Where to put an air purifier — and how to live with it

An air purifier does not clean a room the way a lamp lights one. It has to physically pull every cubic metre of air in the room through itself, repeatedly. That means the machine is only as good as the circulation it can set up, and circulation is the first thing furniture destroys.

The classic failure has a name: short-circuiting. Most purifiers draw air in low down or around the sides and blow it out of the top. Put one in a corner behind a sofa and the clean air leaves the top, hits the wall, curls back down into the intake, and gets filtered again. The machine is working perfectly. It is cleaning a pocket of air roughly its own size while the rest of the room carries on unpurified.

You cannot see this happening, which is why it persists. The display still says the air is good — because the sensor is sitting inside the same clean pocket the outlet is feeding. This is the single biggest gap between a machine’s rated CADR, as measured in an empty AHAM AC-1 chamber, and what it delivers in a furnished room.

Manufacturers typically ask for something in the region of 30 to 50 cm of clear space around the intakes and at least that above a top-mounted outlet. That is not a standard, it is an engineering rule of thumb, and it exists because restricting either end of the airflow path chokes the fan. A machine pushed against a wall with a 5 cm gap is not doing 80% of its job; it is doing considerably less, and the fan is working harder to do it.

Two smaller points that matter more than they sound. Keep the machine off thick carpet if it draws air from underneath — some designs do, and a deep pile will strangle them. And do not put it directly under a shelf or a wall-mounted television if the outlet faces up; you will get a clean patch of ceiling and a dusty piece of furniture.

Height is a genuine trade-off rather than a rule. Fine particles stay suspended and mix throughout the room, so floor level is fine for smog. Larger allergens — pollen, dander, dust stirred by feet — concentrate lower down, which also argues for the floor. Raising a machine onto a table mostly helps by getting the intake away from obstructions, not by finding better air.

This is the most expensive misunderstanding in the category, and the arithmetic settles it quickly. Air changes per hour is CADR divided by the volume being cleaned. Enlarge the volume and the rate collapses.

Take the biggest machine in this catalogue: 600 m³/h, shared by two models. In a 35 m² living room with a 2.5 m ceiling — 87.5 m³ — that is 6.9 air changes an hour, comfortably above the 4 to 5 wanted for smog. Ask the same machine to serve a 90 m² flat, 225 m³, and even if the air mixed perfectly you would get 2.7 ACH. And it does not mix perfectly. Every closed door is a wall as far as airflow is concerned, and even open doorways mix slowly enough that the far room lags well behind.

So the honest framing is per-room, not per-home. Two modest machines in the two rooms you actually occupy will beat one large machine in a hallway, almost always, and at these prices two mid-range machines often cost less than one flagship. The sizing arithmetic, room by room, is in understanding CADR and room size .

Most people buy one machine first. The bedroom is usually the right answer, for three reasons that compound.

1. It is the longest continuous stretch you spend in one room — seven or eight hours, every night, roughly a third of your life.
2. The door is normally shut, so the machine is working on a genuinely closed volume rather than fighting the rest of the flat.
3. A bedroom is small. At a 2.5 m ceiling a 15 m² bedroom holds 37.5 m³, so even the smallest machine in this catalogue at 240 m³/h delivers 6.4 air changes an hour. The same machine in a 35 m² living room manages 2.7.

That third point is the decisive one. A modest machine is transformative in a bedroom and marginal in a living room, so a limited budget goes furthest in the smaller space. The living room argues back on one ground only: it is usually where cooking, the front door and the street are, so it is where particles enter. If you cook a great deal with the kitchen open to the living space, that can outweigh the bedroom case.

The choice also depends on what you are removing. For allergies and for sleep quality, the bedroom wins outright. For a newly renovated flat in China the formaldehyde source is the whole building — flooring, cabinetry, adhesives — so a bedroom machine protects the hours that matter most while ventilation does the heavier work elsewhere; formaldehyde and VOC removal goes into that properly.

Sleep-mode noise across the twelve air machines in this catalogue runs from 22 dB to 35 dB, with a median of 31 dB. For a machine that lives in a bedroom that span is the difference between something you forget about and something you switch off at two in the morning, at which point it is worth nothing at all.

