In depth: air purifiers explained
How we ranked the five purifiers
We do not test air purifiers ourselves. Each model gets a score from 0 to 100 on seven criteria: particle clearing, noise, room size, running costs, features, owner reviews and price. The overall rating is a weighted average calculated automatically, and the ranking follows that rating.
Particle clearing counts for 30 per cent and rests on Expert Reviews' measured times and the stated CADR. Noise and running costs count for 15 per cent each, and room size, features, owner reviews and price for 10 per cent each.
Running costs are scored on how often filters need replacing, their price where known, and power use. Where the filter price is not stated in our sources, we score cautiously rather than guess, which affects the Dyson and Philips.
The Blueair 3250i wins because it scores well on everything: fast clearing, low noise, smart features and the strongest owner reviews, at a mid-range price. Its main weakness is filter cost. The Dyson cleans fastest and covers the largest rooms but is held back by its price.
Below the top two, the Philips and Shark are almost level. The Philips scores on price, room size and owner reviews, the Shark on filter life and particle clearing. If filter costs matter most to you, the Shark may be the better buy despite its rank.
The Philips, Shark and Blue Pure 511 each lead on one point: room coverage for the money, filter life and quietness at a low price respectively. Commission from retailers never affects scores or ranking.
What Expert Reviews' tests show, and where they stop
Expert Reviews first uses each purifier at home against everyday particles such as wood smoke, pollen and pet dander, then runs a standardised test: five seconds of test aerosol in a small room, with an air quality meter recording how long particles take to return to normal. It also measures noise at several settings and power use.
The test is a useful comparison of how quickly each purifier reacts. But starting levels varied from about 140 to about 395 µg/m³ across the models here, so a direct ranking by time alone would be misleading. We also consider CADR and Expert Reviews' written verdicts.
Expert Reviews does not measure gases, odours or formaldehyde, even though the Dyson is designed to target them. Our scores therefore cover particle removal only.
Expert Reviews' roundup also has a few inconsistencies, such as two model numbers for the Philips. Where figures conflict, we note it and use the figure that matches other sources.
Owner reviews at Argos add information about reliability, noise in real homes and filter availability. We read them alongside Expert Reviews' measurements but give them less weight, since owners cannot measure particle levels precisely.
Taken together, the measurements, CADR figures and owner reviews give a consistent picture: the Blueair 3250i and Dyson clean fastest, the Philips covers the most room for the money, and the Blue Pure 511 is the quietest budget choice.
Which models we left out
Expert Reviews' roundup also includes the Hoover H-Purifier 500, Blueair Blue Signature and Levoit Sprout, and mentions the Levoit Core 300S.
The Hoover H-Purifier 500 is a Best Buy at Expert Reviews, but we could only find it out of stock or as a refurbished unit at UK retailers on the assessment date. The Levoit models were available mainly through Amazon marketplace listings we could not verify.
The Blueair Blue Signature is sold in several sizes, and we could not confirm which size Expert Reviews tested, so its measured results could not be matched to a product at a UK retailer.
We also left out Shark's smaller NeverChange5 Compact Pro, sold at Argos, because Expert Reviews tested the larger HP150UK, and the two have very different room size ratings and CADR.
We also searched for air purifiers from Addrevenue advertisers in the UK but found none. All buy links go to UK retailers.
We will review the selection when Expert Reviews updates its roundup or when the excluded models return to stock at UK retailers.
Do you need an air purifier at all?
For many homes, good ventilation and regular cleaning keep indoor air in a reasonable state without a purifier. A purifier makes most sense where outdoor air is polluted, near busy roads, during wildfire smoke or high pollen periods, in homes with wood stoves or heavy cooking, or where pets shed a lot.
A cheap air quality monitor can show whether particle levels in your home are actually high before you spend money on a purifier. Several purifiers here, including the Blueair 3250i, Philips and Dyson, have built-in sensors that show the same information once you have one.
Outdoor pollution data can help you decide. Local air quality forecasts show when particle levels are high, which is when a purifier with the windows closed makes the most difference indoors.
If you decide to buy, start with the room where you spend most time, usually the bedroom or living room. Size the purifier for that room and run it continuously on a low or automatic setting.
Avoid purifiers that produce ozone, which some ioniser designs do. None of the five purifiers here is described as an ozone generator in our sources.
Finally, remember the ongoing costs. A purifier is not a one-off purchase: filters and electricity add up over the years, so budget for them from the start.
If you rent, a purifier is also a portable solution that moves with you, unlike changes to ventilation. Lighter models such as the Blue Pure 511 and Philips are easy to take from one home to the next.
Buying guide: choosing the right air purifier
Size the purifier to your room
The most common mistake with air purifiers is buying one that is too small for the room. A purifier works by pulling air through its filter, and the larger the room, the more air it has to move to clean it in a reasonable time.
Manufacturers state a maximum room size, but the figure depends on how often they assume the air is cleaned per hour. Shark's own site shows this clearly for the NeverChange5: up to 60 m² at one air change per hour, but 12 m² at 4.8 air changes per hour.
