The HEPA filter is a central component of many air purifiers. This air-cleaning stage removes even the smallest particles from your indoor air.  It is effective against dust, pollen, and bacteria. The HEPA filters have a straightforward process, they are robust and user-friendly, with no moving parts.  This is why they are a must in any modern air purifier.

What makes HEPA filters special?  How does a HEPA filter work and what is it effective against?  What to look for when choosing and maintaining a HEPA filter?  

This guide offers a comprehensive overview of HEPA technology, its functions, and what needs to be considered when acquiring and using an air purifier with a HEPA filter.  

Technology and function – What is a HEPA filter good for?

High-efficiency particulate absorbing (HEPA) filters abide by the HEPA standard.  As the name suggests, they are filters that can remove the smallest particles from the air.  These filters consist of finely woven filter mates, in which fibers of different sizes are placed within each other.  These fibers collectively act as a barrier against particles in the air.

The HEPA Filter Process

The basic kind of filter is a sieve.  The sieve has holes that let the air through while trapping particles that are too big to pass through the holes.  Modern HEPA filters follow the same structure.  They have interlaced glass fibers, which are turned and twisted in many directions.  These create a fibrous maze.  The particles in the air are trapped by this maze and thus removed from the air circulation.  This is called the sieving effect.

Medium-sized particles that are up to 0.7 µm in diameter can technically pass through the filter mats.  However, they are too sluggish to pass through the tight curves of the air around the fibers. Instead, they are thrown against the filter fibers by the airflow.  This is called the interception effect.

Smaller particles up to 0.3 µm in diameter are mobile enough to follow the air through the filter. However, as they expand, the fibers are very likely to catch these particles.  This is the so-called blocking effect.

Even smaller particles with a diameter of less than 0.1 µm only move on random paths and do not follow any airflow.  As a result, there is a very high probability that they will impact the fibers and stick there on their way through the filter.  This is called the diffusion effect.

Thanks to the variety of the above effects, a HEPA filter can remove particles of many sizes from the air.  The effectiveness is lowest for solid particles with diameters between 0.1 and 0.3 µm.

Depending on the quality of the filter, 99.97% of all particles with a diameter of 0.3 µm or above are captured by a HEPA filter.

What can a HEPA filter do?

A HEPA filter can employ the above-mentioned methods in removing a variety of particles from the air.  It is the most effective filter to date in air purification.  It can help remove hair, mold spores, pollen, fine dust, house dust, some bacteria, and viruses from the air.  A HEPA filter is effective in cleaning indoor air in all kinds of environments.  Although it is a powerful filter used alone, combining it with complementary filters can lead to even more effective cleaning.

Differences between HEPA Filters and Other Air Purifier Filters

HEPA filters are ideal for removing small particles from the air.  Though isn’t that the claim of any other air filter?  What distinguishes a HEPA filter from its alternatives?

HEPA meaning: High-efficiency Particulate Absorbing

Different efficiency classes and standards exist in terms of cleaning the air mechanically.  In order to qualify a type of filter for a particular standard, the percentage of removed particles and their sizes are examined.  Studies show that particles with diameters between 0.1 and 0.3 µm are the most difficult to filter out.  Accordingly, their removal defines a certain benchmark for filter effectiveness.  The level of efficiency in a HEPA filter is labeled with a number followed by H. 

‘True HEPA’ generally ranges from H10 to H12.  They capture 85-99.5% of all particles that are 0.1 µm in diameter.

For a filter to be called ‘HEPA’ on the other hand, it needs to be classified as H13 or H14.  This is the highest tier of quality and is considered medical-grade quality.  These filters capture 99.5% to 99.995% of all particles that are 0.1 µm in diameter.

Other filter grades include E10, E11, and E12; these are less effective than HEPA filters.  U15, U16, and U17 on the other hand are the so-called ULPA filters (Ultra Low Penetration Air Filters).  These have an even stronger filter effect than HEPA.

These differences in efficiency levels can be significant.  It has been shown in many studies that HEPA filters can reduce the concentration of allergens such as dust or pollen in the air.  In most cases, however, the number of allergens cannot be reduced enough to achieve a noticeable improvement in allergy symptoms or respiratory difficulties.  This is why it is important to choose efficient and powerful HEPA filters.

