Air
Mean Age of Air
The average time since air at a location entered the space as outdoor or supply air. It describes how long that air has been indoors — not how much air the ventilation system is moving.
In more detail
Age of air is a clock, not a flow rate. At any point in a room, the local mean age of air is the average time those air molecules have spent inside the building since they entered as outdoor or supply air. The zone or room mean age is the spatial average of those local values. ISO 16000-8 standardizes tracer-gas methods for measuring local mean age; ASHRAE Standard 129 uses occupied-zone age of air to compute air-change effectiveness.
Under perfect mixing, mean age of air equals the nominal time constant of the space — room volume divided by outdoor airflow, or the reciprocal of the ventilation air-change rate. If supply air takes a short path to the return, the occupied zone can hold air that is much older than that nominal value even though the system is moving the design volume. If outdoor air is delivered preferentially into the occupied zone, local age there can be younger than the room average.
Mean age of air does not identify pollutants, does not measure particles, and does not by itself say whether ventilation is adequate. Older air is more likely to have accumulated occupant-generated contaminants if those sources are in the space, but age is a transport metric. Two rooms with the same mean age can have very different chemistry.
Why it matters
A house can move a large volume of air and still leave the sofa, the desk, or the bed in a pocket of old air. Closed bedroom doors, high-wall supplies aimed at a nearby return, and heating-mode buoyancy all create those pockets. Mean age of air is the quantity that makes that failure visible: the ventilation system’s CFM can look correct while the air at nose height has been indoors far longer than the air-change rate suggests.
The common misunderstanding
That moving a large volume of air guarantees fresh air in the occupied zone. Volume throughput and arrival time are different measurements. A high ACH can coexist with old air at the pillow if the outdoor-air stream never reaches it. The air-change rate is a room-average volume ratio; mean age of air is what a particular location actually received.
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Sources
Last reviewed September 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.