How we test air purifiers

The air inside your home may well be more polluted than the air you breathe on your daily commute. Here's how we test to find the best air purifiers.

Air purifiers suck in air using a fan, pass the air through a filter to capture particles of pollution, and then push the cleaner air back out.

Air purifiers can help reduce the level of certain pollutants in your home, but they can't eliminate the problem of indoor air pollution altogether. The best way to improve your indoor air quality is to remove the sources of pollution and ventilate with plenty of fresh air.

Our air purifier reviews show which models you can rely on to clean the air in your home quickly and effectively, removing common allergens. We also show you the models to avoid.

Only the models that top our tests in each of these areas join the ranks of our best air purifiers

Why our air purifier tests are different

We conduct rigorous lab tests to answer the most important air purifier questions, including:

• How swiftly does it remove common allergens at a range of particle sizes from the air?
• How noisy is it on full and low power?
• How much energy does it use?
• Are the filters easy to remove and replace?
• If it's a smart air purifier, controllable from a smartphone app, how easy is that app to use?
• Should you buy it?

How well does each air purifier remove common household allergens from the air?

We measure the CADR (Clean Air Delivery Rate) of each machine.

CADR is a measure which looks at particle removal over time. It allows us to compare how efficiently different air purifiers can clear the air of different-sized particles.

We base our tests on CADR because it is an internationally recognised test standard. We use small, medium and large aerosol particles, which represent common household allergens such as pollen, dust and smoke.

Some air purifiers do fantastically well in our CADR tests, while others really struggle – meaning they'll trap far fewer particles, and produce far less clean air than a good air purifier will in the same amount of time.

In 2024, we added a new gas separation test that gives an indication of how well the air purifier can remove odours and gas pollutants from the air. We use a gas created from a mix of volatile organic compounds (VOCs).

We also introduced a new filter ageing test at the same time, which shows how well the air purifier works when the filters have already put in a big shift. We simulate the ageing of the air purifier’s filters by exposing them to the smoke from 100 cigarettes. We then repeat the air purification tests using small, medium and large particles.

The performance of some air purifiers drops off significantly when the filter is aged, while others will continue to do a good job of purifying the air.

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How noisy is the air purifier on full and low power?

As air purifiers have to move air, they all make a certain amount of noise. But some are louder than others – and the last thing you want, if you're trying to work, study or just relax at home, is to be distracted by a noisy machine.

You should run the air purifier on its highest setting when you're not in the room, so it can work really hard to clean the air without disturbing you, and then dip it down to low when you're back. Even on low, though, some are louder than you'd expect.

We test air purifiers on both their highest and lowest settings, so we can tell you which ones are most likely to annoy you.

And we don't just measure volume of noise: our expert panel also listens out to see if any make sounds that are really irritating.

How much energy does each air purifier use?

We measure the energy consumption of each air purifier while we’re carrying out our air purification tests. This gives us a reading of the power the air purifier uses, which we use to calculate our energy efficiency rating.

It’s worth noting that portable air conditioners are generally less energy-efficient than split units.

Are air purifier filters easy to remove and replace?

Your air purifier filter will need to be replaced periodically: how often will depend on how much you use your air purifier and on the manufacturer's instructions. With some models, you can clean the filter to extend its life a bit (but check the manual before doing so, in case you damage it). So you'll want know that's not going to be a faff for you each time. 

We remove and replace the filters on all of the air purifiers we test, to see which ones are easy to pop out, and which are a struggle to remove. 

If it comes to our attention that an air purifier has a long-term stock issue with replacement filters, then we add that information to our review, and that product cannot be a Which? Best Buy.

Our additional ease-of-use assessments include how simple it is to use the controls and whether it’s easy to move the appliance around.

We also look at the instruction manual to check that it’s clear to follow and understand. Our ease of use tests combine to generate an overall star rating. You can use this rating to avoid any models that are likely to be a pain to use.

If an air purifier comes with a smart app, how easy is it to use?

We check how easy it is to connect the smart app to the air purifier and set the machine from the app. We check how easy it is to operate the air purifier in a basic way from the app, including turning the air purifier on and off and changing the fan speed.

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Should you buy it?

All of the assessments above contribute to making up a total test score, which is the overall percentage figure we give to each air purifier. Some tests are more important than others in determining just how good each air purifier is, so carry different weightings. For instance, particle removal is more important than ease of use or energy use, because an air purifier’s main job is to clean the air. 

The total test score is made up of the following:

• 60% Performance – particle and gas removal, clean air delivery, filter ageing test
• 15% Noise
• 10% Ease of use
• 10% Energy efficiency
• 5% Cleaning and maintenance

What are Which? Best Buys, Don't Buys and Great Value models?

The air purifiers that impress the most in our tests are made Best Buys. An air purifier needs to earn 78% in our tests to become a Best Buy, but you should still read our reviews to check for any factors that might impact your buying decision.

Air purifiers that receive a poor score of 45% or lower are Don't Buys and should be avoided – these are likely to have performed poorly or only averagely in our CADR tests or they may be very noisy. 

Great Value models do pretty well in our tests – they need to score at least 60% or more in our tests and achieve at least a four-star rating for small, medium and large particle removal. They're also at least 20% cheaper than the average cost of other air purifiers in our tests. These are models highlighted by our experts to go for if you're looking for a bargain. Some Great Value models also score well enough to be a Best Buy.

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