HEPA Filters 101: What They Are & How They Work

HEPA filter

HEPA (High-Efficiency Particulate Air) filters are renowned for their exceptional ability to purify air by capturing a high percentage of airborne particles. Here’s a summary of what HEPA filters are and how they work:

1. HEPA Filter Definition

  • HEPA filters are a type of mechanical air filter designed to remove particles from the air by trapping them within a fine mesh of fibers. These filters are highly efficient at capturing particles of various sizes, including dust, pollen, pet dander, mold spores, bacteria, and even some viruses.

2. Particle Capture Efficiency

  • HEPA filters are required to meet a stringent standard. They must capture at least 99.97% of particles that are 0.3 micrometers (microns) in size. However, they are also effective at capturing particles both larger and smaller than 0.3 microns, making them highly efficient at improving indoor air quality.

3. Filter Construction

  • HEPA filters are typically composed of a dense mat of randomly arranged fibers, often made from materials like fiberglass. These fibers create a labyrinthine pathway for air to pass through, trapping particles as they try to navigate the convoluted path.

4. Air Purification Mechanism

  • As air passes through a HEPA filter, particles in the air are subject to various mechanisms that lead to their capture. These include:
    • Impaction: Larger particles collide with and adhere to the fibers due to their inertia.
    • Interception: Mid-sized particles are captured when they come into close contact with filter fibers.
    • Diffusion: Smaller particles move erratically due to Brownian motion, increasing their chances of contacting and adhering to the filter fibers.

5. HEPA Filter Applications

  • HEPA filters are commonly used in various applications, including:
    • Air purifiers: Standalone units designed to improve indoor air quality in homes and offices.
    • HVAC systems: In hospitals, cleanrooms, and industrial settings, HEPA filters are employed to ensure ultra-clean air.
    • Biological safety cabinets: HEPA filters are used to create sterile environments in laboratories and healthcare settings.

6. Limitations

  • While HEPA filters are incredibly effective at capturing particles, they may not remove certain gases, chemicals, or odors from the air. For these pollutants, other filtration methods, such as activated carbon filters, may be necessary.

0.3 Microns and HEPA Filters

In HEPA filters and other good filters as well, 0.3 microns (in particle size) is a very important “benchmark” and is therefore not a random value. It is the testing standard for HEPA filters.

A particle size of 0.3 microns is referred by scientist as the Most Penetrating Particle Size (MPPS). Particles with a size of 0.3 microns are more likely to “evade” a filter more than large and even smaller particles.

As I mentioned, MEPA filter’s efficiency rating are based on the worst case efficiency rating. The 0.3 microns particle size specification actually responds to the worst case.

Apart from that, it is also worth noting that allergens (substances that cause allergic reactions) are 0.3 microns or larger.

How HEPA Filters Work

HEPA filter design

HEPA filters are made up of randomly arranged sheets of fiber, usually fiberglass or polypropylene. These fibers have very small pore diameters of between 0.5 and 2 micrometers (roughly 0.00002 and 0.00008 inch).

As I mentioned earlier, regular filters work like a kitchen sieve. Large particles are trapped while the small ones are allowed to pass through. HEPA filters work differently.

The sheets/bundles of fiber form a convoluted (folded/twisted) pathway through which air is forced through.  Large particles are trapped by the filter just as they are trapped by the regular filters.

Smaller particles are however forced to twist and turn as the air moves through the convoluted pathway. That motion causes them to collide with the fibers and that is how the embed on them, effectively being removed from the air.

The smallest of the particles, which also have little inertia collide with the air molecules which slows them down forcing them to crash into the fibers.

In a nutshell, HEPA filters remove particles from air in 3 ways. These are:

1. Impaction

Large particles are removed from the air by direct impact. This method is quite similar to how your kitchen sink strainer traps food scraps from the water being drained.

Unlike small particles, the large particles like hair, dust and pollen cannot keep up with the twisting movement of the air and hence trapped by the fibers easily. This is also how regular air filters work.

