HEPA filters explained

-by Sjoerd Post, 29-01-21

In a time where clean air is of more importance than ever, HEPA filters are seen by many as the solution to all our filtration challenges. But what exactly are HEPA filters and how do they work?

HEPA filter stands for High Efficiency Particle Air filter, which is an efficiency standard of air filters. In this blog you will discover:

• The origin of HEPA filters
• Different kinds of HEPA filter
• Will HEPA filters stop Covid?

The origin of HEPA filters

The HEPA filter has a somewhat violent history going all the way back to WWI when soldiers were first confronted with chemical attacks. To protect themselves the troops wore gas masks containing a complex paper which filtered out the airborne toxics.

Three decades later in the 1940s, scientists developed this method further. The first time an HEPA technology was used was during the Manhattan atomic bomb project, where a filter was required that could capture the tiny airborne radioactive toxins emitted during the process.

In this period the 0.3-micron particle size was determined. Radioactive fallout is made of microscopic particles and the 0.3 micron ones are the most penetrable and concerning. This particle size is the key feature that HEPA filters focus on.

Different kinds of HEPA filter

There are many different HEPA filter classes. You have may have heard terms like True HEPA filter, HEPA-like, HEPA-style or something in that direction. Ultimately it all comes down to a filter efficiency of 0.3 micron particles. HEPA filters are classified according to the EN 1822 standard and based on a 0.3 micron particle.

^{Filter groups according to EN 1822}

The difference between a ‘true’ HEPA filter and other types is its efficiency. For example, EPA-class filters have an efficiency of 85% while true HEPA filters offer a minimum efficiency of 99.95%. This capture rate is sufficient by European standards, while American rules demand a 99.97% efficiency.

Most HEPA filters are made of fibre glass or plastic. They capture particles in different ways, namely by:

• Straining
• Impingement
• Interception
• Diffusion.

You can read in detail about how these various mechanisms work in this blog.

Will HEPA filters stop Covid virus?

Yes and no. Although the fibre structure captures the particles in different ways, this does not guarantee every single virus particle will be blocked. Most of the time airborne viruses do not travel as a single virus particle but in aerosols – a small wet bubble of mucus containing multiple particles. This makes them a lot bigger, relatively speaking, and therefore easier to capture.

Wrapping it up

Although not the magical super-filter that protects us from all airborne toxicities and viruses, HEPA comes quite close. With a long history based on nuclear protection the filters were commercialised in the 1950s. Today they are the standard for a filter efficiency focused on particles of 0.3 micron. This is not a randomly chosen size, but the most difficult size to filter out.

There are many different HEPA classes which are distinguished by their efficiency on the 0.3 particle size. To find out which filter is most suitable for your situation, ask one of our engineers. And, as always, stay tuned to heinenhopman.com for everything HVAC.

Sjoerd Post | Senior Engineer QESH

Sjoerd Post has been working at the QESH department of Heinen & Hopman since 2015. With his technical background he is responsible withing the QESH department for implementing standards and performing technical research for the company.