No doubt you have noticed in specifications of HEPA filters the percentage of particles they capture, this is related to their efficiency. But what is HEPA filter efficiency really? Find the answer here.
What does HEPA mean?
The letters HEPA are an acronym that means ‘high-efficiency particulate arrestance’. I have seen this acronym frequently described as having the meaning ‘high-efficiency particulate air’, this is so even on Wikipedia. But as I see this phrase, it has no meaning at all, it looks unfinished and I have no idea who introduced it for the first time.
In any case, as you realize, the word ‘efficiency’ is in it, so this is an important parameter. A filter is HEPA if it captures a minimum of 99.97% of contaminants at 0.3 microns in size. The ability to capture such particles is its efficiency.
You will see the term True HEPA frequently used. This is more for marketing, but if the manufacturer provides information of independent testing, then this term has its real meaning and it is indeed a true HEPA air purifier.
Why 0.3-micron benchmark
This is an interesting topic and it is related to technology and filtering. A filter is normally understood as a net with some pores or openings that capture particles. So the larger the particles, the easier it is to capture them with a filter, and this applies also to a HEPA filter. This process is described as sieving or straining, and it is graphically presented in the picture:
But it turns out that the particles of 0.3 microns are difficult to capture, this is simply a technological problem related to the sizes of pores or the space between the media fibers shown in the picture above. This is why you will always see in specifications the percentage of captured particles of this 0.3-micron size. In other words, this is the ‘most penetrating particle size’ (MPPS).
Larger particles are more easily trapped, and smaller particles either lack sufficient mass to penetrate the filter or they are trapped due to some different physical mechanisms, quite generally you have a combination of diffusion, interception, and inertial impaction.
Now when you know the sizes of pores and understand the meaning of the term ‘efficiency’, you can understand that the efficiency of a HEPA filter increases as the filter gets dirty. This simply means that fewer and fewer particles will be able to pass through it. But hey, this is not the only parameter, there is also something called CADR number. A dirty filter will be less and less able to let the air go through it, so its CADR parameters will decrease in time. This is why such HEPA filters must be changed occasionally.
This is why you normally have a pre-filter as you see schematically in this picture describing the Honeywell HPA300 True HEPA Air Purifier for larger rooms:
Note also that HEPA filters can break down over time and this is also one of the reasons for their recommended replacement every two years even if the device is not used.
How this size benchmark compares with some germs
Here are some sizes of micro-organisms that play a role in our daily life:
- Staphylococcus aureus: about 1 μm.
- The coronavirus species COVID-2019, MERS-CoV, and SARS-CoV: 0.06 – 0.2 μm.
- Mycobacterium tuberculosis: 1.0 μm.
- Influenza A virus: 0.08 – 0.12 μm.
- HIV: 0.08 μm.
- Hepatitis C virus: 0.05 μm.
As you realize, for the Staphylococcus aureus and Mycobacterium tuberculosis, a HEPA filter will behave like a net and it will capture them completely. But what about the others?
It appears that HEPA can filter most of them, although they are smaller than the pores in the filter. The reason is that small particles do not move straight, they perform what is known as the Brownian movement. In short, these tiny objects move randomly and hit filter fibers and they eventually get stuck in them.
There are NASA tests showing that such HEPA filters are effective in capturing up to 100% of nanoparticulate contaminants. Who would say? The picture below shows the close view of the HEPA fibers from a device used on the International Space Station.
Are all HEPA filters really safe?
HEPA filters are usually made of borosilicate microfibers with diameters from 2 to 500 nm, but this is usually proprietary material. Typically you have a thin HEPA layer in your air purifier, but even though it is thin, this means hundreds of layers of fibers as the picture above suggests. This multi-layer structure is partly behind the mentioned filtering of nanoparticles.
Nevertheless, there may be leakage and this is why filters must be individually tested and certified by independent labs. This is the only way to be sure that a purifier works as prescribed.
Thank you for reading. Please check my text on how to choose a good air purifier and also this lost with best rated air purifiers for home. Read more on the issue of air purifier vs air ionizer. You might want also to read about potential HEPA filters fibers health risk.
Let me know if you have any question or comment, there is a comment box below. Have a nice day.