HEPA Filters: How Do They Work?

With many air-moving appliances now coming with a variety of filtering options, especially domestic air purifiers. How do HEPA filters work? What are they and why do we need them?

Air pollution is a major problem, especially for city and big town dwellers and most of the time we think of that pollution as being outside. Poisonous fumes that are continually emitted from large numbers of automobiles, the activities of heavy industry, agriculture and even aviation impact the air quality in most areas of the country these days.

hepa filtersHowever, the air inside our homes can be even more polluted than the air outside. If that comes as something of a surprise, there are a number of reasons why this is, which I will cover in this article along with the measures you can take to reduce the impact on your health and the health of your family.

The Air In Our Homes

Inside a typical modern house, the air is, to an extent trapped within the walls and ceilings of the building. That allows airborne pollutants to increase in density much more so than it ever could outside, where constant air movement helps to disperse the particulates present.

Some of the pollutants present in the air inside the home are more obvious than others. Dust is probably the most obvious because it can be seen as it settles and also when it is highlighted in a beam of light such as a ray of sunshine coming in through a window.

While we strive to remove as much dust as we can, most of it merely gets moved around by dusting, sweeping and even vacuuming (unless the vacuum cleaner is fitted with a very fine filter) and not actually removed completely. Dust gets stirred up from the surface where it as come to rest and is once again dispersed into the air where it will eventually settle once more by the next day.

There are a number of other pollutants lurking in our home's internal atmosphere that are not so obvious. Some of these can actually be harmful to health or exacerbate existing respiratory conditions (such as asthma and other allergies) when breathed in constantly over a period of time.

Chemical Pollutants

Many cleaning products used around the home give off chemical fumes that linger in the air and lack of effective ventilation causes a build-up of the often toxic fumes. Products containing bleach (chlorine) can easily be detected through our sense of smell, because it is a very strong, distinctive and repulsive odor that can also irritate the delicate lining of the lungs and respiratory system in our bodies.

Many other cleaning products contain ammonia, which is also noxious, irritating to the lungs and nasal passages and can lead to respiratory problems.

Vacuum Cleaners and Filters

One of the problems with vacuum cleaners is while they do a pretty good job of removing a good proportion of household dust by trapping it in the bag or dust container (for bag-less vacuums), they still allow most of the finer particles to escape back into the room. Once released back into the air, those particulates simply disperse back into the atmosphere to settle once more later on.

Many modern vacuum cleaners are fitted with filters to trap many of the smaller particles that escape from the bag, but smaller particles still get through the filters. The logically-minded among you are probably thinking that it would be better to simply make the filters finer, or stack filters to trap all of the particles.

While this would work on paper, the problem in practice is that the vacuum's motor would have to work much harder to force the air through dense filters, using more energy that would cost more to run the cleaner. In addition, the filters would quickly become clogged in any case, causing the cleaner to stop working very quickly.

The HEPA Filter

To combat this problem, some high end vacuum cleaners are fitted with a combination of filters to remove a greater proportion of airborne particulates, including a HEPA filter.

HEPA stands for ″High Efficiency Particulate Air.″

how a hepa filter worksHEPA filters employ two particular mechanisms to clean the airstream that differ from regular filters. The first level uses one or more outer filters that trap the larger particles of dirt and dust.

The next level is comprised of a mat of dense glass fibers that do more than filter out smaller dust particles as would the sieve-like outer filer. It uses three different mechanisms that catch smaller dust particles as they pass through the filter in the moving airstream.

Together, the above three mechanisms enable a HEPA filter to effectively trap particles of a variety of different sizes.

The original design for HEPA filtration was created and developed by the nuclear industry for cleaning up dangerous, radioactive particles. In the domestic situation there is not quite the same level of need, but because of their ability to remove a high number of small particles from the air, they are particularly useful in air purifiers for removing a large number of airborne toxins and allergens.

Types of HEPA Filters

According to the ″US National Institute for Occupational Safety and Health″ (NIOSH), a true HEPA filter is one that is able to trap 99.97% of dust particles down to 0.3 microns in diameter. One micron is one millionth of a meter.

To get a perspective on the size of 0.3 microns, a typical human hair is approximately 50–150 microns in diameter. To that end, a HEPA filter traps dust particles several hundred times thinner.

The coarse particulates that are emitted from diesel car exhausts, for example are described as either PM10 (particles smaller than 10 microns), or PM2.5 (smaller than 2.5 microns). HEPA filters work at a scale from 10-30 times smaller.

A genuine HEPA filter is a lot more hygienic than an ordinary filter. That's because it will trap mold spores and even some airborne bacteria and viruses, literally cleaning at the microscopic scale. It also traps some airborne gaseous pollutants such as are discharged from cleaning chemicals and those found in some furnishings, paints, concrete and other building materials.

In respiratory equipment, there are nine different grades that are recognized by NIOSH. These are based on three different efficiency levels (95, 99 and 99.97%) along with three levels of filter degradation resistance (N, R and P):

You may see a filter labeled as, for example N95 (meaning 95% efficient, not resistant to oil) or P100 (99.97% efficient, oil proof).

HEPA filters are also classified with the letters ″A″ through ″E.″ These classifications are based on how well the filter captures particles and resist airflow. Type A are classified as the least effective, while type E are most effective and generally utilized as military grade filters that are capable of coping with chemical, radiological or biological particles.

A further classification bases whether filters are Type 1 (fire resistant) or Type 2 (semi-combustible).

HEPA Filters in Use

If you're an allergy sufferer or have certain respiratory conditions, the above classifications may not to be so relevant if you're merely buying an air purifier or vacuum cleaner. For most people, the key point is to be certain to buy a product with genuine HEPA filtration (99.97% of particles at 0.3 microns, described as "true HEPA" or "absolute HEPA").

You can do this by checking the product's labelling of particulate size as quoted by the manufacturer. Be sure to avoid products with vague descriptions like "HEPA-like" or "HEPA-type" that are not quantified in any way, as these are NOT true, genuine HEPA.

A true HEPA filter will always have its classification figures quoted on the label!

Some professional-grade vacuums along with most high-end air purifiers will contain additional mechanisms that deal with particles even smaller than 0.3 microns. These may be activated carbon granules, or ionizers that are designed to eliminate a number of gaseous pollutants that mechanical filters cannot trap.

It should be noted that a HEPA filter will require cleaning or replacing periodically to maintain the high level of air cleaning performed by the purifier. This is generally made easy with quick access to the filter for removal and replacement when the time comes.


Air purifiers that contain HEPA filtration are among the best options to consider when buying. They may have higher power consumption than non-HEPA purifiers, due to the need for the motor to work harder to force air through the much finer filter, but are almost essential for ensuring the comprehensive cleansing of indoor air to remove almost all airborne pollutants.

This is great news for allergy sufferers or those with respiratory problems that are brought about by breathing in air in the home that is not always as clean as may be believed. Breathing cleaner air helps to improve respiratory health and help to reduce the reliance on inhalers and medications while maintaining a better quality of life.

If in doubt about the effects of airborne pollution in the home and how it interacts with the medications you may be taking, always consult with your doctor about the use of air purifiers with HEPA filtration before altering any medication dosage.