ACH stands for Air Change per Hour.
If a room is 100 sq ft in area, or 10 ft in height, its volume will be 1000 cu foot.
Generally, 20% of the room will be dead space, with closet, desk, sofa, bed, etc.
The volume of the air in the room will be, say 800 cu foot (thus, the effective height of the room is 8 feet, for calculation about air purification).
800 cu foot = 22.65 cu meter is the size of the room (the air in the room, to be precise) to be cleaned.
An air purifier will filter the air in a room or a car at a certain rate, say 100 cu meter per hour. Thus, it will filter the air in the room
100 / 22.65 = 4.4 times an hour
This is called Air Change per hour, or ACH.
It means that the air inside the room will be go through the air purifier 4.4 times every hour.
The question begs to be asked is if the air inside the room goes through the purifier even once, it will be pristine clear, then why would it need further cleaning?
The reason is the air purifier sucks in a little bit of polluted air, cleans it and throws it back into the room every second.
Assuming the clean air mixes with the rest of the polluted air instantly, which will never happen, the air purifier now sucks in some more air, which is marginally less polluted than the earlier instance, and so on.
Thus, the pollution inside the room will be an exponentially decaying function, instead of a linearly decaying function.
However, in real life, some air will go through more than 4.4 times and some air, in farther corners of the room, will go through less frequently. But the average will be 4.4 times.
Why is ACH Important?
ACH is an important factor as its directly related to the air purification capacity of an air purifier for a particular room. It helps you compare purifiers with different CARDs and see which is (if at all) is more suitable.
In many cases, you may feel you have bought a good air purifier, but it may not be powerful enough to clean the air or may take a long time to do so.
As a thumb rule, do not buy an air purifier below 4.5 ACH rating.
But ACH ratings also vary based on the which location you are installing the air purifier.
For example, a kitchen where lots of cooking happen will need a higher ACH than a bedroom.
With all these factors in mind, we have prepared a calculator. Put carpet area and which room you want to install the air purifier and it will give you the ideal minimum CADR you should go for.
As most air purifiers do not give ACH, but provide CADR ratings, you can use this to check if a purifier is good enough for your room or not.