What is a HEPA Filter? How does it work?

Have you ever considered buying an air purifier for your home?

You will come across one name even if you do basic research on air purifiers. There is no need to also buy one. Yes, you have guessed it right. The word is HEPA.

HEPA stands for High-Efficiency Particulate Air.

There are other terminologies as well such as High-Efficiency Particulate Absorber, or High-Efficiency Particulate Arresting, or High-Efficiency Particulate Arrestance. It does not matter.

William Shakespeare had said once, ‘A Rose by any other name shall smell as sweet.’

HEPA Filters – High Standards

A HEPA air filter should be able to eliminate 99.97% of particles of size greater than or equal to 0.3µm from the air that passes through it.

Understand that microns are one-millionth of a millimetre. Let us have a realistic understanding of the size. The diameter of one strand of human hair is between 50µm to 100µm in size. Anything less than 10µm in size is not visible to the human eye.

Now, you have an idea what 0.3µm constitutes. Let us now look at the dimensions of the various airborne particles.

  • Bacteria – 0.3 to 60 microns
  • Car emission – 1 to 150 microns
  • Spores – 3 to 40 microns
  • Mould – 3 to 12 microns

How does the HEPA filter work?

HEPA filters are a composition of a mat of randomly arranged fibres. Usually made of fibreglass they possess diameters between 0.5 and 2 micrometres.

Three key factors namely fibre diameter, filter thickness, and face velocity affect its functions. In the ordinary course, the air space between the HEPA filter fibres is greater than 0.3µm.

Working of HEPA Filter

People assume that the HEPA filter acts like a sieve thereby allowing particles smaller than the openings to pass through. It is an abstract thought. The design of HEPA filters is such that it targets the smallest of pollutants and particles. A combination of the following three mechanisms is at work in a HEPA filter.


Particles that flow along with the air stream come within one radius of a fibre and adhere to it.


The air tends to change its flow as it approaches the filters. The larger particles are unable to avoid the filters because they do not follow the curving contours of the air stream. Hence, they dash on to the fibres and embed into them directly. The higher the velocity of the air and lower the fibre separation results in a higher impaction.


This method caters to the smallest of pollutants, especially less than 0.1µm in size. These particles collide with the gas molecules and slow down. It results in the arresting of the particles by either the interception or the impaction procedure. It is dominant at lower air velocities. This phenomenon is also referred to as Brownian motion.

Thus, you see that the HEPA filters can trap particles less than 0.3µm as well.

HEPA Filters – The Utility

The initial use of the HEPA filters was in the nuclear industry to eliminate the dangerous radioactive particles. However, the Swedish Company IQAir patented the HEPA filters for use in air purifiers. Today, you have the use of HEPA filters in air purifiers, hospitals, aircraft, and the latest Tesla car models.

HEPA Filter – Pros & Cons

  • HEPA filters can clean and purify the circulated air
  • They can keep out 99.97% of pollutants equal to or greater than 0.3µm
  • Dust and debris do not have any chance against the HEPA filter
  • It cannot trap pollutants like gas molecules, fumes, chemicals, and odours
  • The cost is an inhibitive factor. Replacing the HEPA filter is an expensive affair
  • The shape and compactness of the unit depends on the kind of HEPA filters you use
  • Some of the HEPA filters are washable whereas others are not.

Can you use HEPA filters in AC units?

No, it is not possible to have HEPA filters in Air Conditioner units because it will restrict the air flow thereby causing the evaporator coil to freeze and damage the AC unit.

You need to install a blower fan to push the air at high speed through the HEPA filter. Replacing the HEPA filters is also expensive. However, you can have air purifiers and vacuum cleaners having HEPA filters.

A word of caution

HEPA is a new technology in India. You can come across air purifier and vacuum cleaner manufacturers resorting to marketing tricks like using the words HEPA-type, HEPA-style, or HEPA-like while describing their products.

They do not count as HEPA filters. There are five classifications of HEPA filters from A to E with HEPA-A being the least effective. The HEPA-E filters are highly efficient capable of dealing with chemical, biological, and even radiological particles.


  1. Is there really a need of Air purifier in cities of India.
    Can Indian people afford the purifier and later the replacements costs?


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