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What is a HEPA Filter?

The HEPA filter was first developed during World War II by the US government as a way to create an effective gas mask.

Little has changed with HEPA filter media since then. They remain the most effective way to remove particulates from the air.

What does the Acronym HEPA Stand for?

HEPA filter stands for high efficiency particulate air filter

By definition, a HEPA filter removes 99.97% or more of airborne particles that are sized 0.3 microns and larger.  This is considered the HEPA standard and sometimes called a "True HEPA" filter. 

To give perspective on what this means, it will help to define what a micron is.

A micron is short for micrometer.  One micron is one millionth of a meter.  This is much smaller than we can see with our naked eye.

Materials in HEPA Filters

HEPA filter media is often made from one of two materials

  • Fiberglass – also called glass fiber.  We use glass fiber in our Erik Ultra filters and can achieve HEPA performance or better.  Fiberglass tends to have the highest filter efficiencies but also the highest pressure drop (air flow resistance).

  • Nonwoven Fabric Material – usually polypropylene.  This is the type of HEPA air filter media in our air filters.  This type of media commonly has a static charge to give it high performance with a lower pressure drop compared to fiberglass.  Pressure drop is the air flow resistance. So, less resistance means more air flow which is important in cleaning the air in a space.

Why Filters are Measured at 0.3 Microns

Allow us a moment to get nerdy and explain why the standard is based on 0.3 microns.

The reason for measuring the HEPA filter efficiency at 0.3 microns is based on the original test equipment that showed this as the point most difficult for a filter to capture airborne particulates.


More recent research has revealed the most penetrating particle size (MPPS) to be in the range of 0.04 to 0.17 microns. 

It’s the point where the air filter has the lowest efficiency.

It makes sense that performance will increase as particles get bigger but what is not commonly understood is that performance also improves when you move below this point to smaller sized particles.

How Filter Fibers Clean the Air

There are different principles of physics at work that influence particle removal rates and they perform very differently. Taken together, the point where HEPA filters have the lowest efficiency is at the most penetrating particle size.

As seen in the example graph below, diffusion provides higher efficiencies below the MPPS and interception provides higher efficiencies above this point.

HEPA air filter curve

HEPA air filter curve

Understanding Filter Efficiency

The other aspect of HEPA filters is the 99.97% or more removal rate.

This is the standard set by the Department of Energy. What’s important here is that while these numbers all sound impressive there is a huge difference in performance between 99%, 99.97% and 99.99%+.

A 99.97% performer compared to a 99% will remove more of the smallest particles.  This is important size the smallest particle sizes are the most dangerous since they are too small to be captured by your nasal system.   They can pass directly to your lungs.

These tiny particles stay suspended in the air the longest and are more likely to be breathed in.  This is what can then enter your blood stream and cause health issues.

Filter Ratings

Filter efficiencies are rated differently around the world.

In Europe, the primary standard is EN1822.  They use the MPPS (most penetrating particle size) to measure air filter efficiency. Under this standard there are two classes of HEPA filter ratings

  • E11 - 95% efficiency at MPPS
  • E12 - 99.5% efficiency at MPPS
  • H13 – 99.95% efficiency at MPPS
  • H14 – 99.995% efficiency at MPPS

In the US, the most common standard is the MERV Rating (Minimum Efficiency Rating Value) which is set by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers).

  • MERV 16 – 95% or more efficiency at 0.3 microns and larger
  • **MERV 17 – 99.97% efficiency at 0.3 microns and larger

Note: **Technically MERV 16 is the highest level of filtration as defined in the ASHRAE standards. MERV 17 is sometimes used as a way differentiate a HEPA filter although is not part of their standards.

The key difference in these standards is the particle size where the efficiency is measured.  In the US, filter efficiency is measured at 0.3 microns whereas in Europe it will likely be in the 0.04 to 0.17 micron range.

When you go smaller than these particle sizes, filter efficiency usually increases. So, if you see a product marketed as having a certain efficiency at a particle size smaller than this, it is marketing hype.  It sounds impressive and takes advantage of most people not understanding how the science works.

