Air Changes Per Hour Explained

Air changes per hour expresses the volume of air delivered to a space in an hour divided by the volume of the space. Six air changes per hour means a volume equal to the room is supplied six times over. It does not mean every particle is removed six times, because incoming and existing air mix rather than displacing one another cleanly.

The same equipment produces a different air change rate in a different room, because the denominator is the room's volume. A unit rated for a small office delivers a fraction of that rate in an open-plan space, which is why a figure quoted without a room size is not usable.