation may become a source of increased indoor humidity. Mechanical systems will therefore need to remove moisture from the incoming outdoor air or from recirculated air.
Both teachers and students should be aware that appropriate ventilation is essential; that vents must be kept clear of books, papers and other items; and that mechanical ventilation systems should not be switched off. Different areas of the school will require different ventilation rates, although there are both teachers and children can do to improve the quality of air inside the school building.