How do air purifiers work?

How do air purifiers work? Image of woman sitting next to air purifier with cat
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Within this feature, we get into a pressing question: exactly how do air purifiers work? We're spending a lot more time indoors partly due to the COVID-19 pandemic, and have a more general concern over the quality of the air we breathe. 

A recent United States Environmental Protection Agency report revealed indoor air can have up to five times higher levels of pollutants like mold, pesticides and particulates than the air outdoors, so the idea of a device that removes these pollutants, as well as allergens like dust, pet dander, and pollen, seems not only pragmatic, but a healthy choice. But how do the best air purifiers work, what can they remove, and can they protect against COVID-19? 

How do air purifiers work?

Air purifiers sanitize the air, getting rid of pollutants, allergens, and toxins; they're different from air filters, which only remove them from circulation. But how do air purifiers actually work?

Air purifiers have a relatively simple set-up: a fan that sucks in air, and one or more filters. These filters — usually paper, fibers such as fiberglass, or a mesh — capture and neutralize pollutants and particles as air passes over them, before the clean air is recirculated into the living space.

They're effective at filtering out most polluting particles, although some are likely to still remain on soft and hard surfaces like furniture or walls. The particular airborne particles that get pulled out of the air depend on the type of air purifier and filter used.

Most filters trap relatively large, coarse molecules measuring 5 microns or less, like dust mites and pollen. But how do air purifiers work to filter smaller particles? High-efficiency filters use a dense network of fibers and several layers of intricate weaves to remove pollutants and allergens measuring as small as 2.5 microns, around the size of animal dander. Some air purifiers have ultraviolet filters and use light to destroy biological impurities such as mold and bacteria, while those with activated charcoal can remove gases like volatile organic compounds (VOCs) and smoke particles. As such, air purifiers can help with allergies, to a certain extent.

How do air purifiers work? Image of person with mask looking out of window

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Can air purifiers help to protect against COVID-19?

Early in the pandemic, as a surge of shoppers went online in search of air purifiers, many manufacturers implied their machines could protect against COVID-19.

COVID-19 particles are around 0.1 micron in size — that's 1,000 times smaller than a millimeter, and thinner than a strand of spider web silk — but are always bound to something much larger, like a water droplet or aerosol. Just like other airborne viruses and bacteria, these droplets can be removed by air purifiers. 

As such, the novel coronavirus that causes COVID-19 should get filtered out by air purifiers using HEPA. In that way, such air purifiers could help to reduce the transmission of the virus. However, whether that means these purifiers can stop direct transmission — say an infected person sneezes or breathes out viral particles a few feet from you — is not clear. The air purifier takes time to capture these particles and by the time the air gets pulled into the purifier, it might have already made its way up your nose.

"We are still at the beginning of our research on how to manage and protect the population from COVID-19 and its numerous variations," Aneta Ivanova, an allergy nurse consultant at Midlands Allergy Service in the U.K., told Live Science. "COVID particles do fall within the particle-size range that high-efficiency particulate air (HEPA) filters, found in some air purifiers, can capture, which is 0.01 µM/micron and larger. Air purifiers may improve the environment when placed next to a sick patient but will not protect from COVID."

A 2021 report from the Centers for Disease Control and Prevention (CDC) suggests this could be true. Their carefully controlled study showed HEPA air purifiers, which use mechanical suction to pull air across a high-efficiency filter, could decrease exposure to COVID-19 aerosols indoors, in conference rooms for example. However, the researchers didn't consider factors such as open windows, size of the room, and air flow — all of which will affect how an air purifier operates under real world conditions. We did a separate investigation on whether HEPA filters remove viruses.

Can air purifiers help protect against wildfire smoke?

NASA satellite data suggests climate change is linked to an increase in the average number of wildfires globally, and particularly in the US. These fires produce large quantities of smoke — a complex mixture of fine particulate matter, some 30 times smaller than the diameter of a single strand of human hair, and gases such as carbon monoxide, nitrogen oxides, and VOCs.

These particulates and gases can be damaging to human health, so nearby residents are advised to stay inside and keep doors and windows closed when the air is filled with wildfire smoke. But what about particles that have already entered living spaces? How do air purifiers work against these?

Published in the Environmental Health journal, a 2016 study suggested air purifiers, or portable air cleaners, should be at the forefront of the public health response against indoor wildfire smoke exposure. How do air purifiers work to reduce particulates from wildfires? The study recommended devices with HEPA filters to eliminate particles and the acrid smell. It also notes that air purifiers with electrostatic precipitators — which use electrical energy to charge an incoming stream of particles and collect them on an oppositely charged metal plate — could lower the concentrations of fine particulate matter.

How do air purifiers work? Image of open windows getting fresh air

(Image credit: Getty Images)

Would a DIY air purifier work?

But what if you can't get hold of an air purifier? Could you make your own? Actually yes; all you need is a box fan and a HEPA or high-quality filter. There are two types of air purifier you could make: the first uses a fan and one filter, while the second, which is a bit more complex to construct, uses a double filter set up over the fan.

These homemade air purifiers work in the same way as manufactured devices, and are fairly effective, with The New York Times and Los Angeles Times both having made and tested their own versions. Research from the Agency for Science, Technology, and Research (A*STAR) in Singapore considered different filter materials for their DIY air purifiers, including HEPA filters, surgical masks and melt-blown polypropylene. They found each material was around 80% effective at filtering particles and aerosols.

However, box fans aren't designed for this purpose, and their motor has to work much harder to pull air through a filter, so these homemade air purifiers present an increased fire risk and should be used with caution.

So, how do air purifiers work? By trapping most pollutants and allergens, but some will always remain in the air. Therefore, air purifiers are best used not necessarily by themselves, but to complement other air quality improvement strategies such as increased cleaning or cleaning with eco-friendly products to reduce irritants, using indoor plants to help with air quality, and improving ventilation in the home.

Kerry Taylor-Smith

Kerry is a freelance writer and editor, specializing in science and health-related topics. Her work has appeared in many scientific and medical magazines and websites, including Forward, Patient, NetDoctor, YourWeather, the AZO portfolio, and NS Media titles.

Kerry’s articles cover a wide range of topics including astronomy, nanotechnology, physics, medical devices, pharmaceuticals and mental health, but she has a particular interest in environmental science, cleantech and climate change. 

​​​Kerry is NCTJ trained, and has a degree Natural Sciences from the University of Bath where she studied a range of topics, including chemistry, biology, and environmental sciences.