What Are HEPA Filters And How Do They Work? Overview

Here is the basic distinction between HEPA and HEPA filters:

  1. HEPA is a standard, developed by DOE. HEPA stands for High-Efficiency Particulate Air.
  2. Filters that meet that standard are referred to as HEPA filters.

HEPA filters are nothing more (or less) than filters that meet the HEPA standard, put forward in 1983 by the US Department Of Energy:

“(HEPA filters) remove at least 99.97% of airborne particles 0.3 micrometers (μm) in diameter.” (DOE’s HEPA Standard)

These filters were first invented during World War II. The first HEPA filters were designed by Manhattan Project scientists to capture very small particles released during the creation of the atomic bomb.

The industry quickly saw the advantages of such fine particle filters. Today, HEPA filters are the most widespread small particle filters, used in HVAC (air purifiers, air conditioners), biochemical applications, and airlines. Quiet recently, Tesla has announced they will have the world’s first car HEPA-grade filter installed in the Tesla Model X.

Here’s how a HEPA filter looks like when put under the microscope:

hepa filter close up from under microscope
HEPA filters are made out of thousands upon thousands of neatly intertwined fiberglass fibers.

In this overview article about HEPA filters, we will explore:

  • What HEPA filters actually are, and what kind of particles they capture (dust, smoke, pollen, etc.).
  • Different types of HEPA filters, such as HEPA-like, True HEPA, H13 “medical grade” HEPA filters.
  • How HEPA filters work to filter out very small particles (via 4 mechanisms).
  • What HEPA filters are made out of.
  • Can HEPA filters be used to remove viruses?
  • If HEPA filters are washable, and how often they need to be replaced.
  • Devices (along with Tesla Model X and the updated Model S) that use HEPA filters.

All in all, you should get a good understanding of probably the most important filters in existence today. Let’s start by looking at what HEPA filters actually are:

What Is HEPA Filter? 

Here is the HEPA definition, in accordance with the DOE Technical Standard DOE-STD-3020-2015 issue:

“The (HEPA) filter shall exhibit a minimum efficiency of 99.97% when tested with an aerosol of 0.3-micrometer diameter.” (DOE 2015 TS Issue)

HEPA acronym has three formal meanings:

  1. High-efficiency particulate air.
  2. High-efficiency particulate absorbing.
  3. High-efficiency particulate arrestance.

In theory, a HEPA filter is literally any filter that can capture at least 99.97% of very small 0.3 microns in size particles. 

Example of HEPA vs Non-HEPA filter: If you were to shoot 10.000 0.3 micron particles at a filter, and only 2 or 3 particles managed to penetrate it, that’s a HEPA filter. If that filter would let through 5 (99.95% of the total) such particles, it’s not a HEPA filter.

testing hepa vs h13 hepa air purifiers with aerosol
Testing HEPA filters with smoke and a small particle generator.

In all cases, HEPA filters are mechanical filters made out of hundreds of thousands of tightly intertwined fiberglass fibers. These create very small holes that have 2 practical jobs:

  1. Allows for a free flow of air.
  2. Capture almost everything else (primarily, air pollutants).

When building the atomic bomb, this property of HEPA filters was very useful in the separation of Uranium isotope 235 from isotope 238. One isotope was captured by the HEPA filter, and the other managed to get through the filter.

In the 21st Century, however, air purification is the primary use of HEPA filters. That means HEPA filters, by and large, are designed to capture airborne indoor pollutants. Let’s look at which air pollutants HEPA filters are capable of removing from indoor air:

Air Pollutants HEPA Filters Are Capable Of Capturing

By definition, HEPA filters are capable of removing more than 99.97% of all airborne particles bigger than 0.3 microns in diameter. That includes a vast array of domestic air pollutants that lower indoor air quality, and may even be a factor for allergies and asthma.

Here are some of the most common air pollutants HEPA filter air pollutants remove, according to Engineers Journal:

hepa filters can capture dust particles, pollen, mold, lead dust and so on
Common air pollutants HEPA filters capture.

It’s important to understand that HEPA filters are mechanical filters. They can capture both large and small air pollutants if they are in solid form.

