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by Haldane King

Updated by Molekule staff 10/20/22

A HEPA (High-Efficiency Particulate Air) filter can be an important part of common air purifier systems promising to improve the air quality in your home or office. But there are a lot of misconceptions about the HEPA standard, whether or not an air purifier or filter truly meets the HEPA standard, and what a HEPA filter can and can’t do. Let’s take a look at what the HEPA standard means and how a HEPA filter can deal with various types of pollutants in your home.


What is a HEPA filter?

The HEPA standard is defined by the U.S. Department of Energy. It might seem strange that the Department of Energy created an air filtration standard, but HEPA filters were first developed in the 1940s for use in facilities contaminated with radioactive dust. Shielding can be used to contain radiation, but if particles of dust and moisture become irradiated, they can spread radioactive contamination through air ducts and corridors. HEPA filters were developed to keep potentially radioactive particles contained. Since then, it’s been used in numerous industries and applications. By the 1960s, HEPA filters were moving into the consumer market as filters for HVAC units, vacuum cleaners, and stand-alone air purifiers. Now, almost all HEPA filters are partnered with additional filtration technology like activated carbon.

What HEPA air filters are good for

To understand how HEPA filters work, think of it as a dense forest of tangled fibers pressed into a sheet. The sheet is usually folded into pleats to increase the surface area and filter life. Air flows through the fibers and most particles get trapped when they are blown into the fibers, like a car trying to drive through the dense trees or a ball trying to roll through underbrush. Only extremely small particles or molecules of gas can make it all the way through because they are so light they bounce off the fibers and are pushed all the way through by the airflow.

HEPA filters work well on their own and are effective at removing particulate matter like airborne pet dander, pollen, smoke, and dust.

Where HEPA air filters need help

While HEPA filters remove most particles from the air, there are many harmful contaminants in the air that aren’t particles.

VOCs: Volatile Organic Compounds (VOCs) are airborne chemicals that mostly derive from off-gassing of building materials or products in our homes as well as cleaning products aside from other sources like beauty products. The most concerning health effect associated with VOCs is that some are well-known carcinogens. HEPA filters are unable to remove VOCs because they tend to bounce off the fiber media.

Ozone: Ozone (O3) is a gas that forms naturally in the upper atmosphere, but from pollution near the ground, where we may be exposed. Ozone is not only unhealthy to breathe, but it can react with VOCs to produce extremely small particles that can easily penetrate deep into the lungs and pass into the bloodstream. Ozone is a very small and simple molecule that doesn’t stick to HEPA fibers.

Allergens: Pollen, mold spores, pet dander, and many other particles are sources of allergy and asthma triggers called allergens. While the majority of allergenic particles are large enough to be trapped by a HEPA filter, very small fragments can penetrate the filter and remain in the air.

It is important to say that frequent replacement of filters is critical since allergens and other toxic material accumulates on the filter. The trapped pollutants both reduce the filter’s effectiveness and have the potential to penetrate the filter as they dry and crack apart.

Should I buy a HEPA air purifier for my home?

A HEPA filter needs to be understood only to be a part of the solution in improving your indoor air quality. As you can tell above, HEPA filters are more effective when partnered with additional filtration technologies, particularly when removing tiny particles such as allergen fragments and chemicals like VOCs.

For those that are concerned about larger particles such as dust, pollen, and animal dander – an air filter with a HEPA component will help to reduce these in your air. If you are concerned about other sources of indoor air pollution such as VOCs, viruses, and bacteria, you might need more of a solution than HEPA alone.

Compensating for the shortcomings of HEPA

Reducing the pollutants that a HEPA filter is vulnerable to can help to improve air quality by addressing the source of the air pollutants:

  • Limit pets to certain areas of the house.
  • Vacuum and dust frequently to cut down on volume of particles in the room, and to remove pollutants trapped in rugs, drapes, and furniture that an air purifier wouldn’t be able to reach.
  • Open as many windows as possible during and after any cleaning.
  • For those sensitive to allergens, special mattress and pillow covers can greatly cut down on the pollutants you’re exposed to.
  • If you don’t live near a major source of pollution, you can always improve the air quality in your home by frequently opening the windows and letting in fresh air. Houses tend to trap and concentrate pollutants, so the outside air is almost always cleaner.

Learn more about source control, ventilation, and indoor air pollution here.

