Yes. A HEPA filter should be evaluated based its filtering capacity (efficiency) as well as the lowest size of particles (range) it can clean.
A HEPA filter will remove various sizes of particles in its range to differing levels. It is a norm in the filtration industry to quote a filter’s efficiency as its lowest value across its cleaning range.
A generic HEPA filter removes particles above 0.3 µm to a minimum of 99.97% efficiency. A generic HEPA-14 filter removes particles above 0.3 µm to a minimum of 99.995% efficiency.
A HEPA-13 filter, which specifies its range as 0.03 µm and above, may also be cleaning 0.3 µm (or larger) particles better than 99.995%. Such an advanced HEPA-13 filter will effectively be a HEPA-14 filter above 0.3 µm and HEPA-13 filter for particles in the range 0.03—0.3 µm.
Read the complete article to understand how an air purifier’s HEPA filters are rated and how to decide which filter is better.
An air purifier cleans air by removing particles and gases in it. It sucks the dirty air in, filters it and releases the cleaned air back into the room.
This cleaned air mixes with the dirty air in the room, diluting its pollutant content. This semi-cleaned air is again sucked into the air purifier and gets cleaned further.
Do this long enough, and the air in the room will be almost as good as what comes out of the air purifier: clean and pollutant-free.
Our indoor air has three types of pollutants: physical, chemical, and biological.
An air purifier removes physical contaminants using a technology called HEPA filtration, which is the topic of this article.
The purifier removes chemical—or gaseous—contaminants using an activated carbon filter, which is a different technology altogether.
Viruses, bacteria, fungi, and pollen fall under the bio-contaminants category. They are also removed using HEPA filtration. After all, a biological pathogen is a physical particle, too.
Read on this website: Indoor air pollution: An Overview.
What is a HEPA filter?
HEPA is the industry standard for filtering efficiency of an air filter. It is a filter that Arrests Particles with High Efficiency. A HEPA filter should remove at least 99.97% of the 0.3 µm size particles.
As particles get as small as 0.35 µm, the efficiency of the filter starts dropping quickly. So the industry standard is to measure the filtering ability at 0.3 µm. For getting HEPA certification, this capacity should be 99.97% or better.
Any filter that calls itself ‘HEPA-like’, ‘Near-HEPA’, or ‘HEPA-type’ is not really HEPA. Why would someone call a pen ‘near-pen’, or ‘pen-like’, unless it is not a pen?
HEPA-13 and HEPA-14 filters
For starters, these filters have to be HEPA–certified. Then, they are tested for their ability to filter standard particles of various sizes over their entire range.
If a HEPA filter’s cleaning efficiency at all particle sizes across its claimed range is 99.95% or more, it is certified as a HEPA-13 (H13) filter.
If the same is 99.995% or more, it is certified as a HEPA-14 (H14) filter.
A filter’s range includes all particle sizes above the smallest particle the filter cleans. So a filter that claims to clean down to 0.1 µm will clean all particles above 0.1 µm, with efficiency as claimed.
A filter cleans different sizes of particles to varying levels. As particles get smaller, the filtering ability drops. The norm is to quote the lowest efficiency the filter has for any particle size in its range.
So, a filter that claims to clean 99.9% will have the lowest filtration capacity of 99.9% at some particle size. It may be cleaning much better at other particle sizes in its range.
MPPS (Maximum Penetrating Particle Size)
Interestingly, a filter’s efficiency may dip significantly at some particle sizes, before it rises again for still smaller particles. Particle physics at its best.
The particle size at which this happens is called the Maximum Penetrating Particle Size (MPPS) of the filter. You have to quote the filter’s cleaning efficiency as its efficiency at MPPS.
For generic HEPA filters, the MPPS is often 0.3 µm itself, since it is the most difficult particle to remove over the range of 0.3 µm and above.
HEPA filter: Range 0.3 µm and above. Efficiency 99.97%.
HEPA-14 filter: Range 0.3 µm and above. Efficiency 99.995%.
HEPA-13 filter: Range 0.3 µm and above. But, efficiency 99.95%? No, a HEPA filter will need to have efficiency of 99.97% to qualify as a HEPA filter. So, 99.95% efficiency is not allowed for any HEPA filter for sizes above 0.3 µm. It can be HEPA-13 below 0.3 µm.
HEPA-13 filter, 0.1 µm at 99.95%: Range 0.1 µm and above. Its minimum efficiency is 99.95%. This minimum efficiency will be at a particle size between 0.1—0.3 µm. This is because beyond 0.3 µm, the efficiency has to be 99.97% since it is a HEPA filter.
Ok, this is as simple as I can make it, without long–winding explanations.
Grasping EN1822 European Standard HEPA Test is not for the weak of heart. Here is a sample:
EN 1822 explicitly excludes the use of aerosol photometers. With regard to permissibility of PSL leak testing, the standard is not as clear. Part 1, Paragraph 7.3 states: “Possible aerosol substances include but are not limited to DEHS, PAO and PSL”.– EMW Filtertechnik
The Decision Point
You may have realised that a filter that is H14 over a smaller range may be H13 over a wider range.
So the filter manufacturer has a decision to make: whether to claim a wider range and lower filter capacity (H13), or to claim a smaller range and higher filter capacity (H14).
Of course, this is not much of a decision. All top-notch filtration companies accept lower HEPA ratings with bigger range for cleaning. This is because an HVAC engineer will always prefer an H13 filter with much bigger range than an H14 filter with a smaller range.
