Why do we select HEPA Filters at 0.3 micron pore size??

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“Why do we select HEPA Filters at 0.3 micron pore size??”

So today it just came across my mind that why is there a criteria of 0.3 microns particle size for the selection of HEPA Filter & I found something interesting about HEPA filters that I’m going to share with you.

So guys stay tuned with me!!

First of all what is HEPA Filter?

“HEPA” is an acronym which stands for High Efficiency Particulate Air also known as High Efficiency Particulate Absorbing and High Efficiency Particulate Arrestance.

HEPA Filter must satisfy two common standards:-

  1. According to European Standard; HEPA Filter shall exhibit a minimum efficiency of 99.95% when tested at an aerosol of 0.3 microns diameter.
  2. According to ACME, U.S. DOE; HEPA Filter shall exhibit a minimum efficiency of 99.97% when tested at an aerosol of 0.3 microns diameter.


HEPA Filters are made up of a series of randomly arranged fibers. These Filters belong to a class of depth filters, as opposed to membrane filters.

HEPA filters are used with pre filters. Pre filter removes most of the particles thus increasing the life span of HEPA filter.

Capture Mechanism of HEPA Filter:-

There are 3 basic ways media traps particles:-

  1. Impaction
  2. Interception
  3. Diffusion

1. Impaction:-

Inertial impaction works on particles of larger size that are suspended in the air stream. When air changes its path to enter the fibre space, the heavy particle continues to flow in a straight line. It collides with the media fibers and gets trapped instantaneously.

2. Interception:-

Direct interception works on mid range particle size. These particles are not large enough to have inertia & not small enough to diffuse within the flow stream.

3. Diffusion:-

Diffusion predominates below 0.1 microns diameter particle size. Behaviour of smaller Particles is similar to Brownian motion, therefore, enhancing the probability of either impaction or interception. This mechanism works best if air flow rate is lower.

Why 0.3 microns?

Impaction & Interception predominate above 0.4 microns; while Diffusion predominates below 0.1 microns particle size.

So basically 0.3 microns is a benchmark to define the efficiency of HEPA filter.

There is one more thing about HEPA filters that you guys should know..

Do you know what is MPPS?

MPPS stands for Most Penetrating Particle Size.

For HEPA filters MPPS range is 0.15-0.25 microns. At MPPS; both impaction & interception are inefficient. That’s one more reason why we define HEPA filter’s efficiency at 0.3 microns particle size.

Note:- It is possible for particles lower than 0.3 microns diameter to have filter efficiency more than the particles of 0.3 microns size.

So that was all about HEPA filters. For any queries please do comment.. Also if you know anything else then share with us.

Any Question regarding “Why do we select HEPA Filters at 0.3 micron pore size” ask in the comment section.



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