How to choose an air purifier

Buying guide · Last reviewed July 2026

Air purifiers are one of the easier categories to buy well, because one number does most of the work — provided you understand what it assumes and where manufacturers quietly stretch it.

CADR: the number that matters

Clean Air Delivery Rate measures how much filtered air a unit delivers per unit of time, usually in cubic feet per minute. It is normally published as three figures — smoke, dust and pollen — because particle size affects capture. The smoke figure represents the smallest particles and is the most demanding, so it is the one to compare.

CADR combines airflow and filter efficiency into a single number, which is why it is more useful than either alone. A powerful fan with a mediocre filter and a weak fan with an excellent filter can both produce a poor CADR.

Sizing: do the arithmetic yourself

The widely used rule of thumb is that a unit's smoke CADR should be at least two-thirds of the room's floor area in square feet, which delivers roughly five air changes per hour in a room with standard ceilings.

Two adjustments to make:

• High ceilings. The rule assumes about 8 ft. If your ceilings are 10 ft, scale up by roughly 25%.
• Open-plan spaces. Air moves between connected areas. Size for the volume that is actually open, not the nominal room.

Why the box's room-size claim is optimistic

Manufacturers frequently quote coverage at two air changes per hour rather than five, which roughly doubles the headline number. Some quote at maximum fan speed only — a speed most people will never use because of the noise. If a listing gives a coverage area but no CADR, that is a meaningful omission.

Filters: what each stage does

• Pre-filter. Catches hair and large dust. Usually washable. Its job is to stop the expensive filter clogging early — keep it clean and the main filter lasts much longer.
• HEPA. The main event for particles. A true HEPA filter captures at least 99.97% of particles at 0.3 microns, which is the hardest size to catch; both larger and smaller particles are captured more efficiently. Watch the wording: "HEPA-type", "HEPA-like" and "99% HEPA" are not the same standard.
• Activated carbon. The only stage that addresses gases and odours — cooking smells, VOCs, smoke smell. HEPA does nothing for these. Thin carbon-dusted sheets do very little; meaningful odour control needs a substantial bed of pellets, which you can usually infer from filter weight and price.

What HEPA does not cover

Particles only. Gases, VOCs and odours pass straight through. If your reason for buying is cooking smells or a new-furniture smell, carbon capacity is your specification, not HEPA grade.

Technologies to be careful with

Some purifiers add ionisers, UV lamps or plasma stages. Two things to know:

• Some ionising and plasma approaches can produce ozone as a by-product. Ozone is a respiratory irritant. If a unit includes such a stage, look for an explicit statement of ozone-free operation or certification to a recognised limit, and check the feature can be turned off.
• UV lamps need sufficient dwell time to inactivate anything. In a unit moving air quickly through a small chamber, the contribution is usually marginal compared with what the HEPA filter is already doing.

None of this makes such units bad; it means the particle filtration is doing most of the work and you should buy on CADR regardless.

Noise: the reason units get switched off

A purifier only cleans air when it is running. The most common failure mode is buying a unit whose useful speed is too loud for the room, running it on low instead, and getting a fraction of the rated performance.

Check the noise figure at the speed you will actually use, not the minimum. For a bedroom, roughly 30 dB or below is genuinely unobtrusive; the mid-40s will be audible at night for light sleepers. If only a single noise figure is quoted, it is almost certainly the lowest speed.

The running cost nobody budgets for

Filters are consumable and proprietary. Before buying, work out the real annual cost:

• Find the replacement filter price and the manufacturer's stated interval — commonly 6–12 months for HEPA, shorter for carbon under heavy use.
• Confirm filters are available in your country and not only from the manufacturer's own store.
• Multiply out. On some models a year of filters costs a substantial fraction of the unit itself, which changes the comparison between two purifiers entirely.

Also check whether the unit uses a filter-life timer or an actual sensor. Timers count hours regardless of how dirty the filter is, which means you may be replacing early or late.

Before you buy: a checklist

• Measure the room's floor area, and note the ceiling height.
• Target a smoke CADR of at least two-thirds the floor area in square feet.
• Ignore the coverage claim on the box unless it states the air-change rate it assumes.
• Confirm "true HEPA", not "HEPA-type".
• If odours are the goal, check carbon quantity — not just its presence.
• Check the noise level at a usable speed, not the minimum.
• Price a full year of replacement filters and add it to the purchase price.
• Confirm any ioniser or plasma stage can be switched off.
• Check filter availability in your country.