HEPA filters very effectively remove dust particles, pollen (allergen), mold, certain bacteria and viruses, dust mites, PM 2.5, pet dander, and a number of other solid allergens found in indoor air.

What about coronavirus?

A common question in 2020 is if HEPA filters can capture coronavirus. The short answer is that, by definition, HEPA filters can effectively filter out all particles (including solid-form viruses) that are 0.3-micron in diameter. However, it’s vital to understand that the FDA has to approve an air purifier to be used as a medical device against coronavirus. 

Engineering can’t give a definitive answer if HEPA filters filter out coronavirus. Look for this info at FDA, and CDC.

Here’s what engineering can answer: HEPA filters are unable to remove any and all gaseous form air pollutants. Primarily, these are odors like cigarette smoke, kitchen oil smell, the smell of mold, and so on.

Designated filters that can adsorb air pollutants in gaseous form are activated carbon filters. 

When targeting small particles, we measure filters for their capability to remove 0.3-micron in diameter particles. Why is that?

0.3 Micron – Most Penetrating Particle Size

Why are HEPA filters measured for their ability to capture 0.3-micron particles? 0.3 micron does, at first, seem like an arbitrary agreed size of ‘a small airborne particle’. Why not measure the ability to capture 0.2 or 0.4-micron particles?

Here’s why: 0.3-micron is the particle size filters are the weakest at. Pick any small particle filters, they will almost always show a higher filtration efficiency at 0.2 and 0.4-micron levels than for 0.3-microns.

Studies have shown that if you blast a filter with a range of small particles of various sizes, the ones that are best fit to penetrate it are the 0.3-micron particles. 

That’s why 0.3-micron particle size is commonly referred to as ‘Most Penetrating Particle Size’ or MPPS for short.

As we’ll see in the chapter about ‘How HEPA Filters Work’, this is a consequence of impaction insufficiency. Impaction is one of the 4 mechanisms HEPA filters employ to capture particles.

Choosing 0.3-micron size makes sense exactly because the performance of filters is the weakest at that size. It’s stronger at 0.2-micron and 0.4-micron, for example.

In fact, we can differentiate between various types of HEPA filters, based on the air purification efficiency at different particle sizes:

Types Of HEPA Filters

Not all HEPA filters are the same. What is more, some filters that are marketed as HEPA filters, might not even reach the high HEPA standard. 

square and circular hepa filters
HEPA filters can be bent into different shapes. What truly matters is air purification efficiency.

Let’s look at 3 specific terms that apply to different types of HEPA filters:

HEPA-Like

When buying HEPA filters, be aware of these marketing terms:

  • “HEPA-type”.
  • “HEPA-like”.
  • “HEPA-style”.
  • “99% HEPA”.

Such terms are used for filters that don’t reach the HEPA standard. What is more, most of these types of filters aren’t independently tested. All HEPA filters that do reach the HEPA standard are independently tested.

To be fair:

HEPA-type or HEPA-like filters might be very close to HEPA standard. However, it’s equally or even more likely they perform very poorly at 0.3-micron size particulate filtration. Without independent testing, there is no real way to validate air filter efficiency.

99% HEPA filters, for example, seems to suggest that they don’t capture 99.97% 0.3-micron particles. Rather, it’s suggested they capture 99% of such particles. Since they are not validated, we don’t know if they remove 99%, 95%, or even 90% of particles.

These kinds of marketing buzzwords in connection with HEPA filtration gave rise to the “True HEPA” connotation to differentiate between non-validated filters and validated HEPA filters.

Let’s look at what True HEPA filters really are:

True HEPA

True HEPA filters are the standard HEPA filters. Why the addition of “True”? It’s a marketing term to differentiate between HEPA-like (fake HEPA filters) and filters that actually follow the HEPA standard.

Here the deal:

True HEPA filter is a HEPA filter. It’s equal to the term “HEPA filter”. There are no extra special purification qualities True HEPA has over standard HEPA filters.

HEPA filters without “True” are, of course, also standard HEPA filters.

True HEPA filters are the most widespread HEPA filters. You’ll find them in air purifiers, vacuums, furnace filters, are so on.

All of them, by definition, remove more than 99.97% of 0.3-micron airborne particulates.

true hepa replacement filter example
Example of HEPA filter, used in high-performance Coway air purifiers.

