What is HEPA Filter? How does it Work? | Explained!

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Due to the increase in air pollution, the need for breathing in fresh and clean air has become essential. And the HEPA filter which is used in air purifiers is the most effective in purifying the air.

Here in this post I’m going to discuss what HEPA filter is and how it works.

Therefore, give it a reading to know it better.

What is a HEPA Filter?

To begin with, the HEPA refers to High-Efficiency Particulate Air. HEPA is the acronym for this.

Moreover, there are other variations to this acronym. 

HEPA Filter

For example, High-Efficiency Particulate Absorber, High-Efficiency Particulate Arrestance, or High-Efficiency Particulate Arresting.

These filters were invented back in the 1940s. It was during the time when the scientists were developing atomic bombs.

However, the most important part of this filter is that it can effectively capture almost every particle size. These include bacteria, viruses, pollen, allergens, and others.

Moreover, HEPA filters eliminate 99.97 % of particles.

Types of HEPA Filter

HEPA filter types

There are six different types of HEPA filters, according to Institute of Environmental Sciences and Technology (IEST). The table below makes it easier to understand.

Types of Air HEPA filters The Minimum Efficiency Rating
A 99.97% ( at 0.3µm)
B 99.97% ( at 0.3µm)
C 99.99% ( at 0.3µm)
D 99.99% ( at 0.3µm)
E 99.97% (at 0.3µm)
F 99.999% (at 0.1-0.2)

How Does The HEPA Filter Work?

HEPA filter comprises randomly arranged fiber mat. They are generally made up of fiberglass. They have a diameter between 0.5-2 micrometers.

There are three main factors that affect the working of the this filter. Namely, fiber diameter, filter thickness, and face velocity.

Working of air purifier filters

Normally, the air space between the fibers of this filter is more than 0.3µm. The HEPA filter is designed in such a way that it targets the smallest pollutants and particles.

However, people assume that it acts similar to a sieve that allows smaller particles to pass through. But it is just an abstract thought.

The HEPA filter works with a combination of 3 mechanisms- Interception, Impaction and Diffusion.


Here the particles that flow with the air stream come within one radius of the fibre. Then these particles adhere to it.


The air, when it approaches the filters, changes its flow. As a result, the larger particles fail to avoid the filters. It is because the particles are unable to follow the air streams’ curving contours.

As a result, these particles dash on the fibres and get embedded into them directly.

Therefore, with a higher velocity of the air and lower fibre separation, there is a higher impaction.


Diffusion helps in trapping the smallest of particles (less than  0.1µm). These smaller particles tend to collide with gas molecules.

As a result, they tend to slow down.

As they slow down, they are either trapped by the interception or impaction mechanism.

Moreover, it is more dominant at lower air velocities. It is also known as the Brownian motion.

Uses of HEPA Filter

Initially, the Nuclear industry used it to capture and eliminate radioactive particles.

Later on, the Swedish Company IQAir patented the use of HEPA filters in air purifiers.

air purifier

Therefore, now we have HEPA filters in air purifiers, aircraft, hospitals and even in the latest Tesla car models.

Importance of HEPA Filter

HEPA Filter is essential if you have Asthma or anyone in your family has. Using this filters, you can experience a vast difference. 

However, apart from this, there are other circumstances where you can use them.

A High Chemical environment

You can use a HEPA filter if your location is in a high volume of VOCs. It is designed in such a way that it is capable of handling such an environment.


If you find fire around your house, be it forest fire or everyday kitchen smoke, there is an accumulation of smoke particles.

In such a case, HEPA filters easily capture soot and ash particles.

To Ensure a Cleanroom Environment

If you want a clean room environment, then a hyper-HEPA filter is ideal. It captures smaller particles (0.003 microns)- for example, viruses.

In Case of Water Damage

In case of any water leakage or overflow, this filters help to get rid of mold spores, bacteria or microorganisms.

Some Advantages and Disadvantages of HEPA Filter


  • It cleans and purifies the circulated air.
  • With HEPA filters, there is no dust and debris.
  • It keeps out 99.97% pollutants that are equal to or even greater than 0.3


  • It is ineffective in case of gas molecules, odours, fumes, chemicals.
  • HEPA filters are costly.
  • Replacing it is quite expensive as well.
  • Only some of them are washable.
  • The shape and compactness of the unit are dependent on the type of the filter used.

How to Identify a HEPA Filter?

This type of filter has a strict set of requirements to fall under the category. Likewise, in Europe, HEPA filters are considered those that must remove 99.95% of particles.

Whereas, in the US, it is 99.97% of particles.

hand with air purifier HEPA filter

However, according to the ISO rules, the filters capturing particles more than 85% but less than 9.955 are not HEPA filters.

These are EPA filters (efficient particulate air filters).

On the other hand, the ones that exceed the capacity of HEPA filters are called ULPA filters.

ULPA filters are one that captures more than 99.99% of particles. ULPA is Ultra Low Penetration Air.

The table below makes it clearer to understand. Therefore, have a look at it.

Filters Filtration Rate
EPA filters (Efficient Particulate Air Filter) More than 85% but less than 99.95%
HEPA filters (High-Efficient Particulate Air Filter) 99.95%-99.99%
ULPA filters (Ultra Low Penetration Air Filter) Above 99.99%

Use in Air Conditioner

People might think that HEPA filters can be used in AC units. But it is not possible in AC units.

The reason behind it is that it will restrict airflow. As a result, the evaporator coil freezes and damages the AC unit.

However, you can use a blower fan. It will push the air at a higher speed through the filter. Moreover, replacing this filter is expensive as well.

You can have these filters in your air purifiers and vacuum cleaners.

Some Precautionary Tips

  • Do not clean HEPA filters with water.
  • It is not recommended to vacuum the HEPA filters.
  • The products that come with HEPA-like, HEPA-style, HEPA-type, are not HEPA filters. Therefore, do not jump into them.

Final Words!

In conclusion, HEPA filters are a very important and efficient filter. There are different types of this filters.

Therefore, before buying one just check you buy the right one. 

Also, as mentioned earlier in the article, some products say HEPA-like or HEPA-type, these are not HEPA filters.


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