A useful rule is to choose a purifier rated for a larger room than yours, so it can clean effectively on a quieter, lower setting. For a 20 m² bedroom, a model rated for 38 m², like the Blue Pure 511, gives you room to run it quietly.
Air changes per hour describe how many times the purifier can filter the full volume of air in a room each hour. More changes mean cleaner air, especially when new particles keep arriving, for example from cooking, pets or an open door.
Measure your room before you buy. Multiply length by width to get the floor area, and remember that open-plan spaces count as one room. The Dyson, rated for 100 m², is the only model here designed for very large open-plan areas.
If you want clean air in several rooms, two smaller purifiers often work better than one large one, since air does not move freely between closed rooms.
Ceiling height matters too. A room with a high ceiling holds more air than its floor area suggests, so choose a larger purifier for lofts, period homes and double-height spaces.
Doors matter more than many buyers expect. A purifier in a bedroom with the door open is effectively cleaning the landing and hallway too, which can make it feel less effective than its rating suggests.
CADR and particle clearing tests
Clean air delivery rate, or CADR, measures how much filtered air a purifier delivers per hour, usually given in cubic metres. A higher CADR means faster cleaning. The models here range from 147.8 m³/h for the Shark, according to Expert Reviews, to 250 m³/h for the Philips, according to Argos.
CADR is measured by manufacturers, sometimes separately for smoke, pollen and dust. Expert Reviews' own test gives a practical check: it sprays a test aerosol into a small room for five seconds and records how long the purifier takes to bring fine particles (PM2.5) back to a normal level of 25 µg/m³.
The results range from just under 140 seconds for the Dyson to just over three and a half minutes for the Blue Pure 511. The starting levels differed, from 140.7 µg/m³ for the Dyson to 395 for the Blueair 3250i, so the Blueair's 2 minutes 23 seconds from a much higher level is a strong result.
In practice, cooking, candles and wood stoves produce exactly the kind of burst of fine particles that Expert Reviews simulates. A purifier with a fast clearing time brings the air back to normal sooner after such events.
These tests show how quickly a purifier reacts to a burst of particles, such as smoke from cooking. In everyday use, a purifier runs continuously and keeps particle levels low, so the differences matter most when air quality suddenly worsens.
CADR figures are often quoted separately for smoke, pollen and dust. Argos lists the same CADR for all three on the Blueair 3250i, Philips and Blue Pure 511, which makes comparison straightforward.
A simple way to use CADR is to compare it with your room's volume. Multiply floor area by ceiling height to get the volume in cubic metres; a purifier whose CADR is several times that volume can clean the room several times an hour on full power.
Filters and running costs
All five purifiers here use HEPA filters, which trap very fine particles. Most also have a pre-filter that catches larger dust and hair and can be washed, which helps the main filter last longer.
The main filter has to be replaced on most models. Argos lists the Blueair 3250i filter at £35 and suggests replacement about every six months with continuous use. For the Blue Pure 511, an Argos answer suggests six to nine months, and Argos says the Philips filter lasts up to a year.
Shark takes a different approach with the NeverChange5: its HEPA filter can be cleaned and, according to Shark, lasts up to five years. Expert Reviews calls this good filter value but notes the claim has not been verified over time.
Over three years, filter costs can match or exceed the purchase price of a budget purifier. Before buying, check the price and availability of replacement filters, and whether genuine filters are easy to find. Some Argos owners mention difficulty finding filters for the Philips.
Some third-party filters are sold as compatible for popular models. They can be cheaper, but their filtering performance is not tested by our sources, so genuine filters are the safer choice if you want the results Expert Reviews measured.
Electricity is a smaller cost. The Blueair 3250i uses 3 to 19 W in Expert Reviews' measurement, and the Blue Pure 511 and Philips up to 16 and 21 W according to Argos. Even running all day, that is modest compared with most household appliances.
Check whether filters are sold by the retailer you buy from. Argos sells the Blueair 3250i filter, which makes replacement easy; for other models you may need to order from the manufacturer.
Write the date on a new filter or set a phone reminder when you fit it. Some filter indicators may count running hours rather than measure the filter itself, so treat them as a guide rather than an exact signal.
Noise and night use
Purifiers work best when left running, so noise matters, especially in bedrooms. Most are quiet on low and noticeably louder on full power.
Expert Reviews measured 27.6 to 49 dB for the Blueair 3250i and up to 47 dBA for the Blue Pure 511, whose lower settings it describes as almost silent. The Dyson runs from 32 to 53 dBA, and Expert Reviews found the Shark noisier than the others.
Night modes help. The Blueair 3250i's night mode runs the fan on its lowest speed and turns the lights off, and the Dyson has a dedicated night mode. Bright indicator lights can be as disturbing as noise in a dark bedroom.
If you are sensitive to noise, run the purifier on a higher setting while you are out of the room or before bed, then switch to night mode while you sleep.
Owner noise ratings at Argos are low for several models, including the Philips and Blue Pure 511. These ratings probably reflect the top setting, so a purifier sized well for the room and run on a low setting will be much quieter than the ratings suggest.