HEPA filters can be classified as H10, H11, or H12 filters. Most often, the specific classification of HEPA filter is not given, nor is it necessary in the majority of cases.

However, if you’re looking for advanced HEPA filters – the ones used in hospital and clinic – you should look into H13 HEPA filters:

H13 Medical Grade HEPA

H13 are a special advanced type of HEPA filters, commonly used in hospital settings. Hence, they are sometimes referred to as medical-grade HEPA filters.

If you compare True HEPA vs. H13 HEPA filters, you see that True HEPA filters target 0.3-micron particles primarily while H13 HEPA filters target even smaller 0.1-micron particulates.

H13 HEPA filters can capture more than 99.95% particles that are 0.1 microns in diameter. That makes them the most powerful HEPA filters used in common HVAC appliances. Example: You can check a list of the best H13 HEPA filter air purifiers here

People who have prone to allergies and asthma tend to prefer using the H13 HEPA air purifiers. This is because they are very effective at capturing even the smallest airborne allergens.

Additional filter: Medical IQAir purifier, for example, uses Hyper HEPA filters. There are specialized for capturing tiny air pollutants with 0.003-micron in diameter. HyperHEPA filter captures particles that are 100x smaller than what True HEPA filters are designed to remove.

comparison of air pollutants such as allergens IQAir HealthPro Plus hyperhepa filters are capable of catching
True HEPA filters are limited to 0.3 microns. H13 medical-grade HEPA filters are limited to 0.1-microns. HyperHEPA can capture allergens as small as 0.003 microns.

What MERV Rating Is A HEPA Filter?

A common question for an advanced understanding of HEPA filters is the following:

What MERV rating are HEPA filters?

MERV stands for Minimum Efficiency Reporting Value. This is a rating common air filters are measured for and ranked by. Usually, they are used for filters that capture particulates that are bigger than the size of particles HEPA filters are designed to capture.

According to Sy-Klone,

“HEPA air filters are not MERV rated as they exceed the ASHRAE test protocol 52.2 used in determining the MERV ratings.”

As such, we can’t talk about HEPA’s MERV rating.

Let’s look at how the HEPA filters work on the microscopic level:

How Does A HEPA Filter Work? (4 Mechanisms)

In the most basic understanding, HEPA filters are very fine filters, made out of neatly intertwined fiberglass fibers. If you look at a HEPA filter under the microscope, you will see meters upon meters of fine fibers.

These present a mechanical barrier to small particulates. However, to truly understand how HEPA filters work, the simple existence of a physical barrier is not enough.

At the microscopic level, there are 4 mechanisms of how HEPA filters manage to capture these minuscule particulates so well.

4 mechanism by which fiberglass fibers capture small airborne air pollutants
Fiberglass fibers in HEPA filters capture particulates by diffusion, interception, impaction, and electrostatic attraction.

Let’s look at each mechanism HEPA filters employ to prevent particle penetration (all of the photos of the mechanisms are sourced from Wikipedia):

Diffusion

The smallest particles are captured via the mechanism called diffusion. This involves small particles colliding with fibers and gas molecules.

The smallest particles (below 0.1-micron in diameter) move in a random direction. This movement is referred to as Brownian motion, and looks like this:

The Smallest 0.1-micron and smaller particles move in a random direction. When they hit the HEPA fiberglass fiber, they slow down and are captured by impaction or interception.

When you run an air purifier in a low fan speed setting, the airflow is small enough to allow HEPA filters to use a diffusion mechanism to help capture very small particles.

The more significant mechanisms by which HEPA filters work are interception and impaction:

Interception (Adherence)

Particles bigger than 0.1 microns are big enough to directly impact fiberglass fibers. When this happens (even when they come very close to the fiber), they adhere or ‘stick’ to the fibers.

That’s why this mechanism is called interception; HEPA filters intercept small non-Brownian motion particulates:

Small molecules (0.1-1 micron) are big enough to directly hit fibers and adhere to the HEPA filter.

When HEPA filters intercept more and more small particles, it becomes more and more saturated. When HEPA filters become too saturated, they need to be replaced. The new HEPA replacement filter will again be capable of capturing small particles via the interception mechanism.

