An air purifier does something deceptively simple: it moves the room's air through a filter, again and again, faster than the room re-dirties itself. Everything else you read — MERV ratings, CADR, "HEPA-grade" claims, ioniser gimmicks — is detail layered on those two fundamentals: how much air it moves, and how well the filter catches what is in that air. Understand those and you can see through most of the marketing.
681 m³/h CADR, MERV-13, under 35 dB — powerful whole-home clean air — no proprietary lock-in, ships from the UK.
Check price →Use code KITS10 for 10% off at checkout.Airflow: the half most guides skip
A perfect filter is useless if very little air ever reaches it. This is the point cheaper purifiers miss and the reason high-airflow designs perform so well: by pushing a large volume of air through affordable MERV-13 media, they clean a room's entire air volume several times an hour. The best filter in the world sitting behind a weak fan cleans almost nothing, because most of the room's air never passes through it. When you judge a purifier, airflow (expressed as clean-air delivery) is at least as important as the filter grade — arguably more so for everyday home air.
The filter: how MERV-13 catches particles
It is tempting to picture a filter as a sieve that blocks anything bigger than its holes. It is cleverer than that. MERV-13 media captures particles through a combination of mechanisms — interception (particles following the airstream touch a fibre and stick), impaction (larger particles cannot follow the air's twists and slam into fibres), and diffusion (the tiniest particles jitter randomly and collide with fibres). Because of diffusion in particular, the media catches many particles far smaller than the visible gaps between its fibres. In total, MERV-13 captures around 93 per cent of airborne particles: pollen, dust, PM2.5, smoke and a large share of airborne respiratory particles.
What filters cannot do
What about ionisers, UV and ozone?
Why the Corsi-Rosenthal principle works so well
Combining several MERV-13 filters with quiet fans multiplies filter surface area and airflow at the same time. More area means lower resistance and more air moved; more air moved means more clean-air delivery. That is how a modestly priced kit rivals sealed purifiers costing several times more — not by using an exotic filter, but by moving a lot of air through a lot of good, standard media. If you want to build one yourself, the full method is in the Corsi-Rosenthal box guide; if you would rather buy the quiet engineered version, the kits below use the same idea.