Air
Air Changes per Hour
ACH
Also known as air change rate
The number of times a space's volume of air is replaced in one hour. The figure is only meaningful with its context attached, because ventilation, filtration, and enclosure leakage are all reported in air changes and none of them mean the same thing.
In more detail
Three different quantities get abbreviated the same way, and conflating them is the most common error in this vocabulary.
Ventilation ACH describes outdoor air delivered to a space — the rate at which indoor air is actually replaced with air from outside. This is the number that dilutes carbon dioxide and everything else the occupants generate.
Equivalent or clean-air ACH describes recirculating filtration: an air cleaner's clean-air delivery rate divided by room volume. It removes particles without introducing any outdoor air at all, so it can be high in a room where carbon dioxide is climbing steadily.
ACH50 describes enclosure leakage measured during a blower-door test at 50 pascals of induced pressure. It is a diagnostic of airtightness under artificial conditions, not a rate at which the house ventilates in ordinary weather. Natural infiltration is a small fraction of it, varies with wind and temperature, and is not a ventilation strategy.
A room can have a high equivalent ACH from a purifier, a low ventilation ACH because nothing brings outdoor air in, and a tight ACH50 because the enclosure was well built. All three are true at once, and they describe different things.
Why it matters
Air-change figures are the most quoted and least qualified numbers in residential air quality, and they appear on product pages precisely because the ambiguity flatters the product. Reading a specification correctly means asking which of the three it is before comparing it to anything. It also clarifies why a tight enclosure and good ventilation are complementary rather than opposed: airtightness is what makes the ventilation rate controllable in the first place.
The common misunderstanding
That the air-change rate printed on a purifier box is a ventilation rate. It is a recirculation figure — the machine filters room air and returns it, replacing nothing. The mirror-image error treats a house's ACH50 as its ventilation rate and concludes that a leaky house is well ventilated. Leakage happens at whatever rate the weather dictates, through whatever the air passes on the way in.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.