True HEPA vs. HEPA-like vs. HEPA-style

If you do decide that an air purifier with a HEPA filter is right for you, remember that just the term “HEPA” on the box isn’t enough to be sure the filter is effective. Be sure it meets the U.S. HEPA standard, which is capture of 99.97% of particles sized 0.3 microns.

Terms such as “HEPA-like” or “HEPA-style” are meaningless because terms don’t indicate that the filters actually meet the HEPA filter standard. Some manufacturers whose filters do meet the standard have begun using the term “True HEPA.”

The HEPA filter specification (which stands for High Efficiency Particulate Air and also known as High Efficiency Particle Arrestance) is based on single-pass particle capture efficiency of the filter. To meet the standard, a filter must stop 99.97% of all particles that are 0.3 micrometers in diameter in one try. That means that if you sent ten thousand 0.3 micrometer particles through a HEPA filter, only three particles would get through. Particles of this size are used because they are thought to have just the right balance of size and weight to be the most difficult to capture. In theory, all other particle sizes should be easier for a filter to capture.

There’s another aspect of an effective HEPA filter that is not tested for: the construction of the air purifier or HVAC case the filter is installed into must be airtight. Air and the pollutants it carries will always follow the path of least resistance, so if they can get around the filter, then the filter is not going to work very well. It requires more precise manufacturing to create an effective, air-tight filter, and cheap air purifiers aren’t always up to the task.

Effectiveness of HEPA and non-HEPA filters

How effective can HEPA actually be? The chart below, adapted from an EPA report lists the MERV (minimum efficiency reporting value) rating of filtration material corresponding to the typical contaminant that they address, along with the typical filter type found in the MERV rating group (from HEPA to “HEPA-like” filters).

Even though the MERV value is strictly performance based, you can still derive some value from the chart below, as it gives you an indicator of the limits of certain types (e.g. higher efficiency pleated filters vs. true HEPA filters).

Rating Particle Removal Efficiency Particle Size and Typical Contaminant Typical Usage and Type
MERV 1 – 4 Under 20% > 10 µm (micron)
Pollen, sanding dust, textile and carpet fibers
Throwaway Filters
Fiberglass or synthetic media panel, usually 1 inch thick
Washable Filters
Aluminum mesh, foam rubber panel
Electrostatic Filters
Passive self-charging woven polycarbonate panel
MERV 5 – 8 35% to 70% 3 – 10 µm (micron)
All of the above at a greater efficiency plus household dust, lint, and mold spores
Throwaway Filters
Synthetic media panel filters
MERV 9 – 12 90% to 95% 1 – 3 µm (micron)

All of the above at a greater efficiency plus lead dust, coal dust, and legionella or other bacterial spores

Pleated filters (HEPA-like, HEPA-type)
Extended surface with cotton or polyester media, usually 1 to 6 inches thick
MERV 13 – 16 75% to 95% 0.3 – 1 µm (micron)
All of the above at a greater efficiency plus cooking oil droplets, most smoke, exhaled respiratory particles, toner, dander, and auto fumes
High-efficiency filters
Densely packed fibers capturing particles in air down to 0.3 µm in size. MERV of 13 brings 75% efficiency, versus a MERV of 16 with 95%. Molekule PECO Filters are rated MERV 16.
HEPA 99.97% 0.3 µm (micron)
All of the above at a greater efficiency plus all smoke particles, fine dust, and most allergenic fragments
High-efficiency filters
Densely packed fibers in a pleated sheet capturing the most penetrating particles with 99.97% efficiency.

Molekule Tri-Power filters meet HEPA standards.

Reputable air purifier companies that have filtration-based units should clearly state their MERV rating, or state plainly what the particle size and type of contaminant their product can actually address.

Closing it all out

A HEPA filter component as part of an air purification system is a good way to remove particulate matter like pet dander, pollen, smoke and dust. VOCs, ozone, and fine particles can not be completely removed from the air with a HEPA filter. Activated carbon is almost always included as part of HEPA filters sold today, which can help to remove VOCs and ozone, but has its own limitations.

If you are mostly concerned about the larger particles and are looking to buy a HEPA filter, make sure you pay attention to the details when buying a purifier – just because it has the term HEPA doesn’t mean it meets the standard.

Most importantly, make sure to frequently replace the filters to keep them effective and to remove the concentrated pollutants. With higher moisture levels and elevated temperatures, VOCs and other substances can be released from an air filter. Neglecting to maintain the filter may end up introducing high concentration spikes of pollutants into your home – and that is certainly the opposite of what we all want.

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