3 HEPA Air Purifier Myths

3 HEPA Air Purifier Myths

hepa filters

MYTH 1 - The 0.3 micron myth

“HEPA filters can only capture particles larger than 0.3 microns”

Particle size varies from very large to very small.

For example, some pollen grains are very large – some are as large as 100+ microns. Airborne viruses are very small - between 0.02 and 0.12 microns.

HEPA filters are marketed as being as being able to remove 99.97% of all particles that are 0.3 microns in size or larger.

This means that HEPA filters will filter 9,997 out of every 10,000 particles 0.3 microns or larger. Large pollen grains are filtered very well (at 99.97% efficiency). But what about airborne viruses? They’re certainly not larger than 0.3 microns. They are, in fact, quite a bit smaller.

The facts

HEPA filters capture particles of all sizes. HEPA filters capture particles larger than 0.3 microns. And HEPA filters capture particles smaller than 0.3 microns.

In fact, independent testing has shown that HEPA filters may be able to capture small particles at even greater efficiency than large particles.

How is this possible?

First, consider that there are multiple methods by which HEPA filters capture small particles and both of these methods are highly effective.

A HEPA filter captures particles in two ways, depending on the size of the particle:

  • The particle is large enough to get caught between multiple fibers. The technical term for this process is “sieving” – which gives you a good visual of how this method works. This happens to all particles 5 microns and larger.

  • The particle is small enough to fit in between fibers but sticks to a particular fiber. This type of capture comes in three varieties given somewhat difficult to understand technical names:

    • inertial impaction

      A particle has inertia. This inertia keeps it going in a certain direction. This direction lands the particle on a particular fiber. It makes impact with the fiber because of its inertia. This happens with particles 0.5 to 5 microns in diameter.

    • interception

      If a particle travels close enough to a fiber the particle will stick to the fiber – the fiber intercepts it. This happens with particles 0.1 to 1 micron in diameter.

    • diffusion

      If the particle is small enough it can actually collide with and bounce off of gas molecules. Remember, air itself is a gas but air also contains numerous other gases. These gases exist as billions of extremely high energy individual molecules. Super small solid particles collide with and bounce off of these molecules repeatedly as they travel with the air through the filter. The more collisions and bouncing around the greater the probability that the particle will land on and stick to a fiber in the filter. This happens with particles smaller than 0.1 microns.

The above discussion should demonstrate two things:

First – there are multiple ways small particles can be filtered - particles smaller than 0.3 microns can be filtered via interception or diffusion.

Second – both of these methods are highly effective capturing particles smaller than 0.3 microns. Interception is a real process that works. Diffusion is a real process that works. These processes both act to remove particles smaller than 0.3 microns from the air in a HEPA filter.

Furthermore, smaller particles stay airborne much longer than larger particles. Larger particles fall to the ground fast. Small particles do not – they stay airborne for a long time. And this gives an air purifier equipped with HEPA filter much more time to properly filter those smaller particles.

Why does this myth exist?

Why is it a commonly held belief that HEPA filters only filter particles larger than 0.3 microns?

Reason 1: Manufacturers

Here’s the number one reason: Air purifier manufacturers market their products as being able to remove 99.97% of all particles that are 0.3 microns in size or larger.

Read any product description, online or even on product packaging – and you’ll see the phrase “99.97% of all particles that are 0.3 microns in size or larger”.

Most air purifier manufacturer give you a sales pitch along these lines:

“Our air purifier features HEPA filtration which means that it’s able to remove 99.97% of all airborne particles 0.3 microns or larger”

Reason 2: Consumer Publications

Most other consumer publications don’t sufficiently analyze the science behind HEPA filtration. They simply go along with manufacturer claims. That’s why you’ll see the phrase “0.3 microns or larger” mentioned in their air purifier reviews – further convincing you that air purifiers equipped with HEPA filters can only filter particles larger than 0.3 microns.

Reason 3: The Reason Behind the Original Claim

So far we’ve only looked at how and why the 0.3 micron myth is conveyed and spread among the minds of consumers. But where did it start? Where does the 0.3 micron benchmark come from?

The primary reason is this: 0.3 microns is the most difficult size particle to filter. If you study the science behind HEPA filtration you’ll find that particles larger than 0.3 microns are easier to filter. And, that particles smaller than 0.3 microns are easier to filter.

