HEPA Air Purifier

HEPA Air Purifier Introduction

In this article we will explain how a HEPA air purifier works.  Then share the filter ratings system.  You will see the advantages and disadvantages along with some surprising findings from our testing.

At the bottom of this page is the listing of the top HEPA air purifiers.  Or see the best air purifiers from our testing.


HEPA Filter

In the beginning we didn’t know much about the HEPA air purifier. What I mean by this is that we didn’t appreciate the HEPA filter.

Let’s start with what a HEPA filter is.  HEPA stands for High Efficiency Particulate Air. This is a pleated filter that traps airborne particles as they pass through an air purifier. A true HEPA filter removes 99.97% of all particles that are 0.3 microns in size or larger.

A micron, also known as a micrometer is one millionth of a meter. As a reference point a human hair is approximately 60 microns in diameter. A grain of pollen is 5 microns.

What’s amazing is that 90% of all airborne particles are less than 0.1 microns in size.  They are tiny.

The challenge with assessing air purifiers is that most particles are too small to see. It’s hard to tell how well it’s working since you can see it.  That’s why we began to test different models with a particle counter.

The light weight of these tiny particles allow them to suspend in the air for long periods of time. This is what you breathe into your lungs.

The larger and heavier dust type particles will settle to the ground much more quickly. They are harmless to your system.

HEPA air purifiers are the recommended filtration method of leading government agencies. This includes American Lung Association, Department of Homeland Security and EPA. This is due to the high efficiency of the HEPA filter. Plus it’s ability to remove most of the smallest, most dangerous particles. They also have an inherent ability to improve performance over time.

Recommendation.  Therefore find a purifier with a high efficiency filter, good air flow and reasonable noise level.

How a HEPA Air Purifier Works

HEPA Air Purifier Filter

As shown in the picture above a HEPA filter has a series of pleated fibers.  In many cases the fibers are made of fiberglass.  This filter media is held together in a frame.

In testing we see a performance difference.  Larger HEPA filters do better.  Especially when we measure in a mid to large size room.

There are three ways that a HEPA filter captures particles.  These are interception, impact and diffusion.

Interception and impaction catch the large size particles.  Often, larger than 0.4 microns.  Diffusion catches the particles less than 0.1 microns.

A HEPA filter is measured by it’s ability to remove particles at 0.3 microns.  This is the most difficult size particle to capture.  It falls between the ability of interception, impact and diffusion.


Rating System

The filter rating system is known as MERV.  MERV stands for Minimum Efficiency Rating Value.

The scale ranges from a value of 1 to 18.

If you buy air filters for your furnace system you have come across this rating system.  It is printed on the packaging.

MERV 1-4 filters are simple dust filters.  They filter out hair and large dust.  This is the low price filter at the store.

HVAC air filter
Low MERV air filter


There are higher efficiency furnace filters.  But they often top out at a 8 or 11.  Sometimes you will see a 13.

A filter at this performance level is not rated to remove the smallest particles.  And this is important.

The problem is the fan in your furnace system cannot push enough air through a higher efficient filter.  A higher efficient filter has too much air flow resistance.

Without enough air flow your home will not properly heat or cool.

This is why room air purifiers are so popular.  A room air purifier has it’s own fan to power the air through a better filter.  As a result you can get the highest levels of filtration.

MERV 18 is the highest rating.  This is known as ULPA.  Ultra low particulate air.

A filter of this type removes the smallest particles.  And with the highest efficiency.  By efficiency we mean the percentage of particles.  We see few air purifiers with this level of filtration.  An ULPA filter is often found in commercial grade air purifiers.  This is clean room level air cleaning and systems cost well over $1,000.

A true HEPA filter is a 17.  This is the most common rating for home HEPA air purifiers.

And when we test we place a lot of importance on filter efficiency.  A higher efficiency means more of the smaller particles collect in the air purifier.  This is what you want in a good air purifier.  As a result, efficiency is it’s own category in our rating system.


How to Read the Ratings

The chart below is the ratings scale.

See the column for 0.3 to 1 microns.  Below a MERV 13 the filter does not remove any of the particles.  You need to be at a 14 to even get to 75% removal.  And 75% is not a good number if you have allergies or asthma.

A HEPA air purifier is the best solution to improve indoor air since it can achieve the performance for moderate to high allergies.  As well as other indoor air issues.


