The method of measuring air-change effectiveness compares the age of air where occupants breathe to the age of air that would occur throughout the space if the indoor air were perfectly mixed. The standard includes measurement procedures and criteria for assessing the suitability of the test space for measurements of air-change effectiveness. PURPOSE This standard prescribes a method for measuring airchange effectiveness in mechanically ventilated spaces and buildings that meet specified criteria. The air-change effectiveness is a measure of the effectiveness of outdoor air distribution to the breathing level within the ventilated space.