Most air-purifier marketing hides the one number that matters: CADR (clean-air delivery rate), the volume of filtered air the unit actually moves. A high wattage or a big HEPA label means nothing if airflow is low. Here's how to buy well.
Ready to clean up your London air?
Luggable MERV-13 kits deliver up to 681 m³/h CADR — built on the Corsi-Rosenthal principle, from £229. Ships from Oxford in 2–3 days.
Shop the UK store → See all kitsThe three things that matter
- CADR vs room size: aim for 3–4 air changes an hour. Bigger room, higher CADR.
- Filter type & cost: MERV-13 standard media (cheap, no lock-in) vs proprietary HEPA cartridges (pricey).
- Noise: buy a bigger unit and run it slow — quieter and cleaner than a small one at full tilt.
Fans for a Corsi-Rosenthal build
If you're building a CR box, a 20-inch box fan is simplest; a PC-fan array is quieter and tidier. Either way the MERV-13 filters do the work — the fan just moves air through them.
Frequently asked questions
What CADR do I need?
Enough to turn over your room's air 3–4 times an hour. For a typical London living room that's roughly 300–400 m³/h; open-plan spaces want more.
Is a more expensive purifier always better?
No — price often reflects styling and proprietary filters, not clean-air delivery. Judge on CADR per pound and filter cost.