Air changes per hour (ACH)
In one line: ACH is how many times an air purifier can replace all the air in a room in one hour. For clean air at home, aim for roughly 4–5 ACH; for allergy or infection control, 5–6+.
What it means
If a purifier delivers enough clean air to replace a room's entire volume of air five times in an hour, that's 5 ACH. The higher the number, the faster particles are removed and the lower they stay between the sources that create them (cooking, pets, pollen coming in, people breathing). ACH ties directly to CADR: it's simply the purifier's clean-air delivery rate divided by the room's air volume.
How to calculate it
The formula is straightforward:
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 has a volume of 48 m³. A purifier with a CADR of 250 m³/h gives 250 ÷ 48 ≈ 5.2 ACH — comfortably in the target range.
What to aim for
| Goal | Target ACH |
|---|---|
| General cleaner air | 4–5 |
| Allergies (pollen, dust, pets) | 5–6 |
| Smoke / infection risk reduction | 6+ |
Higher ACH is why sizing matters: a unit that hits 5 ACH in a small bedroom might only manage 2 ACH in a large lounge, cleaning far more slowly. Buy for the room you have — and if a space is very large, two units or one high-CADR unit beats a small one struggling.
Next step
Work out the CADR you need for your room, then compare units on measured performance at Best Air Purifier UK.
FAQ
Is more air changes always better? More is generally better for particle control, with diminishing returns and more noise above ~6 ACH in a home. Bedrooms benefit from higher ACH run quietly overnight.
Does opening a window count? Ventilation adds its own air changes and lowers CO2, but it doesn't filter particles. Filtration and ventilation do different jobs and work best together.