Inertial Impaction

Bigger particles (1.0+ micron in diameter) will inevitably hit the fiberglass fibers in the HEPA filter. They are simply too big to penetrate the physical barrier. By the mechanism of inertial impaction, HEPA filters most of the bigger air pollutants such as pollen:

Bigger air pollutants directly impact the fibers in the HEPA filter and are stopped immediately.

Due to a bigger size of particles, the inertial impaction represents the higher percentage of future HEPA filter saturation. To reduce the amount of these and even bigger particles, pre-filters are employed to protect the precious HEPA filter, as we’ll learn further on.

Electrostatic Attraction

Even the smallest air pollutants like dust particles and smoke particles have a small electrostatic charge. Fiberglass fibers also have a small electrostatic charge.

When positively or negatively charged particles come in close proximity to positively or negatively charged fibers, the opposite charge results in electrostatic attraction.

Small electrostatically charged air pollutants (usually with a negative charge) are attracted to positively charged fibers.

The small particles are held in a HEPA filter via electrostatic attraction. 

To reduce the burden larger particles present for a HEPA filter, we usually use pre-filters:

Importance Of Pre-Filter

Air pollutants are combined from small particles (dust, pollen, smoke particles) and larger particles (hair, pet dander). 

If all the particles would be filtered out by the HEPA filters, it would quickly become saturated. In turn, this would reduce the lifespan of a HEPA filter.

That’s why air purifiers use large particle pre-filters that:

  1. Capture larger air pollutants like hair and pet dander.
  2. Protect the precious HEPA filter and prolong its lifespan.

Pre-filters are usually washable. You can take them out, wash the hair out, and insert them inside the appliance.

In all cases, pre-filters are the first barrier air that is to be cleaned has to go through. As such, they present a kind of a shield for the HEPA filters. It is essential to use a pre-filter in order to preserve the immaculate efficacy of a HEPA filter.

Can You Wash HEPA Filter?

HEPA filters cannot be washed with water and soup. Usually, people do ask if they can wash a HEPA filter and how to actually clean it.

In short, HEPA filters are very fine filters that capture very fine particles.

It’s almost impossible to wash out these tiny particles.

When the HEPA filter is saturated, there is only one option:

You have to replace it.

How Often To Change HEPA Filter? 

HEPA filters in air purifiers, for example, need to be replaced every 3-12 months. Some HEPA filters have a bigger capacity than others.

Example: Okaysou HEPA filters need to be replaced every 3-6 months. Alen BreatheSmart, on the other hand, uses advanced filters with higher capacity and thus prolonged lifespan. Those can last for up to 12 months.

How frequently HEPA filters need to be replaced also depends on:

  1. Level of air pollutants. Air with a higher concentration of pollutants (like wildfire smoke) will reduce the lifespan of a HEPA filter.
  2. Use of HEPA filters. The more you use the appliance with HEPA filters, the quicker the filters will reach saturation levels. 

Some producers even give you an estimate of how long HEPA filters last. 

Example: Medify air purifier’s HEPA filters last from 1500 to 2500 hours of use.

It’s vital to replace HEPA filters once they are too saturated. If you don’t, the particles captured by the filter could be blown back into the indoor air circulation and reduce the indoor air quality.

That’s why modern air purifiers come with a detector that tells you when HEPA filters need to be changed.

Devices That Use HEPA Filters

HEPA filters are used in a wide range of appliances. Primarily they are used to filter out air pollutants. Some water filters also use filters that are up to HEPA standards.

Here are some devices for which the use of HEPA filters is essential:

  • Air purifiers.
  • Some air conditioners.
  • Car air purifiers (Example: Newer Tesla cars have built-in HEPA filtration systems).
  • Microbiological laboratories use HEPA filters to prevent the spread of infections.
  • Vacuum cleaners.
  • Air cleaning system on airplanes.

All in all, HEPA filters are one of the most useful filters for air purification. They have an extensive and interesting history, particulate mechanisms of air pollutant removal, and are used in the next-gen futuristic vehicles like Tesla cars.

If you have any questions about HEPA filters, you can pose them in the comments below.

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