Air changes per hour is how many times a purifier can push the entire volume of a room through its filter in an hour.
4.8 ACH, the figure Blueair and Levoit both like to print, means the whole room goes through once every 12 and a half minutes.
That is the definition. The rest of this is why the number on the box rarely survives contact with a real room.
ACH belongs to the pairing of machine and room, which is why it is never a fixed spec
CADR is a flow rate โ clean air delivered, in cubic metres an hour.
On its own it tells you nothing, because a flow has to land somewhere.
ACH is that flow divided by the volume it is working in.
The arithmetic is short. Floor area times ceiling height gives volume.
A 12 mยฒ British bedroom with a 2.4m ceiling holds roughly 29 cubic metres.
A purifier rated at 145 mยณ/h delivers 145 divided by 29, which is 5 ACH.
Move the same machine into a 30 mยฒ through-lounge and it manages barely 2.
Nothing about the purifier changed. Only the denominator did.
The manufacturer picks the room that makes the number work
Every quoted ACH figure comes with a room size attached, usually in small type underneath it.
It is measured at the highest fan speed, which is also the loudest and the one nobody runs overnight.
Drop to the quiet setting you actually sleep with and the flow can fall by two thirds.
So a machine advertised at 4.8 ACH may be managing 1.5 in your bedroom at two in the morning.
Find the room size the figure assumes, then redo the division for the room you own.
The purifier did not get worse when you moved it. The room got bigger, and only the room was ever in the sum.
One air change does not mean clean air, which is why five is the target
An air change replaces the room's volume once, but the clean air mixes with the dirty rather than pushing it out in a block.
One change removes roughly 63% of what was suspended in the room.
Two gets you to about 86%. Three to 95%.
That is why allergy and asthma guidance lands on four to five changes an hour rather than one or two.
How long one full pass through the filter takes at each rate.
At 5 ACH a room recovers within minutes of someone shaking out a duvet or a cat crossing the carpet.
At 2 ACH it never quite catches up with a source that keeps producing.
Two an hour is a fair floor for ordinary dust and cooking smells in a room nobody is allergic in.
The figure assumes a sealed room, and yours is not one
The volume in the sum is the volume behind a shut door.
Leave the bedroom door open and the purifier is quietly working on the landing and the stairs as well.
A trickle vent, a cat flap or a draughty sash adds outdoor air the sum never accounted for.
None of that makes the machine useless. It makes the printed figure optimistic.
Work to a genuine 5 ACH on the quiet setting and the leaks are covered by the margin.
In practice that means buying one size up from the room you intend to use it in.
Our Blueair Blue Pure 511i Max review: the 5 numbers, and why 1,138 sq ft is not one of them follows what happens when a quoted room figure is taken at face value.
Ceiling height is the term people forget, and in a Victorian conversion it is the one that hurts.
A 3.2m ceiling adds a third more volume than the 2.4m the sums assume, and the fan has no idea it happened.