If noise matters most, compare the low setting rather than the maximum. A purifier that is quiet on low but effective enough for your room size will run unnoticed most of the time.
Noise also changes as filters fill up. A clogged filter makes the fan work harder, so keeping the pre-filter clean and changing the main filter on time helps keep a purifier quiet.
Automatic mode, sensors and apps
An automatic mode uses a particle sensor to raise the fan speed when air quality worsens and lower it when the air is clean. It saves energy and noise, and means you do not need to adjust the purifier yourself.
The Blueair 3250i reacted within two or three seconds to changes in Expert Reviews' testing, and its front light changes colour with air quality. The Philips shows a PM2.5 reading and a coloured band, and the Dyson also monitors CO2, temperature and humidity.
App control lets you schedule the purifier, check air quality and get filter reminders on your phone. The Blueair 3250i and Dyson have apps; the Philips and Blue Pure 511 do not.
Apps can also show air quality history, which helps you see when particle levels rise, for example while cooking or when windows are open. That information can be more useful than the purifier itself in changing habits.
Simpler is not always worse. Expert Reviews praises the Philips for being easy to set up and use, and the Blue Pure 511 has just two buttons. If you want to plug in and forget, these may suit you better than a smart model.
Voice control through Alexa is available on the Blueair 3250i, but Expert Reviews found the commands awkward, so treat it as a bonus rather than a reason to buy.
Sensor readings are useful as a guide but are not precise measurements. Expert Reviews found the Levoit Sprout's air quality monitor sluggish, which shows sensors vary between models.
Filter reminders are one of the most useful smart features. The Blueair 3250i alerts you through its light and the app, and the Philips, Dyson and Blue Pure 511 have filter indicators, according to Expert Reviews.
Placement and everyday use
Place the purifier where air can flow freely around it, away from walls and furniture that block the intake. Check the manual for the recommended clearance, and place it near where you spend most time.
Avoid placing it right next to a radiator, in direct sunlight or in a damp corner. Heat and moisture do the filter no favours, and a corner restricts the airflow around the unit.
Keep doors and windows closed while it runs if you want to clean the air in one room. An open window lets new particles in as fast as the purifier removes them.
Run it continuously on a low or automatic setting rather than switching it on occasionally. Particle levels build up quickly, and a purifier that runs all the time keeps them low with less noise.
Clean the pre-filter regularly. One Argos owner of the Philips says it needs cleaning about every two weeks, depending on the environment. A clogged pre-filter reduces airflow and makes the purifier work harder.
Large purifiers like the Dyson need floor space. Argos owners warn that it takes up a lot of room and cannot sit on a table, so plan where it will go before buying.
In homes with pets, hair and dander collect on the pre-filter quickly, so check and clean it more often.
Moving a purifier between rooms is possible with the lighter models. The Blue Pure 511 weighs 1.9 kg and the Philips 3.6 kg, according to Expert Reviews, while the Dyson's 11.9 kg makes it a fixture.
What air purifiers do not do
Air purifiers capture particles in the air, but they do not remove dust that has settled on surfaces, and they do not replace ventilation or cleaning. Regular vacuuming and dusting are still needed.
Most purifiers with a carbon layer can reduce light odours, such as cooking smells, but our sources do not measure this. Argos notes that the Blueair 3250i's filter has a carbon layer that can help with smells.
Purifiers do not control humidity or remove damp. If condensation and mould are a problem, a dehumidifier and better ventilation address the cause.
They also cannot remove all gases. Carbon layers reduce some odours and volatile compounds, but how much varies between models, and our sources do not measure it. The Dyson is designed to target formaldehyde, but Expert Reviews did not measure that either.
We make no health claims for any purifier. If you are considering one for a health condition, speak to your GP or pharmacist, and treat any health claims from manufacturers with caution.
Air purifiers are no substitute for opening windows when outdoor air is clean. Ventilation removes stale air and moisture, while a purifier cleans the air that stays in the room.
Price and where to buy
The purifiers here range from £99 to £900. The mid-range models offer the best balance: the Blueair 3250i at £170 and the Philips at £99.99 cover most UK rooms well.
Prices change often. The Philips was reduced from £139.99 at Argos on the assessment date, and Expert Reviews quotes £130 for it. The Shark was listed at £279.99 on Shark's own site but out of stock there, while Harvey Norman had it for £249.
Add the cost of filters over the years you expect to use the purifier when comparing prices. A cheaper purifier with expensive, frequent filter changes can cost more over time than a dearer one with long-lasting filters.
Argos lists two-year guarantees for the Blueair 3250i, Blue Pure 511, Philips and Dyson. Shark lists two years, with a longer guarantee mentioned on its site without clear terms.
If you buy during a sale, check that the model number matches the one Expert Reviews tested. The Philips at Argos is listed under several model numbers, which can make comparisons harder.
Buying from a UK retailer gives you UK consumer rights, and makes returns and guarantee claims simpler. Check that replacement filters are easy to buy in the UK before choosing a less common model.
