True HEPA Filter: What It Is and What It Isn’t

When you are researching the best air purifier to use in your home, a term you will see again and again is “HEPA.” Some air purifiers use HEPA filters, while others may use terms like “HEPA-like” or “HEPA-style.” Which of these terms should you look for, and which ones are just marketing jargon?

We will take a close look at what HEPA really means, what a filter has to do to meet the standard and what a manufacturer really means when they call their product “HEPA-like.”

The HEPA standard

HEPA stands for “high efficiency particulate air.” A HEPA filter generally uses a mat of dense fibers to trap particles moving through it. In order to meet the HEPA specification, an air filter must trap 99.97 percent of all particles that are 0.3 microns in diameter. It can trap smaller particles (and is generally very effective at trapping larger particles as well), but its effectiveness declines as the particles get smaller. Note that in Europe, the HEPA standard is defined a bit differently (if you see a filter with a rating that looks like H13 or U16, the European standard is being used)—generally speaking, the higher the number (regardless of the letter), the better the filter under the European standard. But in this article we are going to focus on the U.S. HEPA standard.

Official government and industrial HEPA certification (non-consumer)

HEPA is a standard defined by the U.S Department of Energy (DOE). It was originally developed in the 1940s during the Manhattan Project because the DOE needed filters capable of screening out radioactive particles at atomic test facilities. The standard does not  define how the filter must be constructed or what it is made of—it simply indicates that it is capable of meeting the target number for trapping particles.

The important thing to know is that there is no official HEPA certification program for consumer residential air purifiers. It is a government standard meant to ensure proper air filtration in government and military projects. For instance, government contractors must adhere to strict standards when installing HEPA filters in the ventilation systems of nuclear facilities. They would have to prove that the filters used achieve the DOE’s HEPA standard.

Some industrial, non-government facilities, such as clean rooms and protected environments in hospitals, require HEPA filtration  systems. The Institute of Environmental Sciences and Technology (IEST) has created a standard way to conduct tests to make sure that a filter meets the HEPA standard, although the IEST notes that this is, “a basis for agreement between customers and suppliers.”

Is there a certification process for residential HEPA filters (consumer-grade)?

A manufacturer making air purifiers for use in your home cannot go to the DOE and get a HEPA seal of approval to use on their packaging. If an air filter claims to be a HEPA filter, you are basically trusting that the manufacturer had the filters tested and that they meet the DOE standard. The EPA notes that, “there is no widely accepted definition of HEPA performance in consumer products. Thus, they are unlikely to be equivalent in performance to HEPA-designated filter systems used in health care buildings and industrial processes, but still have very high removal efficiency (i.e., usually 99% or higher) for the reported particle sizes tested.”

HEPA facts and jargon: what they all mean

This is a benchmark for filter performance, but there is a certain amount of trust involved. Underwriters Laboratories has their own safety standard testing procedure for HEPA filter units. So an air purifier or filter manufacturer could get their filter tested by UL, and if it passes, the purifier can bear the UL mark (typically, the letters “UL” in a circle, sometimes with the specific safety or performance standard ID listed below it).

True HEPA– A consumer air filter labelled as True HEPA should conform the closest to the DOE standard for a HEPA air filter. At its highest efficiency, it should remove 99.97 percent of all particles that are 0.3 microns in size. InvisiClean, Aviano, Medify, Airthereal, Honeywell, Winix, GermGuardian and Levoit all make air purifiers labelled as True HEPA. Eureka makes vacuum cleaner filters marketed as True HEPA as well. The Aviano claims to filter out particles “as small as 0.03 microns,” but that may be a typo in their marketing materials.

HEPA Type– Calling a filter a HEPA Type filter is essentially meaningless, as it does not conform to any standard. It might be nearly as good as a HEPA filter, or it might be completely ineffective. Holmes, Envion, Febreze and other manufacturers make air purifiers labelled HEPA Type or HEPA-type.

HEPA Like– This is a variation on HEPA Type, although it is less common. It is just a marketing term, and you should not assume any filter with this label conforms to the actual HEPA standard.

UltraHEPA– AirDoctor uses this marketing term, claiming their air purifier is “100x more effective than HEPA air filters,” able to remove particles down to 0.003 microns in size. This is technically possible, but somewhat unlikely in a consumer-grade product.

HEPASilent– This is a trademarked proprietary filter used by BlueAir that combines an electrostatic charge with a mechanical filter. Supposedly the charge makes particles more likely to stick to the fibers of the filter, “which allows the use of a less dense filter.”

Permanent HEPA– These filters are marketed as meeting the HEPA rating, but are also able to be washed and reused rather than replaced. However, we do not recommend washing HEPA filters, even if they are intended to be reused, because the washing process likely causes small amounts of damage to the filter that reduce its effectiveness over time. Envion, Honeywell and Holmes market air purifiers with Permanent HEPA filters.

Absolute HEPA– This is sometimes used to mean the same thing as True HEPA, although sometimes it appears to indicate a claim of even better filtration power, up to 99.999 percent at 0.3 microns, according to Terra Universal, a company that makes filters for vacuum cleaners.

More HEPA related terms you should know

Only the label True HEPA (or sometimes Absolute HEPA) has any real meaning, because it is the only label that even claims to adhere to a standard. But even that should be taken with a grain of salt, since there is no official certification procedure, so you have to trust that the manufacturer has undertaken the tests to ensure their filter meets the standard or look for the UL certification mark. Any other variation on the HEPA label, including HEPA-type or HEPA-like, does not conform to the standard. Manufacturer-specific marketing terms for proprietary filter technology may or may not meet the standard.

They key is to look for specific numbers on labels such as: “99.97 percent of all particles at 0.3 microns in size.” If the marketing claims are fuzzy, such as “more than 99 percent of dust and pollen,” that is not the same as True HEPA. You should also be skeptical of claims that a filter is significantly better than the HEPA standard. These do exist, but mostly in industrial and medical settings, and would probably be too expensive for a consumer-grade air purifier.

There are a few other terms to watch out for that can complicate things when you are shopping for a HEPA air purifier:

CADR– This stands for “clean air delivery rate” and simply means how much air flows through the filter, But it is not a reliable number. For instance, a poorly designed filter with gaps around the filter frame would have a very high CADR number but provide almost no benefit as an actual filter. Meanwhile, a dirty HEPA filter would have a very low CADR rating but would actually be highly effective at trapping particles.

“Down to…” – Some air purifiers are marketed as removing particles “down to” a certain size, such as 0.1 microns or smaller. Without any data on the percentage of those particles are being removed, this number is meaningless. A block of wood will stop particles down to 0.1 microns, because a particle will run into it once in a while, but this alone does not make it an effective air purifier for particles of that size.

Carbon or activated carbon– HEPA filters are great at removing particles from the air, but they do not have any effect on gaseous pollutants, such as volatile organic compounds (VOCs) or odors. Many HEPA air purifiers claim they can remove these pollutants, However they may be hybrid purifiers with an additional carbon filter, or the HEPA filter might be impregnated with activated carbon. In any case, this is an entirely separate method of filtration and has nothing to do with whether or not the filter meets the HEPA standard.

Remember, just because the word “HEPA” is contained in the words on the label does not mean it is a legitimate HEPA filter. Look for the True HEPA designation, a UL certification that can lend more legitimacy to the product, or better still, look at the numbers presented and marketed to you. You want to see that a filter stops 99.97 percent of particles at 0.3 microns in size—anything else stated may just be marketing jargon from an unscrupulous manufacturer. If all else fails and you are still unsure, you may be best served by sticking with well-known national brands.