Types of air in ventilation systems

From the moment it is drawn in until it is expelled back out of the building, the air in a controlled mechanical ventilation system has to travel through an array of different sections. Accordingly, various terms are used to describe the air at different points: outdoor air (is drawn into the ventilation system via intake apertures and filtered), supply air (is filtered and supplied to the relevant rooms, having been cooled or heated depending on weather conditions), extract air (stale air from the interior) and exhaust air (is discharged to the outside again, either through the roof or a vent in the wall, ideally following heat recovery).