Decibels are logarithmic, so the span is wider than it reads. Roughly 10 dB corresponds to a doubling in perceived loudness, which makes the 35 dB machine here clearly more than twice as loud as the 22 dB one. For scale: a quiet bedroom at night has a background of around 30 dB, so anything at or below 25 dB tends to disappear into the room, while anything in the mid-thirties sits noticeably on top of it.

Two things to keep straight. First, these are sleep-mode figures — the lowest fan speed. On maximum, where CADR is measured under AHAM AC-1, every one of these machines is far louder, so a machine sized exactly to its rated CADR is a machine you will have to run loudly to get the rated result. Second, the quietest machine is not automatically the best bedroom machine: it has to be quiet and big enough to hit 4 to 5 air changes an hour while staying quiet. That combination is what you are actually shopping for.

If you tell the air finder that silence matters, it weights heavily towards machines at 25 dB or below — four of the twelve — and penalises anything above 30 dB. That is the same scoring behind the prepared shortlists.

Continuously, on a low or automatic setting. The reasoning is the same infiltration argument that sets the air-change target in the first place.

A home is leaky by design. Ordinary dwellings exchange somewhere around 0.3 to 1 air changes an hour with outdoors through gaps, frames and vents, without anyone opening a window. That means indoor particle levels are not a stock you clear once; they are a balance between what leaks in and what the machine takes out. Switch the machine off and the balance shifts back within an hour or two, so an hour of blasting on high before bed buys much less than it feels like it should.

Running low and constantly also has two side benefits. It is quieter, obviously. And a fan that spends its life at 30% is easier on the filter than one that spends an hour a day at 100%, because filter loading tracks the volume of air pushed through it rather than the hours on the clock — which is the mechanism behind the filter-cost levels in this catalogue and in air purifier running costs .

The exception is a known event. Cooking, a dusty clean, a bad-AQI afternoon with the windows recently open: turn it up while that is happening and for half an hour afterwards, then let it drop back. Seven of the twelve machines here are smart, and most of those will do exactly that on their own using an onboard particle sensor, which is the one smart feature genuinely worth paying for.

A purifier and an open window pull against each other, but not always in the same direction, and the resolution depends on what you are removing.

For smog and PM2.5: close the windows

On a high-AQI day outdoor air is the source. Every open window shortens the air-exchange time with outside, which raises the leak rate the machine is fighting. Keep windows and doors shut on bad days, seal the obvious gaps, and let the purifier work on a closed volume. This is the straightforward case and it applies in Delhi and Beijing alike.

For formaldehyde: ventilation is the main tool

Here it inverts. In a new-build or freshly renovated flat the source is inside the room, releasing continuously from boards, adhesives and finishes, and it carries on for months or years. Dilution by fresh air is the most effective control available, and an air purifier with an adsorbent stage is the supplement rather than the solution — particularly because activated carbon saturates and then stops working, silently, long before the off-gassing finishes.

That gives a sensible pattern for a Chinese new-build during a smoggy winter, which is the awkward overlap: ventilate when outdoor air quality allows it, run the purifier with the windows shut when it does not, and expect to replace carbon filters more often than the manual suggests. Three of the twelve machines here are rated 3 out of 3 for VOC removal, and two of those three are sold only in China — the catalogue reflects where that problem actually lives.

One last note on the sensor readout. A cheap particle sensor sitting on the machine measures the air at the machine, which after an hour of running is the cleanest air in the room. Treat a good reading as evidence that the outlet is working, not that the far corner is clean.

1. 1

   Pick the room you occupy longest with the door shut

   Usually the bedroom. That is where a limited budget converts into the most air changes, and where the hours are.

2. 2

   Stand it clear of walls and furniture

   Aim for roughly 30 to 50 cm around the intakes and above a top outlet. Never in a corner, never behind a sofa, never under a shelf.

3. 3

   Give the outlet somewhere to throw

   The jet from the outlet is what stirs the room. Point it across open floor rather than into a wall a hand-span away.

4. 4

   Shut the door

   A closed door turns an unbounded problem into a 37 to 90 m³ one. That single act is worth more than a fan speed.

5. 5

   Leave it running

   Low or auto, continuously. Turn it up for cooking, cleaning, or after the windows have been open, then let it drop back.

6. 6

   Check the clearance again in a month

   Purifiers migrate. A month later it is behind the chair, and nobody noticed the day it moved.

Placement is the last step of a decision that starts with the pollutant and the room — the whole sequence is in how to choose an air purifier .