How does True Hepa Filter work?

How does True Hepa Filter work?


How True Hepa Filter works? 1
How does True Hepa Filter work? 1

If you are having one air purifier, you may come across the term “True HEPA filter”. In fact, the True HEPA filter is widely acknowledged as the most popular air filter in both classic and modern air purifiers. This air filter is also the most important key factor to decide the performance of one air purifier. In this article, we will know How does True Hepa Filter work? and about the Functions of Hepa filters.

However, in real-world use, you may be misled by many air purifier brands that claim to have the True HEPA filter. The problem is, you don’t know if all the “True HEPA filters” are the same, or if there are specifications to separate the class of these HEPA filters by value.

Other results on the Internet don’t satisfy you, and if you still can’t find one good True HEPA filter don't worry about it.

 How does True Hepa Filter work?
Buy Blueair 480i

At 99Air Purifier, we know all your problems with indoor air quality and air purifiers. We know that you need the best air purifier to improve your quality of life. We are the experts in the field of air purifiers, but more than that, we are just like you, a human.

We also have serious problems with airborne allergies and asthma, and we know your struggle to learn the air purifier and put an end to your misery. Today, you will know everything about the True HEPA filter in our in-depth guide.

If you have concerned about other air filter types as well, we hope that the below guides can help you. 

  1. Ionizer.
  2. Activated Carbon filter.
  3. UV-C light filter.
  4. PlasmaWave.

Get ready to know more about the True HEPA filter. Great! Keep reading.


True HEPA filter, with HEPA, stands for High-Efficiency Particulate Arresting, or High-Efficiency Particulate Air, which is the highest standard for physical air filtration being used by air purifiers. Moreover, HEPA filters are applied in many applications, like vacuum cleaners, automobiles, and even aircraft.

However, to consider a True HEPA filter, one HEPA air filter must be certified to remove 99.97% of all microscopic particles as small as 0.3 microns in size, which is the tiniest particle’s size to get into your lungs. Microns, which are one-millionth of a meter, are how particles are measured.

To help you know the size of a micron, we are unable to visually see anything less than 10 microns. Also, the sizes of particles such as dust, pollen bacteria, virus, and many more areas below.

ParticleParticle Size
Atmospheric Dust0.001 – 40
Bacteria0.3 – 60
Beach Sand100 – 10000
Burning Wood0.2 – 3
Cement Dust3 – 100
Clay, fine0.5 – 1
Coal Dust1 – 100
Combustion0.01 – 0.1
Dust Mites100 – 300
Fly Ash1 – 1000
Grain Dusts5 – 1000
Household dust0.05 – 100
Human Hair40 – 300
Insecticide Dusts0.5 – 10
Lead Dust0.1 – 0.7
Mold Spores10 – 30
Pet Dander0.5 – 100
Pollen10 – 1000
Smoke0.01 – 0.1
Tobacco Smoke0.01 – 4
Viruses0.005 – 0.3

In our list of the best air purifiers, there are only the air purifiers with True HEPA filter. Below are some examples.

99Air Purifier

How does True Hepa Filter work?

The True HEPA filter is mostly made of the highly-dense paper, which is composed of very thin fibers with distances between 0.3 and 2.0 microns. The fibers are distributed randomly and oriented in all directions.

There is a common assumption that the air space between the fibers must be as small as 0.3 microns, so the True HEPA filter can capture everything that is larger than this size. However, it is incorrect and impractical. With the smart design, the True HEPA filter can target ultra-fine particles with a combination of three physical mechanisms:

Inertial Impaction.

The unclean airflow through the filter media of the True HEPA filter, but the heavier particles (or larger particles of pollutants) can not follow the gas flow streamline.

Imagine with their inertia of large sizes, these heavier particles travel along a straight path and will be soon captured by one of these fibers in the True HEPA filter. That’s why the fibers must be distributed randomly. So the larger the objects are, the more chance they will be captured by the Inertial Impaction mechanism of the True HEPA filter.


This mechanism involves particles with a somewhat smaller size, so they can easily follow the streamline, and won’t collide directly with the fibers of the True HEPA filter.

