What exactly does an air purifier do?

Science has proven that air purifiers improve indoor air quality - but what do they actually do?
Updated on May 25, 2024
Written by
Teddy Booth
Teddy joined HouseFresh in 2019 as a senior content producer, producing written content as well as product photography and YouTube videos. He supports Danny as a product tester and is our lead how-to writer.

Air purifiers are simple devices, yet the effects they have on the quality of air we breathe are substantial.

Basically, an air purifier removes harmful airborne particles from a room by drawing in air, passing it through one or a series of filters trapping pollutants, leaving you with fresh, clean air to breathe.

You’ve undoubtedly landed on this page because you have a critical question on your lips…What exactly does an air purifier do?

You may have even seen one in a friend’s house, in your child’s classroom, in a waiting room, on the news or in a store and thought, “Do I need one of those?”

Don’t be put off by all the scientific jargon that comes hand-in-hand with air purifiers like HEPA, CADR and PM2.5. Studies by the EPA show how running an air purifier in your home will drastically increase air quality. Let me put it as clearly as I can for you.

An air purifier cleans the air by removing harmful airborne pollutants.

In this article, I’ll guide you through the ins and outs of how every household would benefit from an air purifier, who particularly needs one and how it cleans our air. But first, check out Danny’s video, simply explaining what an air purifier does.


What does an air purifier do? An air purifier is simply a fan and a filter If I attached this mask to this tiny fan, I could create my own tiny little air purifier – its probably not going to do much good but you get the idea. Most air purifiers you can buy use a combination of filters with a pre-filter, HEPA and activated carbon. The Pre-filter is the first stage of cleaning the air and catches the large dust and hair particles that are easy to see. Next, we have the HEPA filter, designed in 1950 to catch tiny particles so small you can’t see them. HEPA filters pick up tiny dust particles, pollen, bacteria, and viruses and do the bulk of the air cleaning work. A HEPA-grade filter alone won’t mean you will have clean air; you still need a device to push enough air through the filter. For large spaces, you will need a big HEPA filter with a big fan as this will clean the air multiple times in an hour. Smaller rooms can work with smaller devices but don’t go too small. Many small air purifiers are bestsellers on Amazon, but when we tested them, we found them to be too weak for even the smallest of rooms. Finally, we get to the activate carbon filter. While HEPA can deal with tiny particles in the air, it can’t remove any type of gas like VOCs or odors. Its why you can still smell a fart even when wearing a mask. Activated charcoal helps to trap any gasses as the air flows through them. As with HEPA, size matters, so if you have a serious issue with any VOC, Odors or gasses, you will want to get an activated carbon filter measured in lbs. A really simple way to understand what an air purifier does is to build one yourself. With an HVAC filter, a box fan, some cardboard and duct tape you can create your own air purifier that can remove particles from the air in your home. At HouseFresh we have tested over 60 different air purifier models so If your looking for an air purifier Google housefresh to find out what we think are the best devices right now. #airpurifier #airpurifierrecommendation #hepafilter #airfilter

♬ original sound – HouseFresh

Now we know the fundamentals, let’s look further into how an air purifier cleans the air.

How does an air purifier work?

Air purifiers comprise two essential components: a fan and a filter. The fan draws in air, passing it through one or a series of filter(s), then pushes the air back into the room. The filters trap particles and gasses, removing them from the air.

It’s as simple as that.

The filters found most commonly in air purifiers are engineered to remove microscopic particles from your air (the strength of the filter determines what size particles will be trapped). These types of filters can capture airborne:

  • Allergens
  • Dust
  • Pet dander
  • Mold spores
  • Viruses and bacteria
  • Odors and VOCs

Studies have shown that running an air purifier in your home can prevent transmission of COVID-19. SARS-CoV-2, the virus that causes COVID-19.

Many air purifiers are tested by a third-party AHAM Verifide, which clearly indicates how powerful a unit is and what size room it can be used in.

Modern air purifiers commonly utilize a combination of filters to remove airborne pollutants. 

With a pre-filter, a main pleated particle filter and an activated carbon filter, air purifiers can trap up to 99.97% of harmful contaminants from your indoor air. Let’s look at each component more closely.

The pre-filter works like a sieve and picks up the large stuff you can see, such as hair or large dust particles – it will help reduce the number of particles that go into the main filters to prolong their life.

Pleated main filter
The main filter is usually made of synthetic materials or fiberglass and does the majority of the air cleaning:

  • Particles measuring up to 10 microns become stuck in the filter fibers as they can’t pass through the holes.
  • Particles less than 10 microns down to 0.3 microns could pass through the holes in the filter, but due to Inertial Impaction, these particles can’t change direction as quickly as the airflow, so they bump into the filter medium and get stuck.
  • As tiny particles less than 0.03 microns are so small, they constantly bump into air particles. Due to this constant movement, they also keep bumping into the filter material, increasing the chance of being stuck each time as they take a zig zap pattern through the filter. This process is called Brownian Motion, so even non-HEPA-grade filters can block these microscopic particles.

