Does an air purifier need to run all day?

I bought an air purifier in the winter of a year when the flat had single-glazed windows and a neighbour who smoked on the landing. I ran it twenty-four hours a day for three months, because that felt like the diligent thing to do, and then I read the filter price.

That was the moment I started thinking about it properly.

Sizing, which is the part people skip

A purifier is rated for a room volume, and the rating is usually given for a room with a ceiling at 2.4 m and the unit on maximum. Maximum is the loud setting you will never use.

The number to look for is the clean air delivery rate, or the manufacturer's stated room size. Then halve the optimistic one. A unit advertised for 60 m² is a unit for a 30 m² room if you want to run it on medium rather than at a volume that makes television unwatchable.

This is not a trick by manufacturers so much as a difference between a test chamber and a living room. A test chamber has no sofa, no curtains, no bed, and nothing in the way of airflow. Your bedroom has all three.

The practical rule: buy for the room you will actually put it in, and assume you will run it one setting below maximum.

Placement, which is free and matters more

Air is lazy. It goes the easy way, and a purifier creates its own little current that will happily circulate the same cubic metre for an hour.

Not in a corner. Two walls close together is the worst position in any room. The unit pulls from a dead zone and pushes into another one.

30 to 60 cm from a wall. Far enough that it can pull air from behind as well as in front, close enough that it is not in the middle of the floor.

Not behind furniture, not under a shelf, not in a cupboard. Obvious, and people do it because the unit is ugly.

Between you and the door, if you can. A purifier works best when it sits in the path air takes between you and the rest of the flat, rather than at the far end of the room from the source of whatever you are trying to deal with.

One room per unit. Air does not move between rooms with the doors shut. A purifier in the living room does essentially nothing for the bedroom, which is the room where you spend eight hours with the door closed. If you can only buy one, put it where you sleep.

When to run it and when not to

Run it when the room is occupied and the window is shut. That is the entire useful window. Overnight in a closed bedroom, in the evening with the heating on, in the morning before you leave.

Turn it down when the window is open. Filtering air that is being replaced by outdoor air is paying to clean something that is about to be thrown away. Ten minutes of cross-ventilation does more than an hour of filtration, and it is free.

Turn it off when you are out. A purifier in an empty room is cleaning air nobody is breathing. Some people run one continuously to keep a room at a baseline, which is a reasonable choice if the room is sealed and the unit is quiet — but it is a choice, not a requirement, and it doubles your filter consumption.

Use the auto mode if it has one, and check it. Auto mode uses a particulate sensor, and cheap sensors drift. If your unit has been on low for a month in a room where you cook, the sensor is lying. Run it on high manually for an hour after cooking.

The cost nobody puts on the box

Filters are the real price of a purifier, and they are almost never mentioned at the point of sale.

A mid-size unit takes a combined HEPA and carbon filter that costs $40 to $90 and wants replacing every 8 to 14 months, depending on how dirty your air is and how often you run it. Over five years that is $200 to $450 — which can be more than the unit cost.

Some cheap units use small filters that need replacing every six to eight weeks. That is the trap. A $120 purifier with $25 filters every two months is a $870 five-year purchase.

Check the filter price and the replacement interval before you buy the machine. It is one search and it changes the ranking of every unit you are considering.

There is a cheaper version of the same idea, which is to vacuum the pre-filter monthly. Most units have a washable mesh pre-filter in front of the HEPA, and cleaning it is the difference between a filter lasting fourteen months and lasting seven.

A routine that survives a normal week

The reason purifiers end up off is that the routine people are given is too complicated to keep. This is the version I actually do.

Overnight, low, bedroom door shut. Eight hours in a closed room is the single best use of the machine. Low is quiet enough to sleep through and it is the setting that costs least in filter life per hour.

When you get up, turn it off and open the window. Ten minutes of air is worth more than ten hours of filtration, and it is free. The purifier is for when you cannot do this, not instead of it.

Before cooking, on high, then twenty minutes after. Cooking is the biggest particulate event in most homes and it happens in a room that often has no purifier in it. If you have one unit, this is the moment to carry it into the kitchen.

First of the month: vacuum the pre-filter. Two minutes, and it is the difference between a filter lasting a year and lasting seven months.

When the unit tells you to replace the filter, replace it. Not when it looks dirty. A grey filter is often still working; a clean-looking one can be clogged with fine material you cannot see.

That is five habits, none of which take more than a couple of minutes, and all of which are easier to keep than a rule about running it twenty-four hours a day.

What a purifier does not do

It does not cool a room. It does not remove carbon dioxide, which is what makes a closed bedroom feel stale by three in the morning — that is a ventilation problem, and I wrote about it in why your bedroom feels stuffy by 3am. It does not deal with a damp wall, a leaking gutter, or a rug that is still off-gassing.

And it does not substitute for the two cheapest interventions in any home: opening a window, and not bringing the thing into the house in the first place.

What I would buy now

One mid-size unit for the bedroom, rated for a room twice the size I have, quiet on low, with an annual filter I can buy without a subscription. That is the whole specification.

I would run it overnight and in the evening, turn it off when the windows are open, vacuum the pre-filter on the first of the month, and replace the filter when the unit tells me to rather than when it looks dirty — because a HEPA filter that looks grey is often still working, and one that looks clean can be clogged with fine material you cannot see.