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.
Matching CADR to your room
The single most useful habit when buying is to translate clean-air delivery rate into air changes for your actual room. Aim for three to four air changes an hour: multiply your room's floor area by ceiling height to get its volume in cubic metres, then look for a CADR that turns that volume over three or four times. A typical London living room needs somewhere around 300 to 400 m³/h; open-plan or knocked-through spaces need considerably more. Buy short of that and the unit will struggle on the days you most need it. This is also why a physically larger purifier run at a modest speed usually beats a small one at full tilt — it delivers the same clean air far more quietly, which matters if it is running in a bedroom overnight.
For reference, the kits we build span this range: the Luggable 5-Fan (around £229) delivers roughly 369 m³/h CADR, while the Luggable XL 7-Fan (around £279) reaches about 681 m³/h — enough for a large or open-plan room. If odours and VOCs are a concern, the Luggable Carbon (around £299) adds activated carbon on top of MERV-13, because particle filters alone do not remove gases.
Filters, gases and value
The filter decides both your ongoing cost and what the unit can actually remove. A few points worth holding onto:
- Standard MERV-13 media captures roughly 93% of fine particles per pass — highly effective, though not identical to a true HEPA single-pass rating. Our MERV-13 vs HEPA guide covers the trade-off honestly.
- Standard media means no lock-in — you are not tied to one brand's pricey cartridge, so replacements stay cheap. Stock up from our MERV-13 filters range.
- Particle filters do not remove gases; only activated carbon tackles odours and VOCs, so add carbon if smells or fumes are your concern — as our air purifier for smoke guide explains.
The reason high-airflow builds win on value is simple: moving a lot of air through inexpensive media delivers the most clean air per pound, the same Corsi-Rosenthal principle behind classroom filtration. If you would rather compare finished units, the best air purifiers UK 2026 roundup and our cheap air purifier UK guide both weigh airflow against price.
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.