Activated carbon filter 
Finally, while particles of all sizes can be blocked with a pleated filter, gasses will just pass through, which is why many air purifiers use activated carbon filters. 

Activated carbon has many tiny holes that gas can become trapped as it passes through, removing it from the air. This process is called Adsorption.

Eventually, the carbon will become full, so you need to replace it regularly for it to continue to be effective.

Pleated main filters need replacing periodically

When these filters trap pollutants, those particles have nowhere to go. This means over time, the filter becomes full and ineffective. You must replace this filter when prompted to keep your air purifier working to its full potential. 

Most modern air purifiers will have a filter replacement indicator light, which will illuminate when it’s time to change filters.

What types of particles can air purifiers remove from the air?

Air purifiers are the most effective way of removing particles from the air, but there are some things that they can’t do. Let’s look at some of the most prevalent.

Particles air purifiers can remove

Allergens. An air purifier will trap small particles that cause allergic reactions. For example, pollen, dust, mold and pet dander.
Pollution. It’s all around us, especially in towns and cities, from cars and factories to the plague of wildfire smoke in rural areas. Air purifiers will remove these harmful particles from your indoor air.
Gasses and VOCs. The activated carbon filter in air purifiers is designed to adsorb gasses and VOCs, such as odors, extracting them from the air.

Particles air purifiers can’t remove

CO2. A standard air purifier won’t be able to remove CO2 from the air. It’s much easier to ventilate for a short period to let this gas escape than try to use filters to remove it.
Non-airborne particles. Air purifiers can only remove airborne particles. When particles settle on furniture and carpets, use a vacuum cleaner with HEPA-certified filtration.
Moisture. Air purifiers cannot add or remove humidity and water particles from your air. You need either a humidifier (add moisture) or a dehumidifier (remove moisture).

Other questions you may have about air purifiers

So now we know what an air purifier does and how it will improve the air quality in your home. But you’ve probably got a few more follow-up questions.

No. You will still need ventilation in your home. 

Whether that’s opening doors and windows or running an HVAC system, ventilation is key to keeping air circulating throughout your space alongside your air purifier. HVAC systems can even be fitted with pleated filters (the same found in most air purifiers).

A pleated filter could be HEPA grade, but it doesn’t need to be, as lower grade filters can still do a good job of removing pollutants as you can pass the air through the filters multiple times. 

Whilst HEPA is required in scenarios like an operating theatre or a nuclear plant as you don’t want any particles to escape, for cleaning the air in the home, a non-HEPA grade filter can handle a higher airflow and will actually clean your room of particles quicker as more air can pass through the filters.

It all depends on the air purifier. Air purifiers are often expensive because they are either high-performance units for larger spaces or use sophisticated technology like AQI sensors and smart functions.

Unfortunately, this isn’t always the case, as some brands have been known to bump up their prices simply because of their brand status. Others make exaggerated claims about their air purifier’s performance to warrant a higher price tag.

Don’t rely on an air purifier’s price; find the right unit that matches your space.

Overall, air purifiers don’t use large amounts of electricity to run. On average, they contribute less than 1% to household energy consumption.

Air purifiers are manufactured to be energy-efficient, but some are more efficient than others. Many units are tested by a third party, Energy Star and given certification to show their efficiency.

If you want to know more, check out our guide: How much electricity does an air purifier use?

If you have any further questions or comments, just drop us an email and someone from our team will get back to you.

So… Do you need an air purifier?

Air purifiers are amazing yet simple devices. With only a few components, an air purifier can radically change the air quality in your home. 

They are scientifically proven to improve air quality by removing harmful pollutants. All those particles in our air caused by traffic, factories, cigarette and wildfire smoke (the list goes on) make their way into our homes, where we breathe them in.

Introducing an air purifier into your home gives you a better quality of life with clean air. That’s why I strongly recommend that every household has at least one.

But it’s not only pollution particles an air purifier can remove. If you suffer from allergies like pollen, pets, dust or mold, the right unit will trap these allergens in its filters, relieving your symptoms.


Some air purifiers are better at tackling allergens than others. If you suffer from allergies, check out our guide to the best air purifiers for allergies, according to our home lab tests.

Even though air purifiers may seem like an extra expense, they don’t cost much to run, and the benefits far outweigh the cons. You can even make your own air purifier with a box fan, an HVAC filter and some duct tape.


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About the author

Teddy Booth

Teddy joined HouseFresh in 2019 as a senior content producer, producing written content as well as product photography and YouTube videos. He supports Danny as a product tester and is our lead how-to writer.

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