However, the radius of these particles is still larger than the distance between the streamline and the edge of the fiber (which is commonly 0.3 to 2 microns). Therefore, it will be captured by the edge of the fibers.


This mechanism is the most important part of the True HEPA filter and can remove even the smallest particles. Once the particles are as small as 0.3 microns, they don’t strictly follow the streamline and can move randomly due to the collision with the gas molecules (Brownian motion).

So the smaller the particles are, the higher their freedom of movement, and therefore, the higher the probability that they will encounter the fiber. With decreasing particle size, the diffusion mechanism becomes more important.

So to decide the total efficiency of one True HEPA filter, you must count the results of all three filtration mechanisms of it.


No, not all HEPA filters are the same. There are significant differences in the HEPA filters being made by many air purifier brands. Depending on the quality of the materials, as well as the density of the HEPA filter’s fibers, one HEPA filter may be different from one another.

For example, Blueair is using a unique technology named HEPASilent, which improves the efficiency of the ionizer. The ionizer causes an electrostatic charge to build up the particles on the fibers, just like what happens in the cat’s fur. 

Rabbit Air uses the BioGS HEPA filter to prevent the buildup of airborne particles, as well as the growth of bacteria and viruses on the surface, to improve both the efficiency and prolong the lifetime. Or, IQAir invents the HyperHEPA air filter, with the nanofibers to decrease the size of airborne particles to only 0.003 microns.

True Hepa Filter Types

In the air purifier market, there are a lot of terms like “HEPA-type” or “HEPA-like”, and they are totally inferior to the “True HEPA filter”. These filters can only capture particles as small as 2-5 microns, which is more than a 600% decrease in the efficiency of one True HEPA filter. So to say, the best practice is always to look for air purifiers with True HEPA filters.

But how do you know one air purifier has the True HEPA filter? First, it must meet strict standards set by the U.S. Department of Energy. The True HEPA filter needs to remove 99.97% of all particles that have a size greater than or equal to 0.3 microns, as defined by the United States Department of Energy (DOE).

You should search for the HEPA class in your air filter, which either follows the European Standard or the MERV rating. Below is the given efficiency of these HEPA classes for airborne particles down to 0.3 microns, which follows the European Standard:

HEPA classEfficiency

You can also follow the MERV rating, to decide the efficiency of the True HEPA filter:

MERV ratingDust EfficiencyParticle size
20≥ 99.999%0.1 – 0.2 microns
19≥ 99.99%0.1 – 0.2 microns
18≥ 99.97%0.1 – 0.2 microns
17≥ 99.97%0.3 microns
16≥ 99.95%0.3 – 1 micron
15≥ 95%0.3 – 1 micron
1490 – 95%0.3 – 1 micron
1389 – 90%0.3 – 1 micron
1270 – 75%1 – 3 microns
1160 – 65%1 – 3 microns
1050 – 55%1 – 3 microns
940 – 45%1 – 3 microns
830 – 35%3 – 10 microns
725 – 30%3 – 10 microns
6< 20%3 – 10 microns
5< 20%3 – 10 microns
4< 20%≥ 10 microns
3< 20%≥ 10 microns
2< 20%≥ 10 microns
1< 20%≥ 10 microns


If your HEPA filter is not specifically labeled as washable, then the answer is no, you can’t wash your HEPA filters without damaging them. Otherwise, you can wash it, but you need to know how to clean the air filter.


We have learned everything about the True HEPA filter, but you still don’t know if this air filter is flawless or not.

Below are the pros and cons of the True HEPA filter.


  • Highly efficient, capture most of the toxic contaminants
  • Trapped particles will stay within the air filter
  • The physical air filter won’t emit ozone, which causes asthma and other breathing issues


  • Requires routine filter replacement to maintain the efficiency
  • Doesn’t remove odors and smell

Conclusion of using True Hepa Filter:

Yes, it is worth using this filtration system in air purifiers and it helps to make an air purifier stunning and a better performer. How does True Hepa Filter work?

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