One more point: H12 is too low a rating for any non-residential use. So, a company will either claim H14 for a smaller range or H13 for a wider range. But it will never claim H12 for an even wider range, since that precludes its use for any serious air filtration.
In other words, if a company claims H14, it probably means its performance drops significantly for sizes below its range, perhaps down to H12. So it prefers to stay with an H14 claim in a narrower range instead of going for an H13 claim for a slightly wider range.
Authenticity of Certification
Renowned companies never fake their certificates. They have too much reputation to protect. Also, if there is any accident, a court of law can demand to see the certification. Any fraud detected may involve massive liability claims running into tens of millions of dollars.
So, you must trust all the certifications mentioned by reputed companies. This is especially true if you are a small consumer, as you will not get to see the actual certification data.
A test case
I will take a real–life example to show you how the HEPA filtration is to be judged.
Let us say, a company A makes air purifiers having an H14 filter. So its range is 0.3 µm and above. And its minimum efficiency is 99.995% over this range, which is better than 99.97%, the typical HEPA efficiency.
Company B claims its air purifier uses a HEPA filter of 0.00249 µm at 99.99%. So its range is 0.00249 µm and above. And its minimum efficiency is 99.99% over this range. Obviously, the filter is an H13 over the whole range.
Which filter is better at cleaning particles?
Now, 0.00249 µm, or nearly 3 nanometer, is a ridiculously small size. We are pretty much into molecular zone at these sizes. This is the size of a Gold particle just 10 atoms wide.
Incidentally, at today’s rates, you would need to capture 800 million, million Gold specks of that size to get one Indian rupee worth of Gold. Yes, that small!
Company B claims be cleaning particles even at that size with H13 efficiency. This is just insanely high an efficiency. Jaws would surely drop, if a group of HVAC engineers hears that.
So, I requested the manufacturer for a review of their certificates. The filters were tested and certified by an independent test laboratory. Mind you, testing below 0.1 µm is quite difficult and expensive. Only a few labs in the world can do that.
Based on the certification data, which I will present as graphs, here are the filter specifications:
- MPPS = 0.0294 µm
- HEPA-14 compliant filter range: 0.0523 µm and above
- HEPA-13 compliant filter range: 0.0024 µm and above
Let us plot the same data with both the H13 and the H14 efficiency cut–offs shown.
The purple data points in the graph above are the actual certification values. Company B’s H13 filter is almost touching the H14 efficiency, with a slight dip at a few particle sizes. The H13 cut–off is too far even at those particle sizes.
The H13 filter of Company B is cleaning particles 120 times smaller than the generic H14 filter.
In fact, the same filter of Company B would have qualified for H14 rating if it had claimed a coverage of 0.0523 µm and above (green band), which covers almost 5 times smaller particles than the generic H14 filter can catch.
Air Safety: The Real Issue
Corporate entities, businesses, and professionals such as doctors would want the air they breathe in their work–spaces to be safe, or at least safer.
Of course, cleaning or purging the air of viruses won’t offer 100% protection from SARS-CoV-2. This is because the virus transmits through various modes.
However, as per the latest guidelines of the Centers for Disease Control and Prevention (CDC), USA, the virus spreads through person to person contact. CDC says that it does not mainly spread through touching a surface or an object.
Indian Society of Heating, Refrigerating, and Air-Conditioning Engineers (ISHRAE) issued operating guidelines for AC and ventilation during Covid-19 pandemic. Depending on your location, there may be different guidelines for your country. Please check with the local authorities or your government’s website.
As per the ISHRAE guidelines, the SARS-CoV-2 virus is 0.08—0.16 µm in size.
ISHRAE guidelines also say that the SARS-CoV-2 virus transmits through two possible aerial routes:
- Infected microscopic airborne particles (dust particles): The virus can use these as substrates, and settle on them. It can move around in the air resting on the particle. Removing the dust particles can reduce the virus transmission.
- Contaminated aerosol droplets (micro-droplets): The virus can be present in the aerosol generated from mouths and spread around the room. At that sub-micron size, it will stay airborne for hours.
Smallest infectious sizes of aerosol or particulates
There is no consensus on how small the contaminated dust or aerosol droplets can get before they become non-infectious.
So logically, we should assume that the smallest infectious size of a dust particle (with a virus sitting on it) or an aerosol droplet (with a virus in it) should almost be the size of the virus itself.
In other words, we should be prepared to remove dust particles and aerosol droplets of 0.08 µm and larger. This is for additional indoor air safety in the post-Covid world.
Let us see how the above two filters would perform in such a world. For that, let us superpose the virus size range over the earlier two graphs.
It is clear that a generic H14 filter is NOT offering any protection for 0.08—0.3 µm range. The SARS-CoV-2 may transmit through aerosol droplets or airborne dust particles in this size range.
The HEPA-13 filter of Company B is offering HEPA-14 level protection for 0.0523 µm and above, which includes 0.08—0.3 µm—the range critical for SARS-CoV-2 transmission—left out by a generic HEPA-14 filter.
Never evaluate an air purifier’s cleaning efficiency in isolation. Evaluate it along with the range of particle sizes it cleans.
If one wants to use an air purifier for a specific pollutant removal, find out the cleaning efficiency for that pollutant’s size.
False sense of security is worse than no security.
First published on: 25th May, 2020
Image by Roman Koval from Pexels