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What is a HEPA filter and how does it work?

Updated on January 25, 2024
Written by
Marie Cookson
Based in Manchester, Marie is one of the writers at HouseFresh and our resident dust expert. She works together with our in-house researchers and our Managing Editor to produce in-depth articles offering practical advice on how to tackle indoor air quality issues.

If you invest in an air purifier, we advise going for one with a HEPA filter. 

They can help ease and minimize the symptoms of various health conditions, such as asthma and allergies. True HEPA filters are especially effective as they can remove at least 99.97% of harmful airborne particles.

But don’t just take our word for it — various scientific studies back these remarkable filters.

If you’re curious about air purifiers and whether they could improve your health and well-being, you’ve probably encountered the term “HEPA filter.” 

These filter types are used in the highest-quality devices as they’re super effective at capturing toxic airborne particles, such as pollen and dust. 

But how do they do this? Well, that’s what we’ll be exploring in this article. We’ll also break down the different categories of HEPA filters. 

Keep reading until the end, as we’ll look at how they can help with various health problems, including allergies and asthma. 

What is a HEPA filter (and how do they work?)

Developed in the 1940s during World War II to control the spread of radioactive contaminants, HEPA filters are often used for commercial and personal use. 

And air purifiers are one of the most common devices that utilize HEPA (high-efficiency particulate air) technology. This is because they can help to eliminate harmful airborne pollutants and contaminants from indoor air, particularly in the home.

But how do they do this? Imagine it as a maze or a spider’s web catching prey. 

The filters comprise pleated sheets, creating a mat of assorted and randomly arranged fibers. A fan within the purifier sucks in the air, after which the fibers work by capturing and trapping a wide range of airborne particle sizes. Clean, healthier air is then recirculated back into the room.

According to the Environmental Protection Agency (EPA), HEPA filters can theoretically remove at least 99.97% of airborne particles such as dust, pollen and mold with a size of just 0.3 microns (µm).

The particles go through four stages or mechanisms for HEPA filters to work effectively. 

These are:

Stage 1: Diffusion

Molecules smaller than 0.1 microns are so tiny that they don’t immediately pass through the filter. Instead, they randomly diffuse through the filter until they eventually hit a fiber and become trapped.

Stage 2: Interception

Slightly bigger particles of around 0.3 microns are carried through the filter. The airflow causes the particles to collide with and stick to the sides of the fibers, enabling them to be removed from the air.

Stage 3: Impaction

Impaction arises when bigger air particles, such as contaminants like specific types of dust, move in a straight path before colliding with and sticking to a fiber. 

Stage 4: Sieving

Some particles are so big that they don’t fit through the spaces between the fibers. These larger particles are then captured at the fibers, meaning they cannot move further.

Are HEPA filters effective? 

Using an air purifier with a HEPA filter can help ease a variety of health problems. If you suffer from allergies or asthma, you’re probably aware of some of the allergens that can trigger your condition, like pollen, mold spores, dust mites and pet dander. 

The maze-like fibers in HEPA filters can effectively capture the particulates within these allergens and ease your symptoms. 

📖 According to the Environmental Protection Agency (EPA), several studies have found that HEPA filters improved one or more allergy and asthma symptoms in people with these conditions. The EPA also notes that the filters can be beneficial for cardiovascular health as well as respiratory issues.

📖 Similarly, a study published in the American Academy of Pediatrics states that HEPA air purifiers can help to ease asthma symptoms by reducing indoor airborne particles by 25-50%. The same report also found that HEPA purifiers can decrease harmful airborne particulates in tobacco and nontobacco smoke. 

📖 Since the COVID-19 pandemic, breathing clean air has become more important. Reassuringly, scientific studies have found that HEPA purifiers can help stop the virus’s spread. According to the Centers for Disease Control and Prevention (CDC), using masks and HEPA purifiers can reduce exposure to SARS-CoV-2 (the virus that causes COVID-19) by up to 90%. 

📖 Equally, an article published in conjunction with the University of Manchester in the UK, reports that HEPA air cleaners can reduce the risk of exposure to airborne particles that carry the virus. 

While HEPA filters are incredibly effective, one downside is that they often must be replaced regularly. This is because the filters can only trap particles; they don’t get rid of them.

If you use your purifier frequently, the filters become clogged with contaminants over time. To ensure the device works effectively, it’s always recommended to change the filter when it looks visibly dirty or if an indicator light is telling you it’s time to do it.  

If you want to wash the filter, check out our advice.

Different types of HEPA filters 

There are three types of HEPA filters: classic HEPA, medical-grade HEPA and true HEPA. 

But what’s the difference between them all? Let’s take a closer look.

The classic HEPA filter

The classic model can remove 99% of particles as small as 2 microns. Such airborne contaminants include dust, pet dander, pollen and allergens. 

Classic HEPA filters are well-suited to improving everyday air quality and can help to eliminate pollutants from smaller rooms. 

Yet while the 99% figure sounds pretty good (and it is), classic HEPA filters are the least effective type. 

The true HEPA filter

So, what could be better than 99%? Well, the true HEPA filter can trap 99.97% of particles as tiny as 0.3 microns so… this one is considerably better. Moreover, 0.3 microns is the most penetrating particle size (MPPS), and a classic filter cannot catch such particles.

The true filter is the most widespread type and can capture and remove pollen, pet dander, bacteria, mold and viruses.  

The medical-grade HEPA filter

As its name suggests, these filters are often used in hospitals, clinics and other environments where clean air is paramount. 

They are the most effective and powerful form of air filtration, capable of removing up to 99.95% of particles as small as 0.1 microns. This includes contaminants such as dust dander, pollen, mold, bacteria and viruses. Plus, insecticides, asbestos, and anthrax. 

Medical-grade filters are also categorized as H13, whereas true filters are classified as H10, H11 or H12.

Final thoughts

If you spend money on an air purifier, choose one that will work effectively. Opting for a device with a HEPA filter will help to ensure that the air you breathe is clean and sanitized. This is especially important if you suffer from allergies or asthma. 

Be sure to go for one with either a true or a medical-grade HEPA filter, as these will capture the greatest amount of harmful, airborne particles.


About the author

Marie Cookson

Based in Manchester, Marie is one of the writers at HouseFresh and our resident dust expert. She works together with our in-house researchers and our Managing Editor to produce in-depth articles offering practical advice on how to tackle indoor air quality issues.