What Is a HEPA Air Filter? True HEPA, 0.3 Microns, and What to Buy

You might be wondering, “What is considered HEPA, and what is good enough?”

I was recently asked that question regarding HEPA air filters for the home.

A HEPA air filter is designed to capture very tiny airborne particles, which is one reason they are often recommended for people who suffer from allergies. Personally, I like HEPA filtration because it means fewer particulates floating around and potentially making their way into your lungs while you breathe.

You have probably noticed this in your own home at one time or another.

You’re sitting there, and the sunlight is shining through the window at just the right angle. Suddenly, you can see dust and other tiny particles floating through the air. A moment later, when the sun angle changes, you can’t see them anymore. But you know they are still there.

A HEPA air filter helps remove those tiny airborne particles as air passes through the filter. 

What Does HEPA Stand For?

HEPA stands for High Efficiency Particulate Air.

A HEPA filter is a pleated mechanical air filter that meets a specific efficiency standard. In the U.S., a true HEPA filter is generally understood to remove at least 99.97% of particles that are 0.3 microns in size from the air that passes through the filter.

That 0.3-micron size is important because it represents the most difficult particle size range for the filter to capture.

Source: U.S. Environmental Protection Agency / U.S. Department of Energy HEPA definition

How Small Is 0.3 Microns?

0.3 Microns – That’s Really Small

A HEPA filter is commonly rated around 0.3 microns, but how small is that?

One micron, also called a micrometer, is one millionth of a meter. Written another way:

1.0 µm = 0.000001 meter

So 0.3 µm is less than one-third of a single micron. That is extremely small.

For comparison, here are some approximate particle sizes:

Particle or Material	Approximate Size
Wood smoke particles	0.2 – 3 µm
Anthrax spores	1 – 5 µm
Typical bacteria	1 – 10 µm
Asbestos fibers	0.7 – 90 µm
Spider web silk strand	3 – 8 µm
Mold spores	3 – 12 µm
Human sneeze droplets	10 – 100 µm
Human hair	5 – 200 µm
Paper thickness	70 – 180 µm
Dust mites	100 – 300 µm
Pollen	10 – 1,000 µm

So when we are talking about 0.3 microns, we are talking about particles far smaller than what the human eye can see. That is one reason HEPA filtration is so useful. It is designed to capture incredibly tiny airborne particles as air passes through the filter.

 

How Does a HEPA Air Filter Work?

Keeping it simple:

A HEPA air filter is made from a dense mat of randomly arranged fibers, often fiberglass. These fibers are extremely small, commonly measured in micrometers, and they create a complex maze for air to pass through.

One common misunderstanding is that a HEPA filter works only like a screen or sieve, where particles larger than the openings get trapped. That is not quite how it works. In fact, the spaces between the fibers may be larger than some of the particles being captured.

Instead, particles are removed as they move through the filter and collide with, brush against, or are drawn into the fibers. Once they contact a fiber, they tend to stick there. Larger particles may be trapped by direct impact, while very tiny particles move erratically and are more likely to bump into a fiber along the way.

That is why a HEPA filter can capture extremely small particles even though the gaps between its fibers are not simply “smaller than” the particles being filtered.

 

Are All HEPA Air Filters the Same?

No, not all HEPA air filters are the same. Some companies use marketing terms that can be confusing or even misleading.

Products described as “HEPA-type,” “HEPA-like,” “HEPA-style,” or “99% HEPA” may not meet the actual HEPA filtration standard. In some cases, they may not have been tested to the same requirements, or tested by an independent laboratory at all.

That does not necessarily mean every one of these filters is useless. Some may perform reasonably well, while others may fall significantly short of true HEPA performance. The problem is that the wording alone does not guarantee that the filter meets the recognized HEPA standard.

This is why many manufacturers now use the term “True HEPA” to reassure consumers that their filter is designed to meet the HEPA standard: capturing at least 99.97% of particles at 0.3 microns from the air that passes through the filter.

For practical purposes, when shopping for an air purifier or replacement filter, look for clear wording such as “True HEPA” or a stated filtration efficiency of 99.97% at 0.3 microns. Be more cautious with vague marketing phrases that sound similar to HEPA but do not clearly say the filter meets the HEPA standard.

 

Which HEPA Air Filter/Purifier Should I Buy?

The best starting point is to choose an air purifier or filter that is rated True HEPA, or one that has been tested and shown to meet the HEPA standard.

Many HEPA filters can meet the same basic filtration requirement, so “best” often comes down to the way you plan to use it. Once the filter itself meets the standard, the more practical differences are room size coverage, fan speed options, noise level, filter replacement cost, design, portability, and price.

There are small tabletop air purifiers, larger room-size units, whole-house filtration systems, furnace filters, and even HEPA filters for vacuum cleaners. The right choice depends on whether you are trying to clean the air in a bedroom, living room, workshop, entire house, or a specific appliance.

For most people, a good room-size True HEPA air purifier is the most practical choice. Match the purifier to the size of the room, check the replacement filter cost, and choose a model quiet enough that you will actually run it regularly. A HEPA filter only helps when air is moving through it.

As an Amazon Associate I earn from qualifying purchases (no extra cost to you).

→ Search for all “True Hepa” Air Purifier choices

There are lots of great choices for a True Hepa air filter. There are various sizes depending on room size and volume, and there is a range of corresponding prices. I will list several air purifiers that I own or have owned:

GermGuardian Air Purifier

I have the following ‘True HEPA’ room air filter and it has been great for us:

→ Check Price & Availability(Amazon)

LEVOIT Air Purifer

Another brand of which I am a fan is LEVOIT. I own several of them too. I personally recommend the LEVOIT Core 400S-P Air Purifier.

→ Check Price & Availability(Amazon)

Medify Air Purifier

I purchased the following 2-pack of the Medify MA-14 Air Purifier. They seemed the perfect size for a small room, and the lowest fan speed is super quiet.

→ Check Price & Availability(Amazon)

HEPA Air Purifier for Preparedness

Besides the everyday benefit of improving indoor air quality, a good HEPA air purifier can also be useful from a preparedness standpoint.

Why? I can think of at least two reasons:

1. Airborne radioactive particles
2. Illness or pandemic situations

Airborne Radioactive Particles

A HEPA filter may help capture radioactive particles that are attached to dust, smoke, or other airborne debris. This is important because radioactive contamination from a nuclear accident or fallout event is often carried on very fine particles.

To be clear, a HEPA filter does not block all forms of radiation. It will not stop gamma radiation, x-rays, or radioactive gases. But if radioactive material is attached to airborne dust or fallout particles, a HEPA filter can help remove some of that material from the air passing through the purifier.

This is one reason high-efficiency filtration is used in certain controlled environments where airborne particulate contamination is a concern.

Illness or Pandemic Situations

A HEPA air purifier may also be helpful when someone in the home is sick.

Viruses themselves can be extremely small, but they often travel in respiratory droplets and aerosols produced by coughing, sneezing, talking, or breathing. A HEPA filter can help capture many of those airborne particles as room air circulates through the purifier.

It is not a guarantee that others in the home will not get sick, and it should not replace common-sense precautions. But running a properly sized HEPA air purifier in the room where a sick person is resting may help reduce the amount of airborne particulate matter circulating in that space.

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