You may have come across a High Efficiency Particulate Air (HEPA) filter at some point, whether it was in an air purifier for your home or car, vacuum cleaner, or air filter for your HVAC system.
But how exactly does a HEPA filter compare to other standard filters, and how does the HEPA rating system work anyway? HEPA filters are actually broken up into several classes of filters, and the two standards most widely known are the United States Department of Energy (DOE) and European Union standards. Most people, however, consider a HEPA filter to be a mechanical filter that’s able to trap 99.97% or more of particles 0.3 micrometers (µm) in size or larger. This is what’s widely known as a True HEPA filter.
There are several nuances here that can make this more technical and complicated than it needs to be, so let’s take a look at what really matters and simplify it.
The United States and European HEPA Rating Standards
In the United States, the Department of Energy (DOE) now uses the standard known as the Institute of Environmental Sciences and Technology (IEST)-RP standard to validate its HEPA filter testing program. Filter manufacturers who want to claim that their filter is a HEPA filter must go through independent laboratory testing following the this independent method of testing.
The European Union uses a different standard called the EN 1822:2009, or European Norm.
The US standard test starts off with an arbitrary particle size, which is 0.3 µm for most filters. The European test, however, uses something called the Most Penetrating Particle Size (MPPS) to determine the particle size to test.
The US standard also requires that a minimum of 99.97% of those pollutants be filtered, while the European standard requires a minimum filtration of 99.95% for the most widely known type of HEPA filter.
Manufacturers often use the term “H13 HEPA filter” to denote a filter that meets the above criteria under the European jurisdiction.
The H13 HEPA filter is somewhere in the middle on the scale of the different types of HEPA filters. The H13 filter in fact is just part of a broader series of filters that we call HEPA filters. As of now, it is the most practical and realistic type of filter for everyday, residential applications.
The European standard that makes up the H14 HEPA filter is generally what most consider a True HEPA filter in the United States.
|HEPA class||retention (total)||retention (local)|
|H13||> 99.95%||> 99.75%|
|H14||> 99.995%||> 99.975%|
|U15||> 99.9995%||> 99.9975%|
|U16||> 99.99995%||> 99.99975%|
|U17||> 99.999995%||> 99.9999%|
HEPA filter types, according to European standards (source).
On one side of the spectrum is the E series of HEPA filters, which isn’t as discerning as the H13 filter.
The E10 (or once known as the H10) can actually only trap 85% of the 0.3 µm and larger particles! It’s still a type of HEPA filter, though.
On the other side, however, are the U series of filters, also known as Ultra Low Penetration Air Filters (ULPA) filters. These are typically regarded as completely different kinds of filters from the H13/H14 level HEPA filters.
While ULPA filters may in theory be able to remove a greater percentage of pollutants, they require a powerful air flow and a rapid air exchange rate. Otherwise, the blocked air may just sit there without the power behind it to be moved through the space. This is why they’re typically used in cleanrooms and high biohazard laboratories only.
In the United States, filter guidelines are now set by the The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Institute of Environmental Sciences and Technology (IEST). The Department of Energy (DOE) plays less of a direct role now, although it still continues for its original trademarked HEPA name to be used for quality testing and validation.
For residential applications in the US, it’s just widely accepted that a True HEPA filter needs to be able to filter out 99.97% of particles 0.3 µm or larger. This eliminates the confusion of the other HEPA filters out there, as well as filters that claim to be similar to a HEPA filter, but aren’t.
Which brings us to manufacters that use the HEPA name without actually meeting the standard…
True HEPA Vs. HEPA-Like
I’ve already stated it multiple times –
A real HEPA filter must at minimum be able to filter out at least 99.97% of particles 0.3 µm or larger in a laboratory setting. There are, however, a lot of filters that are marketed as HEPA-like, HEPA-style, or similar.
Most of those filters in reality can only filter out 99.0% of particles 2 µm in size or larger.
The reason why their manufacturers can use words like HEPA-like or HEPA-style on the packaging is because these filters are made from the same media used to make HEPA filters, but the filters themselves don’t end up being manufactured the same way. These filters don’t undergo HEPA testing since they’d fail the test.
The difference between 99.97% and 99.9% may not sound like a lot, but it is when you consider the billions of particles floating in the air and constantly being cycled out.
The difference in size between 0.3 and 2 µm doesn’t sound like it’s a lot, either. But, it becomes significant when you consider that even HEPA filters can’t filter out small particles like viruses at all, and only partially can filter out other pollutants like bacteria, dust, cooking smoke, and dust, if at all.