Advantages and Disadvantages of HEPA Filters

Contrary to activated carbon filters, ionizers, ultraviolet lamps, and photocatalytic filters, HEPA filters only provide a mechanical air cleaning method.  Thus, HEPA filters do not offer biological or chemical purification of the air.  There is little downside to this.  HEPA filters consist of fibers that are so finely woven that they form a mechanical barrier against hair, mold spores, pollen, bacteria, and even some viruses.  Without any supportive filtering methods, HEPA filters can effectively remove pathogens from the air.


As the particles in the air get smaller, it is easier for them to pass through the HEPA filters.  When chemical solvents or adhesives evaporate, they extract volatile organic compounds, which are molecular pollutants.  These pollutants are typically too small to be captured by HEPA filters.  Such pollutants can be hazardous and also lead to an unpleasant smell.  Ultraviolet lamps, photocatalysts, ionizers, and ozone generators may be more effective in dealing with such pollutants.  


The lack of biological or chemical cleaning methods in HEPA filters can also be considered an advantage.  The reactive substances used by chemical methods may lead to health hazards and damage.  A good example is ozone generation through ionizers or ozone cleaners.  Ozone is very effective against odor molecules and volatile organic compounds.  However, if humans inhale ozone, it may lead to severe irritation of the mucous membranes.  In addition, if ozone is generated in a smoking household, the reaction of ozone with nicotine forms very harmful aerosols.  

Air Washers with HEPA Filter

An air washer is an air cleaner with a humidifier.  Air passes through wet rollers.  Certain dust particles get stuck in the liquid, getting separated from the air that comes back out.  The filtering effectiveness of air washers is significantly lower than that of HEPA filters.  It is difficult to remove fine dust particles from the air through this method.

It is however possible to combine the advantages of a humidifier with the effectiveness of a HEPA air purifier.  There are devices that do exactly that; combine humidifiers and HEPA technologies.  Please note that air washers with HEPA filters require regular cleaning and disinfection.

Are HEPA Filters Effective Against Mold?

Is your house, office, or shop a victim of mold? Be careful.  If you inhale mold spores, it may lead to chronic respiratory infections or allergies.  You should highly consider getting an air purifier to combat mold.  HEPA filters offer an effective measure to reduce the concentration of toxic mold spores in the air.  As the size of mold spores typically ranges between 2 µm to 10 µm, they are bound to get caught in the finely woven HEPA filter mats. 

Please note: ridding the air of mold spores will not solve the source of your mold problem.  The dark spots ornamenting your walls will not get smaller or disappear through the use of a HEPA air purifier.  It requires proper cleaning of the affected areas. Most importantly, you need to control the humidity of your enclosed area.

HEPA Filter Maintenance

Air purification through HEPA filters is a passive method as it is completely mechanical. The only need for moving parts in the HEPA air purifiers is due to air intake and extraction through fans.  Nevertheless, HEPA air purifiers need to be maintained on a regular basis in order to ensure their effectiveness.

The filter mats in the HEPA air purifiers absorb all the particles filtered out of the air.  Naturally, the longer the filter is used, the more pores are clogged and blocked by dirt particles.  Thus, the cleaning performance worsens.  It is therefore necessary to replace the HEPA filters in a timely manner to get the best performance.

Filter Change for HEPA Air Purifiers

How often do HEPA filters need to be replaced?  It depends on the operating time of the air purifier.  If a device is only switched on for a few hours a day, the filter lasts much longer than if the device is on continuously.  In any case, the filter replacement time differs from product to product.  This period is typically every four to nine months. 

Before you replace the filter, the device should be turned off and disconnected from the power source.  The used filter mats house a concentrated amount of hair, pollen, dust, bacteria, and viruses.  It is best not to come in direct contact with these mats during replacement.  You should wear a protective mask in order not to come in contact with these sensitive particles. 

Cleaning HEPA Filters

In order to prevent the filter mats from clogging or blocking up quickly, HEPA air purifiers typically have a pre-filter installed in the device.  These pre-filters filter out the air for larger dust particles and dirt before the trapped air passes through the HEPA filter.  These pre-filters should be regularly cleaned.  A good practice is to clean them on a monthly basis.  

These pre-filters are often removable and can be easily washed with water.  Pay attention that the pre-filter completely dries off before placing them back into the air purifier.  Otherwise, the moisture may lead to the formation of germs in the device.  Moreover, leftover water may destroy the electronic device completely.

The HEPA filters themselves should not be cleaned or wiped.  Doing so may destroy the tightly woven fiber structure and reduce its effect.  If you see concentrated dust on the HEPA filters, replace them promptly.