Air changes per hour (ACH)
In one line: ACH is how many times an air purifier delivers a roomful of clean air in one hour. For clean air at home, aim for roughly 4–5 ACH; for allergy or infection control, 5–6+. But it does not mean what most people assume it means — see below.
What it means
If a purifier delivers enough clean air to equal a room's entire volume five times in an hour, that is 5 ACH. It ties directly to CADR: it is simply clean-air delivery divided by room volume.
How to calculate it
ACH = CADR (m³/h) ÷ room volume (m³)
Room volume = floor area (m²) × ceiling height (m).
Example: a 20 m² room with a 2.4 m ceiling holds 48 m³. A purifier with a CADR of 250 m³/h gives 250 ÷ 48 ≈ 5.2 ACH — comfortably in the target range.
The part nearly every guide gets wrong
It is tempting to read "1 air change per hour" as "all the air in the room gets cleaned once an hour". It does not work that way, and the difference matters.
The purifier cannot tell which air it has already cleaned. Room air mixes continuously, so some of what it draws in has just come out of the filter, while some has been sitting in a corner untouched. The machine is always working on a mixture. The result is that removal is exponential, not linear: each hour takes away a fixed proportion of whatever is left, never a fixed amount.
So one air change per hour removes about 63% of airborne particles in that hour — not 100%:
| Air changes per hour | Particles removed in 1 hour | Still airborne |
|---|---|---|
| 1 | 63.2% | 36.8% |
| 2 | 86.5% | 13.5% |
| 3 | 95.0% | 5.0% |
| 4 | 98.2% | 1.8% |
| 5 | 99.3% | 0.7% |
| 6 | 99.75% | 0.25% |
| 8 | 99.97% | 0.03% |
Assumes a well-mixed room and no new particles entering. The proportion remaining after one hour is e−ACH.
This table explains the usual advice better than the advice explains itself. Going from 1 to 3 ACH is transformative — 63% becomes 95%. Going from 6 to 8 moves 99.75% to 99.97%, which no one in a living room will ever notice, while the fan noise needed to get there is very noticeable indeed. That is where "diminishing returns above 5–6 ACH" comes from: it is the shape of the curve, not a rule of thumb.
How long until the room is actually clear?
The same maths answers the practical question. Time to remove 90% of what is in the air is about 2.3 ÷ ACH hours:
| Air changes per hour | Time to clear 90% | Time to clear 99% |
|---|---|---|
| 2 | 69 min | 2 h 18 min |
| 3 | 46 min | 1 h 32 min |
| 4 | 35 min | 69 min |
| 5 | 28 min | 55 min |
| 6 | 23 min | 46 min |
Useful in ordinary life: if you burn the toast, or someone with hay fever is arriving, 5 ACH has the room back to normal in under half an hour, while 2 ACH needs most of a morning. It is also why running a purifier for twenty minutes before bed does very little in an under-sized room, and quite a lot in a well-sized one.
Filtration, ventilation and settling all count
A purifier is not the only thing removing particles. Three processes run at once and their rates add together:
- Filtration — the purifier's CADR divided by room volume.
- Ventilation — outdoor air displacing indoor air. Typically well under 1 ACH in a modern sealed home, higher in a draughty one or with a window open.
- Deposition — particles settling onto floors and surfaces on their own. For fine PM2.5 this is slow, often only a few tenths of an air change per hour; coarse dust settles much faster.
Add them for the total removal rate. This is why filtration and ventilation are not rivals: on an ordinary day, an open window and a running purifier reinforce one another. The exception is a high-pollution day, when ventilation brings particles in — then the window becomes a source and the purifier has to work against it.
Why ACH matters while something is still producing particles
Everything above describes a room clearing after a one-off event. Cooking, a candle, a wood burner or a pet moving about are continuous sources, and then the useful result is different: the level the room settles at is roughly the source strength divided by the total removal rate.
In other words, doubling ACH roughly halves the steady level you live with while the source is running. That is the real argument for buying more capacity than the clearing-time table alone suggests — not to clear faster after the fact, but to hold the room lower while the frying pan is still on.
What to aim for
| Goal | Target ACH |
|---|---|
| General cleaner air | 4–5 |
| Allergies (pollen, dust, pets) | 5–6 |
| Smoke / infection risk reduction | 6+ |
Sizing is the whole game: a unit that hits 5 ACH in a small bedroom may manage only 2 ACH in a large lounge — which, from the first table, is the difference between 99.3% and 86.5% removal in an hour. Buy for the room you have, and in a very large or L-shaped space two units beat one, because clean air has to reach the far end rather than merely exist somewhere in the room.
Where the number flatters reality
ACH assumes the room is well mixed. Real rooms are not. A purifier tucked behind a sofa, or blowing straight at a wall a foot away, can recycle its own clean output while the far corner stays untouched — so the effective air-change rate is lower than the arithmetic says. Give the machine an open path, and in a long room put it where the air has to travel back to it rather than in the nearest corner to the socket.
Free ACH calculator
Enter your room and a purifier's CADR to get the air changes per hour — or find the CADR you need to hit a target:
Live UK air quality — why ACH matters today
Indoor particle levels track the air outside. When the reading below is high, more air changes an hour keep your rooms clear:
Next step
Work out the CADR you need for your room, then compare units on measured performance at Best Air Purifier UK.
Looking for a unit that does this well? The Luggable XL is #1 of 247 on the US ENERGY STAR Most Efficient list, lab-tested by Intertek, and uses standard 3M MERV-13 filters.
FAQ
Is more air changes always better? Better for particle control, but with sharply diminishing returns and more noise above about 6 ACH in a home — the first table shows why. Bedrooms benefit most from a unit with enough headroom to hit a good ACH quietly.
Does opening a window count? It adds air changes and lowers CO₂, but it swaps indoor air for outdoor air rather than filtering it. Great on a clean day; counterproductive when outdoor PM2.5 is high.
Why do two units sometimes beat one bigger one? Because ACH assumes mixing. Two moderate units at opposite ends of a long room get closer to the ideal than one large unit in a corner, even at the same total CADR.
Does filter choice change ACH? Only through CADR. A denser filter lowers airflow, which lowers CADR, which lowers ACH — see HEPA vs MERV-13.