What is an hepa filter ?

"HEPA" is an acronym for « High-Efficiency Particulate Air ». This standard, defined by european standards EN 1822 and EN ISO 29463, came into effect in 2009. This standard is applied to any ​unit able to remove at least 99.97% of particulates measuring 0.3 micrometer diameter in a lab setting.

They have been in use in laboratories, operating theatres and clean rooms for decades.

To get the HEPA standard, a filter goes through a « DOP » test (Dispersed Oil Particulate). This test uses oil particulates (of 0,3 μm) to evaluate the filtering level. A such, there are five levels of HEPA filters, ranked depending on efficiency :

AIRVIA Medical exclusively uses HEPA H13 filters. While there are slightly more performing filters ( 0,05% gain) like the HEPA H14 or the ULPA filters, they slow the flow of purified air and are, as such, counterproductive to quickly suck in droplets and aerosols.

What does an HEPA filter catch ?

HEPA filters catch a large variety of atmospheric pollutants, especially particulate and biological pollutants. ​They filter all fine particulates (up to PM0.1), allergens (pollen, acarid), and germs (bacteries, viruses, mould, spores).

How does an HEPA filter work ?

The HEPA filter is mostly composed of various intermingled natural fibers, like the FFP2 masks with which it shares many similarities although it does offer a better filtration: The standard for the FFP2 masks that are being recommended at the moment is to filter at least 94% of aerosols and particulates with an average diameter of 0,6 (with a variation of 0,1 to1 μm) 9. As such, a HEPA H13 filter offers more filtration than a FFP2 mask.

Filtering mechanisms:

There are three different filtration mechanisms that allow the HEPA filter to intercept particles:

  1. Inertial impaction : Particles and droplets bigger than 1μm are big enough to be caught directly in the filter’s fibers.
  2. Direct impaction : Particles and droplets bigger than 0,1μm are intercepted by the Van der Waals force (​distance-dependent interaction between atoms or molecules​) when they pass through the filter.
  3. Brownian diffusion : The smallest particles, smaller than 0,1μm (like aerosols) have a trajectory called brownian. Their random motion makes them hit the filter’s fibers where they are consequently caught by the Van der Waals force.
  4. Electrostatic forces : they attract viral aerosols to the filter’s fibers where they are caught via process 2 or 3.

Filtration of 0,01μm particles - The myth of the 0,3 micrometers barrier

While it is often said that HEPA filters are only capable of catching 0,3 μm particles or bigger, this is completely wrong. This mistaken assertion is based in part on an incorrect understanding of the operation of HEPA filters. If the HEPA standard distinguishes between 0,3 μm particles and others, it is because these are - and this is certainly counterintuitive – the most difficult to filter. This is the reason why 0,3 micrometers is used as a standard to calculate the efficiency of HEPA filters.

However, particles up to 0,01 μm will truly be caught by an HEPA filter. A NASA1 study from 2016 clearly demonstrated that. It showed that HEPA filters are highly efficient to capture both a high percentage of nanoparticles, as well as particles bigger than 0,3 μm.

Sources :
  1. Perry JL, Agui JH, & Vijayakumar R. (2016, May). Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials. Retrieved from source.
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