MERV Rating Scale
MERV Rating Scale




HEPA air filters are rated best due to their ability to remove a high percentage of particles. They do this with each pass of air through an air purifier.

If you suffer from allergies or asthma a HEPA air purifier is always the best choice. Its filtration removes the tiny particles that trigger an allergic reaction.

We tested it. And found the particle removal performance to be much better with a HEPA filter.  Yet when we test other technologies like the ionic and electronic the result is not close.  Rather than high performance for small and large particles we observe the electronic air cleaners do 30-40% less.

In addition, HEPA air filters often improve performance over time.  We saw that as the filters became dirty, they removed a higher percentage of particles.

In contrast, a problem with filterless technologies is the plates become dirty. When we tested them after a few days of use we saw a decrease in performance.  As a result of the dust building up, the ability to collect particles is less for a filterless system.

Finally, a HEPA filter does not generate ozone. The state of California Air Resources Board requires all air purifiers to be at a safe ozone level. Because of this standard most air purifiers are HEPA based.



The primary downside to a HEPA air filter is the noise level from moving enough air for a large room.  And you need to replace the filter.

Noise level can be subjective. Some people comment positively on the white noise effect. But some do not like the noise level on the higher fan speeds of a HEPA air purifier.

The noise level measurement is decibels (dB). They provide a good reference point when comparing HEPA air purifier models.

Since the HEPA filter media is dense it can generate various levels of white noise. Running a HEPA air purifier on a low fan speed will result in lower noise levels.  In some cases silent.

However you also get a less air flow with the lower fan speed.


bedroom room size

Room Size

Congratulations!  If you have made it this far you have a degree in HEPA filters and efficiency 101.

But there is more.

A high filter efficiency is not enough.  Because the air purifier needs to clean the air in your room.  Or your whole house.

So, to do this you need good air flow.  If the air flow is poor you will not get the clean air you expect.

We see this in testing.  This is why we have a rating category for air flow.  It’s the combination of filter efficiency and air flow that produce the results.

The Association of Home Appliance Manufacturers created a metric for this.  This is the Clean Air Delivery Rate and we explain the CADR here.

The rating system we use is similar.  By this I mean we factor in air flow and filter efficiency.  The difference is we give air purifiers more credit for achieving a higher efficiency.  This is necessary for anyone with allergies or asthma.

Our rating scale also differs in that we include ease of use and customer feedback.



We are still learning.  Especially in the testing.  But what I can share is that for cleaner air in a room, nothing comes close to a room air purifier.

Another option is the whole house air purifier.  We go into a little detail here.

Or you can consider a commercial air purifier.  These cost about $1,000 or more and deliver better than HEPA performance.

We have done initial testing in a bedroom.  Here we compared a room air purifier with a whole house air filter.  The particle count with the HEPA air purifier was much less than the whole house air filter.

There are two reasons for this.  One is the efficiency of the HEPA filter is higher.  And second, the room air purifier was sized properly and had more air flow in the room.

However, the downside to the room air purifier is that it only cleaned the one room.

Finally, I will share an interesting finding.  Within 15 minutes of turning on or off the air purifier, the air quality changes fast.  We see this with the particle counter.  The change in air quality is from a mixing of air from other rooms.


Other Considerations

A HEPA filter is amazing when it comes to particle removal.

However, there is one area where it fails to deliver.  This is in the removal of odors.

An odor is a gas.  And too small to catch in the fibers of the filter.

If you need odor removal in additional to particles you will also need a carbon filter.  We go into this in some detail in the article activated carbon air filter.

In a nutshell, activated carbon absorbs gases.  Not all gases but most of them.  It is a general gas and odor remover.

If you need smoke removal then a HEPA filter and carbon filter will work well.  This is also a good solution for mold or mildew where this is a light odor.

One gas where carbon is not so good is formaldehyde.  This is given off from building materials, carpeting, furniture and flooring.  To remove this you need a specialized gas filter media.  An example of this is a substance called potassium permanganate.  Anyways, if you have this issue please check to make sure the gas filter can address your concern.



If you are looking for a solid solution to improve your indoor air then a HEPA air purifier is the best choice. The better HEPA air cleaners cost more than $200.

In this article we show how a HEPA filter works.  The rating system for filters was explained as well as the pluses and minuses of a HEPA system.

We test many types of air purifiers. Most of all, HEPA filters consistently outperform the others.  The reason is they remove more of the smallest particles.  Most of all, they do not introduce pollution.


Most Popular HEPA Air Purifiers: