Air Purifier Buying Guide UK: HEPA, CADR and Room Size Explained

Hay fever, city dust, pet dander, or just stuffy rooms — air purifiers have become a UK household staple. The spec sheets, however, read like exam papers. Here is what actually matters.

True HEPA or nothing

The filter does the work. Look for True HEPA (H13 or HEPA-13 class), which captures 99.97% of particles down to 0.3 microns — pollen, dust mite debris, pet dander and smoke. Beware the phrase “HEPA-type” or “HEPA-like”: these filters meet no defined standard.

CADR: the honest number

CADR (Clean Air Delivery Rate) tells you how much purified air the unit actually moves. To size a purifier, aim for a CADR that cycles your room's air about 4–5 times per hour. A quick rule: for a typical UK bedroom (12–15 m²), a CADR around 150–200 m³/h is comfortable; for open-plan living rooms, look at 300+ m³/h.

Noise is the spec people regret ignoring

You will run this machine at night. Check the decibel rating at the lowest speed — under 30 dB is whisper-quiet, and a dedicated sleep mode is worth having. Max-speed noise matters less; you will rarely use it.

Count the filter costs

Replacement filters every 6–12 months are the real price of ownership. Before buying, check the cost of the replacement filter and multiply by your yearly usage — a cheap purifier with expensive filters is not cheap.

Features worth paying for

• Activated carbon layer — the only thing that tackles odours and VOCs
• Air quality sensor with auto mode
• Filter-change indicator
• Timer and night light off switch

See our air purifiers, replacement filters and the wider heating, cooling & air quality range — free UK delivery and free 30-day returns.