(You can learn more about what size which pollutant is and whether a HEPA filter will be able to remove it in this post.)
Luckily, most HEPA filters are supplemented by activated carbon filters, but it still takes a True HEPA filter to remove the majority of those pollutants.
HEPA Filters Are Compared Using Minimum Efficiency Reporting Value (MERV) in the United States
The Minimum Efficiency Reporting Value (MERV) is another test that was developed by ASHRAE. Its purpose is to create a universal testing method that can be used to compare filters that otherwise use different laboratory testing methods. The MERV is a great resource for US consumers who want to get the most realistic comparison between filters.
The MERV rating scale ranges from 1 to 20. Most HEPA filters are said to fall in the MERV 17 to 19 range, although some more discerning HEPA filters that can filter out particles down to 0.1 µm in size fall under the MERV 20 rating. This is where the ULPA filters would generally fall.
But all this comes with a great caveat – HEPA filters aren’t tested using MERV testing methods at all.
Instead, the data that result when HEPA filters undergo the actual IEST tests are compared to the MERV rating benchmarks had the actual test been performed. This is a major confusion. HEPA filters do not undergo any actual MERV tests, but they are given a MERV rating in order to “put them on the map.”
This is because HEPA filters go beyond the scope of the MERV test. The MERV scale in reality goes from 1 to 16. There is no such thing as a real MERV-17, 18, or 19 rating.
I know, confusing.
But it still helps us as consumers compare a HEPA filter to the other filters out there.
Compare a MERV-17 to MERV-19 HEPA filter rating to the inexpensive fiberglass filters that are found in most average American homes. The latter only have a MERV rating between 1 and 4. These cheap filters ironically aren’t tested either since they fall into the lowest category anyway.
I’m of the opinion that you can absolutely use the MERV rating for HEPA filters. The MERV rating won’t necessarily reveal whether a filter is of the HEPA filter type or of an inferior imitation, but it will help you determine how efficient a potential filter is if you first start by focusing on what kind of pollutant you want to remove and its size.
Be mindful that highly MERV-rated filters aren’t always a good thing, however.
It will be hard to all of a sudden start using a True HEPA filter on an already existing appliance like a HVAC without special modification of the entire system first. Usually, when we talk about True HEPA filters, we’re talking about smaller items like air purifiers and vacuum cleaners.
Most Penetrating Particle Size (MPPS) and Why It Matters
Circling back to Europe again, the Most Penetrating Particle Size (MPPS) is used instead to set HEPA filter standards. Here, the MERV rating system is less known. It makes sense considering that the MERV rating system doesn’t even go high enough to properly rate a HEPA filter.
But what is the MPPS then?
When it comes to air filtration, the MPPS could be considered as a gray zone, or weakest point. It’s the exact particle size where a filter struggles to neither use diffusion nor inception to successfully trap a pollutant.
The MPPS that’s usually used by European standards is in the range between 0.12 to 0.20 µm when it comes to HEPA air filtration.
For this reason, the HEPA filtration testing requirement has been set right above this range in the United States, at 0.3 µm. It’s just the right size to still be a potent air filter and cover as many of the pollutant sizes as possible, yet close enough to the MPPS to provide a challenge for manufacturers to improve upon.
Now, this small gap may not be as bad as it sounds, but it isn’t great either since this is right where many pollutants like viruses, bacteria, and smoke fall.
The good thing about HEPA filters is that they use four methods of filtration simultaneously – diffusion, intertial impaction, interception, and electrostatic attraction, meaning that the MPPS is still kept to a minimum, relatively speaking.
This is how the fibers on a fiberglass mesh of a HEPA filter could in reality be spaced between 0.5 and 2.0 micrometers across, but the filter may still be able to successfully filter out a pollutant that’s 0.5 µm across and have a 99.97% chance at it. I could imagine some of this probably occurs with some of the pollutants that fall in the MPPS range.
Different parts of the world have their own standards of testing for HEPA filtration, which can cause some confusion. Using terms like HEPA-like and HEPA-style can also be confusing for consumers.
So while we wait to see if the world will adopt a universal rating standard (known as the ISO 29463), just know that if a filter is being marketed as being able to remove at least 99.97% of pollutants 0.3 µm in size or larger, it’s a real HEPA filter.
Combine it with a heavy activated carbon filter and it should be able to trap the majority of indoor pollutants in your home.