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What ACH actually measures
Air changes per hour is how many times the purifier processes the entire volume of the room in one hour. It is a measure of treatment rate, not of cleanliness, and it is the number that connects a purifier’s CADR figure to your actual room.
It matters because CADR alone is meaningless without a room. A 135 CFM purifier is excellent in a 10 m² nursery and marginal in a 40 m² open-plan living space. ACH is the conversion that makes the comparison honest. our air quality hub covers this in more detail.
It is also the number consumer advice usually skips. Brands quote room size, which is derived from ACH at maximum speed — the setting you will almost never run overnight. Our sizing chart covers the same ground from the practical direction. Related reading: how often to change a baby-room filter.
Calculating it from CADR
Two steps. First, convert the room to cubic feet: length × width × ceiling height, in feet. A 10 ft × 10 ft room with an 8 ft ceiling is 800 cu ft. Second, divide CADR in CFM by that volume, then multiply by 60 to get changes per hour. the small-bedroom guide goes deeper on this point.
So a purifier with 135 CFM CADR in an 800 cu ft room gives 135 ÷ 800 × 60 ≈ 10 air changes an hour at full speed. The same unit at its quiet setting, moving perhaps 40 CFM, gives about 3 — which is the number that decides whether the quiet setting is enough. the purifier-vs-humidifier comparison covers this in more detail.
That last step is the important one, and it is where most sizing goes wrong. Always calculate ACH twice: once at the CADR the box advertises, and once at the speed you will actually run overnight. Related reading: our HEPA nursery picks.
What target to aim for
Four to five air changes per hour is a solid target for a nursery or bedroom. It is enough to meaningfully reduce particulate in a small room without demanding a loud fan. Higher is better up to a point, and there is no meaningful safety ceiling for a HEPA-filtered unit. Related reading: Blueair purifiers for a nursery.
Three is workable but slow to clear a spike — if someone cooks or a candle burns, the room takes a while to recover. Two or fewer means the purifier is decoration: it will not meaningfully change the air in the time you spend in the room. Related reading: the Coway nursery guide.
The reason to aim at the low setting rather than at maximum is that the low setting is the one running at 3 a.m. A unit that hits 10 ACH on turbo and 3 on low is not a 10-ACH purifier for your purposes.
Getting your room volume right
Use the actual room, not the flat. If the door is open to the rest of the apartment, the effective volume is much larger and the purifier’s performance drops accordingly — which is why running with the door closed overnight matters.
Measure rather than estimate. A room that looks like 10 ft × 10 ft is often 9 ft × 9.5 ft once you account for a wardrobe or a chimney breast, and that is a 15% difference in volume and therefore in ACH.
Ceiling height varies more than people expect, particularly in converted buildings. A 2.4 m ceiling and a 3 m ceiling are the same floor plan with a 25% difference in volume.
The turbo trap
Advertised room sizes are usually calculated at maximum fan speed. That produces a healthy-looking number and a purifier you cannot sleep next to. Running the same unit on low can drop you to two or three air changes, which is not enough.
The fix is to buy one size up, so the quiet setting delivers four or five. This is the same rule that appears in the quietest purifier comparison and the silent bedroom guide, and ACH is the reason behind it.
If you already own a unit that is marginal on low, run it on a higher setting for 30 minutes before bed to clear the room, then drop to low for the night. That pattern trades a brief louder period for better overnight air.
Applying it to a real nursery
Take a typical small nursery: 3 m × 3 m with a 2.5 m ceiling. That is about 265 cu ft — roughly 9.4 m³ — wait, in imperial terms about 9.8 ft square is 96 sq ft, times 8.2 ft high is 790 cu ft.
To hit five air changes an hour you need 790 × 5 ÷ 60 ≈ 66 CFM of filtered airflow. Many compact purifiers deliver that on their second-lowest setting, which is quiet enough for a nursery. So a correctly sized small nursery purifier can hit the target on a near-silent setting — as long as you checked the ACH at that setting rather than trusting the box.
If your nursery is larger, or the door stays open, do the arithmetic for the larger volume. Our sizing chart and small-nursery options both walk through real examples.
ACH target by room and what it means
| ACH (on the setting you use) | Verdict in a nursery | Practical effect |
|---|---|---|
| 5+ | Excellent | Clears a spike quickly, room stays low |
| 4–5 | Good | Solid target; the realistic goal |
| 3 | Marginal | Slow to clear cooking or smoke |
| 2 | Poor | Barely changes the air |
| Under 2 | Pointless | Buy a bigger unit or a smaller claim |
Sources: AHAM (CADR certification program), U.S. EPA (Residential Air Cleaners guide), ASHRAE Standard 62.2 (ventilation rates), WHO (air quality guidance).
Frequently asked questions
How many air changes per hour should an air purifier provide?
How do I calculate air changes per hour from CADR?
Why does the advertised room size seem too big?
Does ceiling height matter for air purifier sizing?
Should I run a purifier at high speed to get more air changes?
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