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HEPA Filters - The Basics

Introduction to HEPA Filters

HEPA filters
What does HEPA stand for?

HEPA stands for High Efficiency Particulate Arresting. This means that HEPA filters have a high efficiency at arresting, or capturing particles. HEPA is a type of air filter. Many people often mistake this for believing it to be a brand or manufacturer, and this is not true. It is important to note that HEPA is a type of air filter.

How did HEPA filtration originate?

During WWII scientists working on the Manhattan project sought to create a filter that would remove radioactive particles from the air. It is from this work that the HEPA air purifiers that we know today originated. Kept secret until after the end of the war, the filtration technology has a long history of application and uses. From labs and pharmacies to hospitals and industrial settings, HEPA filters make possible many of the things that we now take for granted.

How effective is a HEPA air filter?

True HEPA air filters are 99.97% effective in capturing particles as small as 0.3 microns. This is as efficient and effective as an air filter gets. If you suffer from allergies or simply want to breathe clean air, then note that a HEPA air filter is what you are looking for.

What are some examples of particles that HEPA filters will capture?

  • Dust Mite Debris: 0.5 to 50 microns
  • Household Dust: .05 to 100 microns
  • Human Hair: 70 to 100 microns
  • Bacteria: .35 to 10 microns
  • Spores from plants: 6 to 100 microns
  • Mold: 20 to 200 microns
  • Smoke: .01 to 1 microns

What is a micron?

A micron is a measurement that is one-millionth of a meter or 1/25,000th of an inch. Invisible to the naked eye, our world is full of organism and particles that are but tiny fractions of an inch.

What is so special about the size, 0.3 microns?

HEPA is measured against particles of all sizes but the standard is the 0.3 micron particle. Testing has shown that particles smaller and larger than this are more easily trapped by a HEPA or HEPA-style filter. The 0.3 micron size particle is unique. It hits something of a "sweet spot" when it comes to the ability to pass through filter media. It is considering the MPPS or Most Penetrating Particle Size, and is the MOST difficult particle size to effectively filter with any type of filter media. So when crafting a filtration standard, such as HEPA, the standard is based upon how well filters can remove the most difficult particle there is to capture or trap - 0.3 microns.
Additionally, particles around this size are small enough to get past the tiny hairs that line our breathing passages and too large to be easily exhaled. Larger particles can be trapped by these tiny hairs (cilia), while smaller particles can more easily be inhaled and exhaled right back out.

How to HEPA filters trap particles?

HEPA filter media is often a non-woven fiber. Sometimes ceramic or synthetic in nature while often made from glass fiber, sheets of this non-woven fiber trap particles in four ways.

  • Impaction - As air flows through the filter media, larger particles (larger than about 0.5 microns) are heavy enough that while the air changes direction and flows around the fiber, the inertia of these heavier particles propels them forward, causing them to impact and stick to the filter media.
  • Diffusion - For the smallest particles (smaller than about 0.1 microns), they are so light they do not actually flow with air, instead diffuse through the air. Because they do not simply follow the air path around the filter fibers, they instead are most likely to impact the fiber and become trapped.
  • Interception - For particles not heavy enough to have inertia but not light enough to diffuse through the air stream, mid range particles follow the air stream as it goes around the filter fibers and are trapped when they touch the fiber as they pass by.
  • Sieving - This is what occurs when fibers are so closely spaced that particles simply cannot fit through the air space between fibers and become lodged. It works best with particles about 1 micron and larger.

The combined effect of all four methods of filtration demonstrate exceptionally high filtration of microparticles, with the weakest point being 0.3 microns. With a HEPA filter, even at this weakest point of filtration, the filter traps 99.97%.

What uses a HEPA filter?

HEPA filters are often found in air purifiers and vacuum cleaners. These air filters help to protect you and the environment when picking up hazardous material. HEPA filters trap and prohibit particles as small as 0.3 microns from being exhausted or released from the air purifier or vacuum back out into the air.

What about air filters that are not HEPA? Are those air filters just as good?

No. A standard air filter, unlike a HEPA air filter, will not capture particles and prohibit them from re-entering the air. However, a HEPA filter will capture particles as small as 0.3 microns. This is part of the reason why recent research on products like vacuum cleaners has shown that vacuuming with a traditional style filter or a vacuum that has only a dust bag, can dramatically worsen the air quality in a home. Instead of filtering these fine microparticles, allergens and particulate are simply redistributed back into the air you breath. Also note that standard filters are not certified. A certified HEPA filter must pass stringent government standards in order to be qualified as a certified HEPA filter.

Are all air purifiers and vacuums that claim to be HEPA really true HEPA?

HEPA filters must contain a certified label with a written efficiency test result measurement. The manufacturer should be able to provide this data.

To learn more about air filtration, visit our Air Purifier Buying Guide.