The one number that matters in an air purifier

Air purifier boxes talk about filter layers, ionisers and smart sensors. The one number that actually cleans a room is the clean air delivery rate.

Read it against the room

The recommended room size assumes five air changes per hour. Fine for a bedroom; a living room needs the next size up.

Filter cost

You pay for the device once and for filters every year. A unit without affordable, easy-to-find filters costs more over two years.

Noise

For night use, the decibel figure at the lowest setting decides. Under thirty decibels is a whisper.