What is a HEPA Filter?

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Written By Jamila W.

A HEPA filter is a High Efficiency Particulate arrestance filter. It is a pleated mechanical air filter that can remove 99.97% of airborne contaminants and particles that are 0.3 micrometers in size. These airborne particles can include dust, mold, bacteria, viruses, pollen, and many other pollutants.

air pollution

But, to answer the questions “What is a HEPA filter?” in totality, we’ll have to dive deep into some other relevant topics. Today, I’ll be using my extensive experience and understanding of air purifiers to answer questions like:

  • Why HEPA filters were invented
  • How do HEPA filters work
  • What makes them so efficient
  • What is our indoor air quality like
  • And the effects of poor indoor air on our health. 

Let’s get started!

High-Efficiency Particulate Air Filter: A Short History

Fascinatingly, we can trace the history of HEPA filters back to the Second World War. At that time, the US undertook the Manhattan Project to develop the atomic bomb.

The goal was to develop a filter that could protect the people working on the Manhattan Project from the hazardous, radioactive particles in the air.

Nobel Laureate Irving Langmuir worked on this project and eventually created a filter now known as the HEPA filter. Even though these filters failed to protect against radioactive material, they were very good against mustard and chlorine gas.

Fast forward to the 1950s, HEPA filters were trademarked and introduced commercially for air purifiers.

How Do HEPA Filters Work?

HEPA filters use randomly aligned and intertwined glass fibers in a pleated construction. These fibers create a trap for particulate pollution coming in from every direction. This works much like a net we use to catch things, only at a much smaller scale. 

But that’s not all.

HEPA filter

From what we know, particles entering a HEPA filter are treated differently depending on their size. Let’s see how big, small, and minute particles are stopped by HEPA filtration.

Level 1: Straining

For a HEPA purifier, particles larger than 10 microns are considered big, though they’re microscopic for the human eye to see. As these particles pass through the HEPA filters, they get stuck between the intertwined linings of the fiber. This process is also known as straining. 

Level 2: Interception
Also known as “inertial impaction,” this 2nd layer stops particles in the range of 0.3-1 microns, such as bacteria, lead dust, oil smoke, etc.

This happens because the air carrying these particles passes over the fiber of the filter with relative ease. However, the particles are much heavier than the air and, so, cannot make such a sharp turn due to their inertia and get caught in the fibers as a result.

This is a similar principle to industrial sawdust filters, just on a much smaller scale.

Level 3: Diffusion

The concept of Brownian Motion comes into play when discussing particles smaller than 0.3 microns. Because of their size, these particles are so light that they constantly bounce off gas molecules upon impact — causing them to move randomly.

Even though they can pass the HEPA filter fibers because of their smaller size, their zig-zag movement becomes their undoing and the reason for getting stuck.

Most Penetrating Particle Size-MPPS and Efficiency

Household dust contains many common allergens and ranges from 0.05-100 microns. But if HEPA filters remove only 0.3 microns particles, how can they remove particles as small as 0.05 microns?

We’ve seen many people doubt the efficacy of HEPA air filters due to this common misunderstanding.

dust in the filter

A HEPA filter can remove particles of 0.3 micrometers with 99.97% efficacy. This is known as the Most Penetrating Particle Size(MPPS). It essentially means that particles with a size of 0.3 micrometers are the worst case scenario or the most difficult for a HEPA filter to clean. Even then, it can still wipe out 99.97% of them with ease.

Particles larger or smaller than 0.3 microns thereby are a lot more easier for HEPA filters to clean.

Indoor Air Quality

Effects of Poor Indoor Air Quality on Human Health

Indoor air is a pestilent combination of pollutants and allergens such as carbon monoxide, radon, pet dander, mold, human hair etc. Smaller particles like viruses and bacteria may also linger in the indoor air.

Alarmingly, research suggests that indoor air may be 2-5 times more polluted than outdoor air. Moreover, we’re also well aware of the negative effects of poor indoor air quality.

man coughing

According to Environmental Protection Agency, poor indoor air quality can lead to health issues such as:

  • Allergies and irritation of the eyes, nose, and throat
  • Respiratory and pulmonary issues
  • Heart disease
  • Cancer

Effects of HEPA Filters on Indoor Air Quality

Fortunately, HEPA filters are a proven effective measure against most indoor pollutants that affect human health.

We also found a study that shows how air purifiers fitted with a HEPA air filter can substantially reduce the amounts of airborne particles and allergens.

air filter, purifier

Another conclusive study showed that HEPA air purifiers can reduce fine particulate matter by an impressive 55 percent!

If you don’t know, fine particulate matter, or PM 2.5, are small particles that can lead to respiratory problems and aggravate respiratory conditions such as COPD and asthma.

What is True HEPA and How is it Better?

True HEPA filter is a marketing term used to differentiate HEPA filters based on whether they have been rigorously tested.

For example, terms like “Absolute HEPA,” “True HEPA,” and “Ultra-HEPA” are used specifically with filters that have been tested to meet the DOE’s standards of cleaning 99.97% of the 0.3 micron particles.


On the other hand, filters like “HEPA-type” are only 99% efficient and good at capturing particle sizes of 0.2 microns. While this may seem more impressive, we have to remember that the MPPS (most challenging particle to capture) is the 0.3 microns one.

So, my personal advice is to always look go for “True HEPA filters”; they’re more effective and provide the best bang for your buck.

Final Words

There you have it, the lowdown on HEPA Filters.

HEPA filters work reliably well when cleaning the indoor air of allergens, bacteria, viruses, and other harmful substances. Their intricate maze of glass fibers can tackle particles larger and smaller than 0.3 microns without skipping a beat.

The question is, have you made up your mind about HEPA filters? If yes, check out the best selection of filters and breathe better!