The blue “diffusion” line in the chart above shows that performance can increase with smaller sized particles.

HEPA vs ULPA Filters

Environments that require the cleanest air sometimes use an ULPA filter.  ULPA stands for Ultra Low Particulate Air.  By definition, an ULPA air filter removes 99.999% or more of airborne particles sized 0.1 microns or larger.

So, the ULPA filter removes a higher percentage and it does so at particle sizes smaller than a HEPA filter.

An ULPA filter is what you might see in a cleanroom or a top-level biosafety cabinet.  Basically, environments that need to be free of any airborne particles.

This level of air filtration is more than you would need in a home. 

HEPA vs HEPA-Type Filters

"True HEPA" filters are fairly difficult to manufacture and therefore higher priced.  When HEPA filter media is pleated there is a phenomenon where the filter efficiency decreases. This can cause a 99.97% HEPA filter media to perform slightly less such as 99.7%. If the filter is not sealed properly the efficiency can be reduced further.

With a push to provide affordable air purifiers many are marketed as HEPA or True HEPA filters, when in fact many are not technically when pleated into an air filter. In this case they are sometimes referred to as HEPA-Type filters.

True HEPA filters are used in specific industrial applications such as a clean room where every particle needs to be captured. In the consumer market, the use of the term HEPA filter is generally used to imply a high efficiency filter as opposed to the literal definition used in industry. It's also a way to differentiate from the lower efficiency filters such as home air filters for an HVAC system.

More recently some Chinese air purifiers have started to market their air filters as H13 or H14. This is quite aggressive for a consumer air purifier filter. The filter media may by H13 however when pleated is most likely E12 or E11. Acheiving an H14 level for a consumer air purifier is not believable. It doesn't make sense since the pressure drop increases which lowers the CADR.


Air Purifiers: HEPA Filter Efficiency vs CADR

So, what is most important: the filter efficiency or the CADR?

For the consumer air purifier market, the CADR (Clean Air Delivery Rate) shows the overall system performance and is the number you should focus on. It takes into account the air filter efficiency and the volume of air through the air filter.

If an air filter is 100% efficient with an air flow of 300 cfm (cubic feet per minute) the CADR is 300 (100% x 300). If the filter efficiency decreases to 99.8%, the CADR decreases to 299.4.

From a performance standpoint where it becomes more problematic is if the filter efficiency at 0.3 microns drops much lower since its ability to capture the fine particles can drop quickly. For example, if the efficiency at 0.3 microns and smaller drops to 50%, the 300 cfm air purifier would effectively only have a CADR of 150.

The other extreme is for air purifiers marketed as H14. While the filter efficiency is nearly 100%, the air flow through the filter will likely be reduced. So, if the filter efficiency is 100% and the air flow is 250 cfm, the overall CADR is 250. A better solution would be 99.8% at 300 cfm.

What's most important here is transparency into an air purifier's performance. Fortunately there is the CADR standard to give consumers a way to fairly compare performance.


Are HEPA Air Purifiers Safe?

Yes, air purifiers that use only HEPA filters are safe since they do not emit pollution into your home. These filters collect airborne particles and hold them in the fibers.

However, if the air cleaner uses other technologies like electronic filtering it is possible it can produce ozone. This is a lung irritant and something you will want to avoid.

There are standards for ozone emissions and the California Air Resources Board makes sure that air purifiers that are sold into the state of California emit no more to 0.05 ppm of ozone.

HEPA Filter Limitations

HEPA air filters are designed to clean the air of airborne particles.  This includes things like pollen, mold spores, fine dust, pet dander, etc.

What is not included in this list are odors and gases.  So, if you need to remove cigarette smoke the HEPA filter can catch the tobacco smoke particles but not the smell.

As we show above, HEPA filter fibers work like a sieve to catch small particles. But, gases are much smaller.

To learn how to remove odors please see our guide on activated carbon.  This article explains in simple terms what is needed to make your room smell fresher.