How does an air purifier work?
In one line: a fan pulls room air through one or more filters that trap particles (and, if fitted, carbon that soaks up gases), then blows the cleaned air back out — repeating this until the whole room's air has passed through many times an hour.
The stages
- 1. The fan. It draws room air in. How much air it moves, combined with the filter, sets the clean-air delivery rate — the number that decides how fast a room is cleaned.
- 2. Pre-filter. A coarse first layer catches large debris and hair, protecting the main filter and extending its life. See what a pre-filter is.
- 3. Main particle filter. A HEPA or MERV-13 filter traps the fine PM2.5 particles — dust, pollen, pet dander, smoke — that matter most for health.
- 4. Activated carbon (optional). If fitted, carbon adsorbs gases and odours a particle filter can't catch.
- 5. Clean air out. The filtered air returns to the room, and the cycle repeats.
Why it has to keep running
Cleaning a room isn't a one-off — new particles arrive constantly (from outside, cooking, movement, pets). A purifier keeps the level low only while it runs, which is why the numbers that matter are how much clean air it delivers (CADR) and how many times it turns the room's air over per hour (ACH) — and why quiet, low-cost operation matters, so it's never switched off.
What about "filterless" purifiers?
Some devices use ionisers or ozone instead of a filter. These work differently and come with trade-offs — see ionisers & ozone. A mechanical filter remains the safest, best-evidenced approach for a home.
Next step
See well-designed units compared on clean-air delivery, noise and running cost at Best Air Purifier UK.
FAQ
Do air purifiers use much electricity? Efficient units draw only a few watts to a few tens of watts; check the wattage and estimate with a running-cost calculator.
Where should it go in the room? Somewhere with clear airflow around it, not tucked behind furniture or curtains that block the intake.