How air purifiers work - we reveal how air purifiers leave your home cleaner and fresher

Wondering how air purifiers work and whether you need one? Here’s what you need to know

how air purifiers work, air purifier on green background
(Image credit: Getty Images)

How air purifiers work might seem complex - but it's actually pretty straightforward. The best air purifiers can help you breathe more easily and clean your room - plus so much more.

Many of us might not count air purifiers as an essential item in our home. But air purifiers are there to get rid of the 'stuff' that we can't see, but that may be harming us in different ways - causing us to cough, sneeze, sniffle, or be something that may even be hurting our lungs. We can’t see most of it, but the air around us is full of particles, and some of that can be a problem - because microscopic allergens and other irritants can affect our bodies.  

Air purifiers promise to remove those irritants to help us breathe cleaner and fresher air. So, how do they work and what do the various numbers and acronyms in the product descriptions mean? Let’s find out.

How air purifiers work: the ins and outs

What is an air purifier?

At its simplest, an air purifier is just a filter with a fan attached to it. The fan moves air through the filter, the filter traps particles, such as dust, pollen, dust mite residues, pet dander and so on, and the fan moves the newly cleaned air around the room. It’s like a vacuum cleaner for the air you breathe. 

How does an air purifier filter things?

The best air purifiers for home use will contain a HEPA filter. HEPA stands for high-efficiency particulate air and it’s a paper-like filter made from lots and lots of tiny interwoven fibres. Those fibres are woven so closely that large particles can’t get through, but air can – so the air passes through the filter and comes out the other side with many of its impurities removed. 

Some air purifiers supplement their HEPA filter with an ioniser. That fills the air with negatively charged ions and their job is to attract positively charged particles, such as dust and dander. When they bump into those particles, each pair of positive and negative particles becomes a single, larger particle – and larger particles are much easier for the filters to trap.

How effective is a HEPA filter?

A HEPA filter must be able to remove 99.97% of airborne particles that are 0.3 microns or bigger. Dust, pollen and dander particles are all much bigger than that. 

Can air purifiers filter everything?

No home air purifier can remove everything unpleasant from the air and some cope better with some things than with others. Large particles, such as pollen and dust, are fairly easy to filter out, which makes the best air purifiers for dust and the best air purifiers for allergies an effective choice. But volatile organic compounds (VOCs) or gases such as those produced from cigarette smoke are too small for HEPA filters to trap them effectively. If you need to reduce those kinds of particles, you’ll need an air purifier with activated carbon filters. But whichever purifier you have will help improve the air quality in your home.

how air purifiers work, air purifier filter

(Image credit: Getty Images)

Can air filters get rid of coronavirus?

It’s possible, at least theoretically. The airborne droplets that spread coronavirus are the perfect size for most air purifiers to trap, but for that to happen, you need to get the particles into the filter. While an air purifier might help reduce the amount of droplets in a small room used by someone who is positive and whose coughs are putting more droplets into the air, you can achieve very similar effects by opening the windows. Consumer Reports has a good round-up of the current thinking regarding air filters and the virus here. The short version: Assume it doesn’t make any difference.

It's also important to remember to clean your air purifier regularly, to ensure that your air purifier is working as best it can to protect you against any pollutants and particles that could be harmful.

I’ve seen some purifiers listed with CADR ratings. What are they?

CADR is short for Clean Air Delivery Rate. It tells you how much filtered air a purifier produces in cubic feet per hour, but it doesn’t tell you how well the filter works. In other words, it’s a measure of cleaning capacity, not cleaning performance. 

How can I tell whether an air purifier is any good?

An air purifier needs to be powerful enough for the room you’re in. Many air purifiers detail their maximum recommended room size, and you can use that to see whether that particular purifier has enough air power for the place you want to put it. 

If you don’t know the size of your rooms, the LABC (which represents councils’ building control teams) has a good rule of thumb – it says that the average UK room sizes are 17 square metres for a living room, 13 square metres for a master bedroom and 13 square metres for a kitchen. 

A second thing to look at is how many times an hour the air purifier changes the air. Some will change the air five times an hour; some less than twice. To really feel all of those good air purifier benefits, the former is generally better.