How Do HEPA Filters Work? (Everything Explained)

HEPA filters seem to be all the rage these days.

These filters get a lot of press for being the only material that can strip microscopic particles out of the air you breathe.

But how do HEPA filters work exactly?

And are all HEPA filters created equal? Or are their varying degrees of efficiency with each HEPA filter that’s manufactured?

Those are the questions we’ll answer in this post.

So, keep reading.

Defining HEPA Filters

The acronym “HEPA” is short for High Efficiency Particulate Air.

And HEPA filter gets its name when it meets a certain level of efficiency set out by the U.S. Department of Energy and the European Union.

In the U.S., HEPA filters must be able to remove 99.97% of contaminant particles down to 0.3 microns in diameter. In the European Union, the standard only requires an 85% efficiency.

While it may seem random to set specifications at 0.3 microns, this size is actually the most difficult to filter out. We’ll explain why below.

For both the U.S. and European Union, a HEPA filter must also meet a maximum airflow standard and minimum pressure drop when filtering the air. This regulation prevents a manufacturer from just creating a super-dense filter that doesn’t allow any air to pass through.

HEPA filters are the best type of filter used in an air purifier. This material traps the widest range of contaminants from the air you breathe. That’s why you’ll find this type of filter in all of the air purifiers we recommend on our list of the top air purifiers.

HEPA filters are also excellent for stripping contaminants out of the air in bedrooms to help you sleep better. To see what’s available, check out our small air purifier for bedroom buying guide to get tips on choosing the top model.

How HEPA Filter Material Works

HEPA filters can technically be made of any type of material; however, the most common media is borosilicate glass fibers or plastic fibers (e.g., polypropylene).

HEPA filters contain thousands of randomly arranged fibers that form a dense mat. When air goes through this filter, the fibers trap contaminant particles that flow through them.

HEPA filters work to capture particles in three primary ways:

  • Internal Impaction — Particles that are 0.4 microns and large flow directly into the path of a HEPA fiber and get trapped in the collision.
  • Interception — Particles that pass between the opening between HEPA fibers eventually brush up against other fibers. Eventually, these particles drag along a fiber and get stuck which removes them from the airflow.
  • Diffusion — Particles that are 0.1 microns or smaller travel in an erratic fashion due to interaction with gas molecules. These erratic movements cause the particles to get stuck within the HEPA fibers.

Now that you understand the internal workings of how a HEPA filter traps pollutants, it will be much easier to comprehend why particles that are 0.3 microns in size are the most challenging to trap.

As you learned, diffusion traps particles that are 0.1 microns and smaller while internal impaction traps particles that are bigger than 0.4 microns. The 0.3 micron size is both too small and too large for either of those methods to always work, and so the efficiency drops.

Therefore, a HEPA filter that captures 0.3 microns is really describing a variant of the filter’s minimum efficiency.

HEPA Classifications

At the beginning of this guide, we mentioned that the U.S. and European Union have a different set of standards when it comes to the efficiency of a HEPA filter.

The U.S. sets a minimum standard of 99.97% efficiency at trapping particles down to 0.3 microns while the European Union is only 85%.

What this means in a real work application for air purifiers is that the device may actually meet or exceed those standards. The full efficiency of the HEPA filter depends on the classification that it’s under.

United States HEPA Standards

The U.S. uses an A through F ranking system for their HEPA filters and each one goes through a slightly different testing method. Since the differences are so small, any HEPA filter classification will do for household use.

The ranking system includes:

  • A > 99.97% at 0.3 microns
  • B > 99.97% at 0.3 microns
  • C > 99.97% at 0.3 microns
  • D > 99.999% at 0.3 microns
  • E > 99.97% of 0.3 microns
  • F > 99.999% at 0.1 to 0.2 microns

European Union HEPA Standards

The European Union uses number classification to rate their HEPA filters. As you can see below, there’s a big difference between the lowest (H10) and highest (H14) rating.

  • H10 > 85% at 0.3 microns
  • H11 > 95% at 0.3 microns
  • H12 >99.5% at 0.3 microns
  • H13 > 99.950% at 0.3 microns
  • H14 > 99.995% at 0.3 microns

When you’re purchasing an air purifier that’s regulated by European Union standards, you’ll want to find out what number rating the HEPA filter uses. That way you can know exactly the level of air filtration you’re getting for your money.

HEPA Filter Disadvantages

While HEPA filtration may seem like the perfect solution for trapping contaminants, these filters are not perfect.

The drawbacks to HEPA filters include:

  • You can’t wash HEPA filters and they degrade over time. They must be replaced every 6 to 12 months.
  • As a HEPA filter gets more contaminated, the efficiency drops.
  • If you don’t change a HEPA filter on a schedule, manufacturers can void your air purifier’s warranty.
  • HEPA filters can be delicate and easily damaged during installation. If you puncture a hole through the material, it essentially becomes inactive in that spot.
  • The best HEPA filters are not cheap.
  • Some HEPA filters are cheaper and washable, but you sacrifice quality and efficiency.
  • HEPA filters can remove bacteria and mold spores, but those microorganisms can also grow inside the filter and multiply.

Not All HEPA Filters are Created Equal

Now that you know the science behind how do HEPA filters work and what you can expect out of this material, you may think that you can just go buy any air purifier that uses a HEPA filter and be fine.

However, there are actually lower-grade HEPA filters being sold today disguise themselves are high-quality HEPA filters.

When you start looking for HEPA filters or air purifiers that use this media, you may see something called HEPA-like or HEPA-type. These two things are nothing like the HEPA filters which are certified by the U.S. and European Union.

Some manufacturers use the term “HEPA” to mislead their buyers into thinking that they’re buying a high-quality product. However, you’re actually getting an inefficient copy.

HEPA-type or HEPA-like filters only trap between 25% to 80% of particles down to 0.3 microns in size which is very inefficient for most people’s uses.

This is why HEPA-like and HEPA-typer filters are cheaper to buy.

If you find an amazing deal on an air purifier, you’ll want to take a closer look at the type of HEPA filter it uses or ask the manufacturer for the specifications. A rock bottom deal often means that the product is using a far inferior version of a HEPA filter.

To help avoid confusion, many manufacturers use the words “True HEPA” filter to indicate to consumers that the HEPA filter is certified by the U.S. or European Union. So, always look for the word “True” when shopping for an air purifier that uses a HEPA filter.

To see what air purifiers are available at a low price, take a look at our guide on the best inexpensive air purifier models that use HEPA filters. Those air purifiers can save you a ton of money on a new unit.

HEPA Filters are Not That Complicated

Hopefully, you came away from this post with a full understanding of how do HEPA filters work.

As you learned above, the inner workings of HEPA filters are not too hard to comprehend. These filters are basically made up of tiny fibers that capture contaminate particles as they pass through so that they don’t remain in the air you breathe.

If you’re interested in getting an air purifier for your home that can clean the air to its fullest, take a look at our list of the top air purifiers. It includes the best air purifiers you can buy today that use the highest quality HEPA filters.

Now, if you’re looking specifically for an air purifier to remove dust and other allergens from your bedroom so you can sleep better at night, you’ll want to check out our small air purifier for bedroom buying guide. It includes a few tips on how to select the best device for this purpose as well as stresses the importance of how a HEPA filter works to create the freshest air possible.

So, take a look at those resources to find out more about HEPA filters and their uses inside air purifiers.