Recall that a HEPA filter captures particles in two different ways: if the particle is large enough it gets caught between multiple fibers (the filter acts like a sieve). If the particle is small enough it will stick to a fiber as it travels through the filter.

0.3 micron particles are just small enough to fit between fibers (to not get caught) and just large enough to not stick as effectively to remaining fibers.

Don’t worry though. HEPA Filters are still 99.97% effective at capturing particles of this size – only three in every 10,000 particles make it through.

0.3 micron particles are so difficult to filter that they have their own name: “baby bears”. These “baby bears” are quite a handful to filter. So a HEPA filter being capable of filtering such particles at 99.97% efficiency is actually a tremendous accomplishment. This is why the claim is made that HEPA filters can filter particles 0.3 microns or larger.

Most consumers and most consumer publications just put emphasis on the wrong part of the statement – they focus on the “or larger” part of the statement because they simply don’t know enough about the “0.3 microns” part of the statement.

Furthermore, most particles that consumers tend to care about are larger than 0.3 microns. Particles like pollen grains, mold spores, most dust particles, and even wildfire smoke particles are all larger than 0.3 microns. The “or larger” part of the statement gives consumers confidence that a HEPA air purifier will meet their needs in removing most of the particles they’re concerned about.

Finally, particles smaller than 0.3 microns are much more difficult to test for (than particles 0.3 microns and larger). Numerous tests and sufficiently accurate testing equipment exists for testing for initial and final concentration of particles 0.3 microns and larger. Third party independent testing is rare while standardized testing is almost nonexistent for particles smaller than 0.3 microns.

That is not to say that there isn’t any testing at all. As we showed earlier, certain tests do exist and those tests have shown that HEPA filters are highly effective filtering particles smaller than 0.3 microns.

These claims are further substantiated in real world applications. Airborne pathogens like bacteria and viruses are combated in hospitals by, you guessed it, HEPA filtration.


1. You’ll often hear it said that 90% of particles are smaller than 0.3 microns, making HEPA filters ineffective at filtering the majority of particles in the air. Now you know this simply isn’t true. A HEPA filter is perfectly capable of filtering particles smaller than 0.3 microns and therefore is perfectly capable of filtering 90% of particles.

2. You’ll also hear it said that small particles are the primary concern – larger particles fall to the earth while only the smallest particles stay suspended in the air. Again, HEPA filters have no trouble with these smaller particles.

MYTH 2 - The old technology myth

“HEPA filters have been around for decades. Newer filtration technology is better.”

You may be well aware that HEPA filtration “technology” is old. And yes, it is old. It’s been around since the 1940s -that’s over seven decades old!

You may be inclined to think that because it’s old, it’s not as good. This is certainly true for most “technology” implemented in electronic devices – phones, TVs, etc.

Furthermore, you may have been exposed to some consumer publications and certain (higher end) air purifier manufacturers belittling HEPA filtration – emphasizing its limitations while praising other newer technology as being superior to HEPA.

Here’s the bottom line – HEPA filtration “technology” may be old but it’s still highly effective.

It does have some limitations but air purifier manufacturers have found ways to combat those limitations quite effectively as well. Let’s take a look.

HEPA’s only limitation – gases

Air purifiers purify air. Air is itself a gas but contains solids, liquids, and gases.

HEPA filters can only remove solids (and liquids) – particles/particulates - from the air. It cannot remove gases. This means that it’s ineffective for VOCs, odors, etc.

Air purifier manufacturers have solved this shortcoming by adding carbon filters to their air purifiers. Activated carbon adsorbs gases. The carbon filter is placed right before (or sometimes right after) the HEPA filter.

The air enters the air purifier. Unwanted particles are filtered by the HEPA filter. Unwanted gases are filtered by the carbon filter.

Problem solved.

Do not doubt the efficacy of HEPA

Other technologies may make bold claims about their effectiveness and efficiency. But that shouldn’t diminish the efficacy and long track record of HEPA filtration.

  • HEPA filters are extremely effective in removing particles from the air. A HEPA filter only let’s through three 0.3 micron or larger particles per 10,000 particles – THREE.
  • HEPA filters are and have been commonly used in industry for years. Hospitals use HEPA filtration. If hospitals use HEPA filtration, shouldn’t that be good enough for your home?

Not to mention HEPA advantages like

  • mechanical filtration – the part of the air purifier that filters – the actual filter - can’t break
  • low cost – compared to newer tech HEPA filters and HEPA equipped air purifiers are usually much cheaper

MYTH 3 - The Antibacterial myth

“A HEPA filter needs an antibacterial coating to more effectively filter airborne bacteria”

You don’t need to kill airborne bacteria to filter airborne bacteria.

