The rate at which inside air is replaced by outside air. The rate may be expressed as the number of changes of air per unit of time (e.g. Air Changes per Hour - ACH) or the volume of air exchanged per unit of time (e.g. Cubic Feet per minute (CFM)

The rate at which outside air replaces indoor air in a space. Expressed in one of two ways: the number of changes of outside air per unit of time air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time - cubic feet per minute (cfm).