Do you actually need an air purifier at home?
Most pages answering this question are trying to sell you something, and it shows in the way they answer. This one starts from what a purifier physically does, which turns out to rule out a lot of the problems people buy one for.
What a purifier actually does
A portable air purifier is a fan and a filter in a box. It pulls room air through filter media that traps particles, and pushes the filtered air back out. Everything it can do for you follows from that one sentence, and so does everything it cannot.
It works on what is floating in the air right now. It does not reach into a carpet, a mattress or a sofa, where a large share of household dust and dander actually sits until something disturbs it. It does not ventilate: the air in the room is the air it works with, and carbon dioxide from the people in it keeps climbing regardless. And it does not change humidity at all, which matters more than it sounds.
One more limit worth stating plainly, because almost nobody does: particle filtration and gas or odour control are different jobs done by different materials. The dense particle stage does effectively nothing for smells or fumes. That is the activated carbon stage, and how much carbon a unit carries varies enormously between models — a thin carbon-coated sheet and a packed cartridge are both sold as "carbon filtration" and they do not last remotely the same.
Three questions before you spend anything
FIRST: can you remove the source? Damp is the clearest case. If a room smells musty or the windows run with condensation, the issue is moisture, and a purifier will not lower it by a single percentage point. Humidity control comes first, and no amount of filtration substitutes for it. The same logic applies to a gas hob, a litter tray, or a smoker indoors: extraction and ventilation beat filtration, and they are usually cheaper.
SECOND: can you ventilate instead? Opening a window is the highest-capacity air treatment most homes have, and it costs nothing. The case for a purifier gets stronger exactly where ventilation makes things worse — during a pollen peak, when outdoor air is smoky, or on a busy road. If your problem disappears when you open a window, you may not have a filtration problem.
THIRD: does the particle load keep coming back? This is the question that actually justifies buying one. Pollen in spring with the windows open, dander in a flat with a cat, smoke drifting in during wildfire season, dust from a nearby construction site: these keep arriving, cannot be removed at the source, and are exactly what a filter is for.
What the numbers on the box are worth
Two figures dominate the marketing, and only one of them is comparable between brands.
CADR — Clean Air Delivery Rate — is the useful one. It combines how well the media captures with how much air the fan actually moves, measured separately for smoke, dust and pollen. A superb filter behind a weak fan gives a poor CADR, which is why the filter grade alone tells you very little. The figure is worth most when a third party measured it: AHAM Verifide means a laboratory did. A CADR with no AHAM entry is the brand’s own number, and nobody outside the company has checked it.
The square footage on the box is the one to distrust, because it is not comparable until you know the air-change rate behind it — and manufacturers pick different ones. The same machine can honestly be sold as "1,051 sq ft" at one air change per hour and "219 sq ft" at 4.8. Both numbers are true. They differ by a factor of five. AHAM’s convention is 4.8 air changes an hour, and when a listing quotes a much larger room without stating a rate, that is usually why.
And a caveat that the specification sheet will not give you: those figures are measured at maximum fan speed, which on most units is the setting nobody can sleep through. A purifier only works while it is running. The decibel figure at the speed you would actually use is, for a lot of people, the specification that decides whether the machine gets used at all — and a fair number of manufacturers do not publish one.
What we are not going to tell you
We do not publish claims about what a purifier will do for anyone’s health. Not because the question is unreasonable — it is the reason most people are reading — but because the honest answer requires evidence we do not have and cannot generate. Reviewing specifications does not qualify anyone to say what a machine will do for a person, and the sites that do say it are almost always converting a laboratory figure into a promise it was never measured to support.
The most common version of that trick is worth learning to spot, because once you see it you see it everywhere: a filter efficiency figure, measured on a test rig at a fixed flow rate against a specific particle size, restated as though it described the air in your room. A filter grade describes what happens to air passing THROUGH the media. What happens in a room depends on how much air actually gets through it, how big the room is, how much leaks around the unit, and what keeps coming in. Those are different questions, and the answer to the second one is CADR.
Where a manufacturer does not publish something, our reviews say so rather than filling the gap with an estimate or a sibling model’s figure. Missing data is information too: it tells you what the company was willing to stand behind.
Questions people ask about this
Are air purifiers worth the money?
They are worth it when you have a particle problem that keeps returning and cannot be fixed at the source — pollen, outdoor smoke, dander, road dust. They are a poor answer to damp, to a smell with a findable source, or to stuffiness, which is a ventilation problem. Work out the yearly filter cost before deciding: on some units it is a substantial fraction of what the machine cost.
Should I leave an air purifier running all the time?
A purifier only affects the air while it is running, and the particle load returns once it stops, so continuous running at a low speed is generally more useful than bursts at maximum. That is also why the noise figure and the wattage matter more than they first appear: a machine that is too loud at the speed your room needs is a machine that gets switched off.
Will an air purifier help with damp or mould smells?
No. A purifier does not change humidity, and damp is a moisture problem. If a room smells musty or the windows are running with condensation, humidity control and ventilation are the answer. Filtration does not substitute for either.
Do I need one in every room?
A portable unit treats the room it is in, and an open door makes the effective volume much larger than the floor area you measured. Most people are better served by sizing one unit properly for the room where they spend the most time than by spreading undersized units around the house.
Where this comes from
Written from published standards and manufacturer documentation. We do not bench-test units, and nothing here is a health claim — where the honest answer is that something is not documented or not measured, the page says so.