HEPA filters capture particles, including biological particles like viruses and bacteria alive or dead. The HEPA filter’s efficiency doesn’t change depending on whether the particle is alive or dead. In other words, it’s equally efficient filtering dead or alive particles.

This means that bacteria and viruses do NOT need to be killed by some type of antimicrobial coating in order to be filtered. Again, the HEPA filter will capture bacteria and viruses just the same whether they are alive or dead.

A quick word on different types of HEPA filters

True HEPA.


They’re exactly the same thing.

Different organizations and industries in different countries define HEPA in different ways. But no organization or industry distinguishes between HEPA and True HEPA. It’s the same thing.

You do not need to limit yourself to air purifiers that come equipped with a “True HEPA” filter. If the unit simply comes equipped with a HEPA filter that’s good enough. A regular HEPA filter is exactly the same thing.

Where does the term “True HEPA” come from?

What you definitely do need to look out for are “HEPA Type” filters. These are not HEPA filters.

HEPA Type filters are the very reason why the term “True HEPA” filters exists. “True HEPA” is added to “HEPA” to further distinguish it from a “HEPA Type” filter.

Again, HEPA Type filters are not HEPA filters. HEPA filters have to to meet the 99.97% 0.3 micron standard. HEPA Type filters do not. There is, in fact, no standard for HEPA Type filters.

In conclusion,

  • True HEPA is the same thing as HEPA – 99.97% effective removal of particles larger than 0.3 microns. Recommended.
  • HEPA Type is not the same thing as HEPA – no standard removal rate. Not recommended.

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Our Recommendations for HEPA Air Purifiers

The real concern – CFM

HEPA filters do a great job of filtering any size particle. When buying an air purifier you absolutely need to make sure that it comes equipped with a HEPA filter. But this is easy to do – just check the model’s product description – is it HEPA? If yes, you’re good to go. If no, then it’s not even an option.

The next step is much more difficult. Once you’ve found a HEPA air purifier you need to make sure it’s going to work for your particular application. In order to do that you need to find the unit’s CFM.

Most manufacturers do not list CFM. They list CADR. Don’t worry about what CADR means. Just use it to estimate CFM. The CADR will normally be about 5% lower than the CFM. For our purposes CADR essentially equals CFM.

Generally, you want to buy the highest CFM air purifier you can buy, no matter the room size. Why? Because the higher the CFM, the more chances of capturing particles.

Imagine a block of air. You want to clean that block of air as quickly and as well as you can. Putting it through a HEPA filter is going to clean most of it. But new contaminants are always being added to air, contaminated air is always mixing with clean air, etc. so the more times you process that block of air per unit time, the better.

This is why we recommend higher CFM units. The higher CFM, the more processing of any particular block of air per unit time. Through extensive research and testing we’ve found that the “sweet spot” for CFM for a stand-alone air purifier is about 250 CFM.

The two best rated units in this category are the Winix 5500-2 and the Coway Mighty air purifier. Either one of these units are our recommendation if you’re looking for the best HEPA air purifier currently on the market. Both units are equipped with HEPA filters for particle filtration and both units have sufficient CFM for most applications.

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Comments (2)

Larry Wangsays...

I found your site doing a search on Youtube. The more I read the websites out there on air purifiers and then cross check against the content on your site, the more impressed I become with our site.

I am currently testing out the Winix 5500-2 and have a Levoit LV-PUR131 that I had gotten earlier in the summer based on Amazon reviews. I am running the Winix 5500-2 without their plasma wave mode (since don't want to deal with any ozone emissions). Found the Levoit to be good in small bedrooms and dealing with pet litter odors and other pet odors due to the significant amount of carbon pellets in their filters. The Winix 5500-2 seems to have have the amount of carbon pellets in their filters.

I will be getting a particle counter as well to check on air quality. A lot of good research and material on your site.


I'm quite impressed, this is the first site that actually describes the physics correctly! Just one comment: HEPA-Filters can break. Touch them a bit too hard and the local effectiveness will decrease dramatically, even you don't see a break. Also thank you for that Winix-recommendation! If was looking a long time for a quiet purifier with a high enough volume flow (and reasonable prices), but never looked at winix before! In Europe they don't sell the exact same models as in the US, but still I found some good ones.