2. Interception

This is how mid-sized particles are removed from the air. Although particles in these category are smaller than the gaps in the filter fibers, they are not as light as the air molecules and will therefore not keep up with the air movement through the sheets of fiber.

And that is how they are “intercepted” by the filter fibers.

3. Diffusion

The smallest particles are also very light and will therefore move more erratically than big particles when they collide with air molecules (Brownian motion) increasing their likelihood of hitting filters fibers and that is how they are trapped.

Actually, the movement of particles in this category increases their chances of being trapped either by impaction or interception.

True HEPA vs HEPA-Type Filters

true HEPA filter

True or absolute HEPA filters are used by marketers and manufacturers to differentiate between American and European HEPA filters.

For a filter to be HEPA-certified in Europe, it only needs to remove at least 87% of particles sized at 0.3 microns while in the US HEPA filters need to remove at least 99.7 of the same-sized particles.  Due to that discrepancy, American HEPA filters are usually referred to as “True HEPA Filters’.

These filters (which fall short of the 99.7% requirement) are commonly known as HEPA-type/HEPA-like filters. HEPA-type filters look like and are even made like true HEPA filters just not as effective.

To be sure which type of HEPA filter you are buying, look for the test results (or even serial number) printed on the filter. The test results for true/absolute HEPA filters are 99.7% for particles sized 0.3 microns.


The filters used in most HVAC systems are MERV filters. MERV stands for Minimum Efficiency Reporting Value, a rating designed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

MERV ratings simply determines how effective a filter is in removing particles from the air (on a range from 1 to 20). The higher the rating the more effective a filter is.

All HEPA filters have a rating of MERV 17 or higher. HEPA filters have a better efficiency than a MERV 17 filter since they are rated at the worst performance.

Filters with a high MERV rating are however not the best filters for residential HVAC systems. That is because they have a higher pressure drop and also need to be replaced frequently.

In fact, the best MERV rating for home filters is between 8 and 13. Check out why that is the case in this post.

Below is a table showing MERV filters (1-20) and their effectiveness in removing particles sized 0.3 to 1 microns from air:

Type of PipeProsCons
Cast IronStrongInfiltrated by roots
Easy to install
Not durable
Attacked by roots
Tricky to cut
Attacked by tree roots
Low strength
ConcreteCost effective
Not DIY-friendly
PVC and ABSNot attacked by tree roots
Easy installation
Smooth interior (few clogs)
Lasts long
Cracks under pressure

HEPA Filters and Pre-Filters

As you can imagine, HEPA filters are quite pricey. In a system that uses many of these filters, it can get quite expensive running operations since they have to be replaced every 6 to 12 months.

To save on cost, HEPA filters can be used in conjunction with pre-filters. The pre-filters are usually cheap carbon-activated filters which trap the big particles before they get to the HEPA filters.

That ensures that only the smallest particles reach the HEPA filter and it can therefore last up to 3 years. The carbon-activated filters will however need to be replaced after every 3 months.

Where are HEPA Filters Used?

HEPA  air filter

The most common use of HEPA filters (in residential applications) is in vacuum cleaners. Vacuum cleaners that use HEPA filters are designed in such a way that all the air it sucks is expelled through the filter.

Note: There are vacuum cleaners labelled “HEPA” which actually have a HEPA filter but not all of the sucked-in air flows through it. Be on the lookout especially if you or someone in the household suffers from allergies or asthma.

HEPA filters are also used in air purifiers. As I mentioned, the MERV filters used in residential HVAC systems are not as effective as HEPA filters. Having a HEPA filter in an air purifier helps to remove the particles not removed by MERV filters.

HEPA filters are also widely used in sensitive commercial HVAC systems like hospitals, laboratories or pharmaceutical companies to avoid contamination.

Commercial airlines have found HEPA filters to be effective in avoiding the spread of airborne pathogens in the recirculated air. Motor vehicles have also started appreciating the use of HEPA filters in their air filtration systems.

Wrap Up

And basically that is everything about HEPA filters including how they work. I hope that this guide was helpful.