Close-up of a HEPA particulate filter

How does a HEPA filter work?

HEPA, which stands for High Efficiency Particulate Air, est une désignation utilisée pour décrire les filtres capables de retenir 99,97 % des particules de 0,3 micron.

Bien que la norme HEPA et son processus de certification n’aient pas été établis avant 1983, le développement du filtre HEPA remonte à la Seconde Guerre mondiale. Ce filtre de qualité était alors utilisé pour capturer les particules radioactives libérées par la bombe atomique.

Why 0.3 microns?

This micron size (0.3) is the most penetrating particle size. Scientists have found that particles of this size escape air filters more than larger or smaller particles. To get a better idea, the following table shows the size of particles such as dust, pollen, bacteria, viruses, etc.

Particle sizein microns (µm)
Sand100 - 10 000
Mites100 – 300
Human hair40 – 300
Animal hair10 – 1000
Mould spores10 – 30
Cement dust3 – 100
Bacteria0.3 – 60
Lead dust0,1 – 0,7
Household dust0,05 – 100
Smoke0,01 – 4
Pollen0,01 – 0,1
Virus0,005 – 0,3
1 µm (micron) is 10-6 metres or 0.000 001 metres

How do HEPA filters work?

Most HEPA filters are made of interwoven glass fibres, which are twisted and turned in a myriad of directions to create a fibre maze. As particles pass through this web, they are removed from circulation by the following four processes:

HEPA filter: principle of fine particle filtration by impaction
  • By direct impaction Large contaminants, such as certain types of dust, mould and pollen, travel in a straight line, collide with a fibre and stick to it.
HEPA filter: principle of filtering fine particles by sieving
  • By sieving The air flow carries a particle between two fibres, but the particle is larger than the space, so it gets trapped.
HEPA filter: principle of filtration of fine particles by interception
  • By interception The air flow is fluid enough to bypass the fibres, but due to inertia, the particles continue to flow and stick to the edges of the fibres.
HEPA filter: principle of filtering fine particles by diffusion
  • By dissemination Ultrafine particles move more irregularly than large ones and are therefore more likely to hit and stick to the fibres.

Are there many particles that are not captured by the HEPA filter?

Don't worry. There are other technologies that work in conjunction with the HEPA filter to repel many very small contaminants, such as cigarette smoke, fumes and other chemicals.

Activated carbon filters use small pores to capture certain chemicals, odours and fumes that a HEPA filter would not. Combining different air filtration systems gives you superior indoor air quality and can even improve your sleep.

Are there different types of HEPA filters?

There are a number of terms that marketers use to describe air filters. Although True HEPA is technically a marketing term, it is used by many to differentiate between US and European HEPA standards.

In Europe, a filter must capture only 85 % of pollution particles of 0.3 micron in size, while the US standard is 99.97 %, to be HEPA certified. The US standard is therefore often referred to as "True HEPA". HEPA-type", "Ultra HEPA" and other HEPA variants are not recognised by accreditation bodies in the US or Europe.

Home Air Guides | What is a HEPA filter and a HEPA air purifier? (with French subtitles)

Is a filter capable of capturing 99.9 % of particles effective?

Not necessarily: a filter that claims to be able to suck up 99.9 % of large particles may not be able to capture ultra-fine particles. Similarly, a filter that touts its ability to trap the smallest particles doesn't tell you how well it captures the 0.3 micron particles, which we know are the most troublesome.

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Where are HEPA filters used?

The HEPA filter was originally intended for use in laboratories and factories to ensure cleaner air. Particularly effective against airborne particles, it is now used to capture pollutants in cars, hoovers and you guessed it, the air purifier.

Can HEPA filters be washed?

If your HEPA filter is not specifically labelled as washable, then the answer is no, you cannot wash your HEPA filter without damaging it. If it is not, you will need to follow the instructions for your air purifier or hoover to find out how to clean your filter.

Advantages and disadvantages of HEPA filters

It is highly effective in capturing most toxic contaminants.It requires regular maintenance/replacement of the filter to maintain its efficiency.
Trapped particles remain in the air filter of your air purifier or hoover.Does not eliminate bad odours.
The physical air filter does not emit ozone, which causes asthma and other respiratory problems.