Do air purifiers actually work, and how do I choose one?

An air purifier is a fan and a filter. It is very good at what that combination can do: pulling particles out of the air in one room, continuously, as long as it runs. Independent testing supports that clearly for particulate. The disappointment comes from the things people expect it to fix that are not particles at all. Carbon dioxide, which is what makes a room feel stuffy, passes straight through. Water vapour, which is what causes condensation and feeds mould, passes straight through. Carbon monoxide passes straight through, which is why a purifier is never a substitute for an alarm. Knowing the boundary is most of the buying decision.

What you'll need

• The floor area and ceiling height of the room it will live in
• An honest statement of the problem you want solved
• Prices for the replacement filters for the models you are considering (optional)

Step by step

1. Name the problem before looking at any machine.Hay fever in a bedroom, a cat in a flat, cooking smells in an open-plan kitchen, smoke from a nearby source, or dust in a room used by someone with asthma — a purifier addresses all of those. Stuffiness, condensation, a musty smell from damp, and a smell that returns instantly are not filter problems.
2. Size it by clean air delivery rate against room volume.CADR is the volume of filtered air a machine delivers per hour and it is the specification that means something. A common rule of thumb is to look for a CADR of at least five times the room volume per hour, so a 35 cubic metre bedroom wants around 175 cubic metres per hour or better. Many machines quote a room size instead — treat it as optimistic and check it is quoted at a realistic air change rate.
3. Insist on a genuine HEPA filter, and read the wording.A true HEPA filter is tested to capture at least 99.95 percent of particles at the hardest size to catch. HEPA-type, HEPA-style and HEPA-like are marketing phrases for filters that were not tested to that standard. This is the one specification worth being pedantic about.
4. Check whether it has activated carbon, and how much.HEPA catches particles; only carbon adsorbs gases and smells. A thin carbon-impregnated fleece bonded to the HEPA filter does very little. If odours or fumes are the reason you are buying, look for a substantial separate carbon stage measured in hundreds of grams.
5. Look at the noise figure at the speed you will actually use.Every machine is quiet on its lowest setting and most are unusable at maximum. The number that matters is the decibel level at the speed that delivers the CADR you need. In a bedroom, a machine you turn down to silent is a machine doing very little.
6. Price the filters for five years.HEPA filters typically run 6 to 12 months and carbon 3 to 6, and for some models a year of filters costs a meaningful fraction of the machine. This is the real cost of ownership and it is where cheap machines with proprietary filters become expensive.
7. Be sceptical of ionisers, plasma and UV features.In domestic machines their contribution is small next to the fan and the filter. Ionisers can produce ozone, which is a respiratory irritant, and some are marketed as filterless for exactly that reason. If a machine has an ioniser, check it can be switched off.
8. Accept that it treats one room.A purifier cleans the air that passes through it, and air does not travel far through a house. One machine in the bedroom of the person with hay fever does more good than one in a hallway. Doors closed, as with a dehumidifier.
9. Run it continuously on a moderate speed rather than in bursts.Particles are continuously replenished by movement, cooking, pets and outdoor air. A machine run for an hour a day makes very little difference; one run continuously on a low or medium setting holds the level down. Draw is typically 15 to 60 watts on those settings, which is small.
10. Do the free things first, and keep doing them.Extraction at the hob, a bathroom fan that works, brief wide-open ventilation, and removing the source where you can. A purifier is a supplement to those and works badly as a replacement for them.

Tips

• For hay fever, the bedroom is the highest-value place for a machine, because that is where you spend the most consecutive hours and where pollen brought in on clothes and hair settles.
• Vacuum and dust before assessing whether the purifier is working. It filters air, not surfaces, and a room full of settled dust keeps re-supplying the air.
• Filter indicators on many machines are timers rather than sensors. Note the fitting date on the filter itself and judge by that plus how the machine sounds and smells.
• If a machine has a particle sensor and a display, that alone is educational — watching the reading spike when you fry something reorders most people's view of indoor air.

Common mistakes

• Buying one to fix a musty or damp smell — Damp is water vapour and the smell comes from mould growing in the fabric of the room. A filter removes neither. The fix is ventilation, a dehumidifier and dealing with the source.
• Buying an undersized machine for a large room — It will run permanently on maximum, be noisy, and still not deliver enough air changes. Sizing to CADR is the difference between a machine that works and one that is decoration.
• Treating it as a substitute for a carbon monoxide alarm — No filter removes carbon monoxide. If there is any fuel-burning appliance in the house, that needs an audible alarm and nothing else does the job.

Alternatives

• Better ventilation: Working extractors and short bursts of wide-open window handle CO2, moisture and cooking fumes, which no filter can. Cheaper and addresses more of the problem.
• Removing the source: Cooking with the lid on and the fan running, fewer aerosols and scented products, and keeping pets out of one bedroom often do more than any machine.
• A HEPA filter on the vacuum instead: If settled dust rather than continuous airborne particulate is the problem, a sealed vacuum with a HEPA exhaust is the more useful purchase.

Questions people ask