How we test air purifiers
The air inside your home may well be more polluted, and more harmful to your health, than the air you breathe on your daily commute. Here's how we test to find the best air purifiers.
Air purifiers are devices which suck air in using a fan, pass the air through a filter or several filters to capture particles of pollution, and push the cleaner air back out.
Air purifiers can't eliminate the problem of indoor air pollution. The best way to improve your indoor air quality is to remove the sources of pollution and to ventilate with plenty of fresh, outdoor air. When that's not possible, or not sufficient, air purifiers can help by reducing the level of certain pollutants in your home.
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? And how secure is your data?
- Should you buy it?
How well does each air purifier remove common household allergens – dust, pollen and smoke particles – from the air?
We measure the CADR (Clean Air Delivery Rate) of each machine.
CADR is a measure which looks at both particle removal and air flow. It allows us to compare how efficiently different air purifiers can clear the air of dust, pollen and smoke particles.
We base our tests on CADR because it is an internationally recognised test standard. Dust, pollen and smoke are common household allergens and represent a range of particle sizes.
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.
How noisy is the air purifier on full and low power?
Air purifiers need to be run pretty much constantly to be effective: you can't just run your air purifier for an hour or two and expect it to clean your air.
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.
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.
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.
We also check how secure each smart air purifier is.
Are there any security issues?
While it might sound random – even a bit ridiculous – for a cybercriminal to want to target your air purifier, it's a genuine, and scary, possibility.
There are several ways in which a hacker could harm you and others by gaining control of your air purifier. For example:
- The personal data gathered about you could be used to build a detailed picture of your daily routine, including when you're usually away from home
- The settings could be changed, say, by a vindictive ex-partner, to confuse and intimidate you
- The air purifier could act as an entry point to your entire home internet network, potentially compromising other devices such as laptops and smartphones
- The air purifier could be weaponised – used as part of an army of botnets to carry out a large-scale attack on another organisation by flooding that organisation with requests.
We check that your personal data is encrypted: encoded so that it's no longer in a readable format.
We also check where data is being sent, particularly if that’s outside of the EEA (European Economic Area).
Firmware (the software or set of instructions programmed into the hardware of the air purifier) that's out of date or hasn't been maintained could leave gaps through which a hacker could access your air purifier.
Data transparency (permissions)
It's important for apps to only request data that's genuinely necessary. We check what data the app is asking for, and we assess the reasons behind it, particularly when it comes to things like your location or access to your camera.
We look at whether air purifiers force you to enter strong, difficult to guess passwords, or allow you to choose ones that could be easily cracked.
Some connected devices might still allow basic passwords such as ‘password’ or ‘1234’ – although this is less common than it used to be. Or they might have a default password, such as ‘admin’, and won’t oblige you to change this.
If that's the case, we’ll highlight this in our reviews and give you advice on how to set a good password.
Our tests also look at how easy it is to reset the product and wipe all of your data when it’s time to get rid of your smart air conditioner. If you’re selling your air conditioner on to someone else – which, from an environmental perspective, is much better than getting rid of it – then you’ll want to be sure none of your data is still attached to it.
This is when a hacker snoops on information passing to and from your air purifier. We check if every smart air purifier that goes through our lab tests is vulnerable to this kind of attack.
Other types of attack
Cybercrime is always evolving, so we'll be updating our test programme as necessary in the future.
We put every smart air purifier we test through a full privacy and security test programme, to check for security loopholes that hackers could exploit.
While it’s impossible for us to say that any smart device is 100% impenetrable to hackers, we check for the most common vulnerabilities and flag anything untoward with manufacturers.
The air purifiers that we recommend had nothing that worried us at time of testing.
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, we think that 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% CADR – particle removal and clean air delivery
10% Ease of use