Where to put an air purifier, and why the corner is the worst place for it

Put it in the room where you spend the most consecutive hours, out in the open, with at least 30cm clear on every side that draws air.

That is the whole answer. The rest of this is what each part of it costs you when you get it wrong.

Most purifiers pull air in through the sides or the base and push it out of the top.

Block either path and the machine works quietly against itself.

The corner is the worst place, and it is where most people put it

A corner puts walls on two sides and, usually, furniture on a third.

The intake is left with one open face instead of four.

Airflow drops, the fan works harder for the same result, and the clean air never travels far enough to reach the part of the room you are in.

Behind the sofa is worse again, because the sofa muffles the fan as well.

You lose the one cue that tells you the thing is actually running.

Thirty centimetres of clearance is the number that matters

Manufacturers typically ask for 15 to 30cm on the intake sides, and the larger figure is the safer one to work to.

Curtains are the usual culprit in a British bedroom, because curtains move.

A curtain that hangs clear at eight in the evening can be pressed flat against the grille by a draught at two in the morning.

Give the top the same thought.

A shelf 20cm above a top-venting purifier sends the exhaust straight back down into the intake.

If the intake is on the base, keep the unit off deep-pile carpet and put it on a hard floor or a thin mat.

The bedroom earns it before the living room does

A living room gets an hour or two of occupancy at a time, with the door opening constantly and cooking drifting in.

A bedroom gets seven or eight unbroken hours behind a shut door.

That is the longest single stretch you spend breathing the same air anywhere in the house.

It is also the only stretch long enough for a purifier to turn the room over several times at a quiet fan speed.

One machine goes in the bedroom. A second belongs wherever the source is — the kitchen, the wood burner, or the room the cat sleeps in.

Sizing decides whether the room turns over quickly enough to matter, and our Blueair Blue Pure 311i Max review works through the numbers on one mid-size unit.

A purifier in a corner spends the night recycling its own exhaust while the far side of the room goes untouched.

The shut door is part of the placement

A purifier is rated for a room, not for a floor of a house.

Leave the bedroom door open and it is trying to clean the landing, the stairs and whatever the kitchen did at seven.

Shut the door and the same machine turns over a much smaller volume far more often.

Windows are the harder rule, because ventilation and filtration pull in opposite directions.

On a still day with poor outdoor air, keep the window shut and let the purifier work.

On a mild day after rain, open it and switch the purifier off. You cannot out-filter an open sash, and there is no point trying.

Checking the outdoor forecast first takes about ten seconds and settles which of the two you want.

Check today's UK air quality forecastDefra, UK-AIR →

Height and noise decide the last metre

Floor level is fine for most machines, and it is where the manufacturer tested them.

Lifting a small unit onto a chest of drawers brings the clean air nearer your face, which helps in a large or awkwardly shaped room.

It also brings the fan nearer your ear.

Somewhere between one and two metres from the pillow is the usual compromise.

A machine you switch off at midnight because it whines filters nothing at all, which is the most expensive placement error of the lot.

Keep it away from the television and the radio if either picks up interference, and off the same extension lead as a fridge or a dehumidifier.

Then leave it where it is. Moving a purifier around the house every few days is how people end up concluding it never did anything.