There are three types of indoor air pollutants that can be reduced or controlled by portable air purifiers. These are particulate, gaseous and biological.

HEPA (High Efficiency Particulate Air) filtration air purifiers are designed to remove extremely small particles from the air. The term "particle" refers to the size of the pollutant and particle pollutants may include solids, liquids (in the form of mists), organic and inorganic compounds, living and dormant organisms.

Because they filter extremely small particles, a wide range of biological pollutants as well as non-biological particles can be removed from indoor air by HEPA filtration. This includes fairly large biological contaminants such as pollens and pet dander as well as highly potent and very small allergens such as found in dust mite feces.

HEPA filtration also has the ability to capture bacteria and viruses that are 0.3 micrometers in size or larger.

Therefore, a HEPA filtration air purifier is an important tool to minimize the effects of allergens and other indoor air pollutants, whether they aggravate existing conditions or to prevent future health problems.


The HEPA Filtration Specification

The HEPA filtration specification consists of two numbers that designate a very high level of filtration of very fine particles. The first number represents the filtration efficiency as a percentage and the second number is the particle size retained. For HEPA, filtration efficiency must be 99.97% and particles retained must be as small as 0.3 micrometers (formerly microns).

The phrase "as small as" is important because it means that if all the particles were 0.3 micrometers in size, the filter would still capture and retain 99.97% of them ("filtration efficiency"). The term "down to 0.3 micrometers in size" does not refer to the same filtration performance, as it may refer to a mixture of particle sizes for the stated efficiency.

The ability to filter very small particles is a very important specification because small particles can easily reach the deepest recesses of the lungs and, according to the U.S. EPA, may cause a wide range of extremely negative health effects, such as aggravated asthma, acute respiratory symptoms including aggravated coughing and painful or difficult breathing, eye, nose, and throat irritation, respiratory infections and chronic bronchitis, decreased lung function, shortness of breath and lung cancer.

And, for allergy sufferers, one of the most potent allergens, which is thought to be responsible for a significant percentage of allergies worldwide, is found in dust mite feces, which are generally 10 to 24 micrometers in size and even smaller when disturbed.

To give you some idea of how small these fine particles are, a human hair is approximately 100 micrometers thick. HEPA filtration is designed to capture 99.97% of particles one-third of one micrometer in size.

HEPA filtration is an important and valuable feature to have as part of your air purifier. So how can you be sure that you are getting true HEPA filtration? Let's start with certified HEPA filters.


HEPA Filter Certification Standards

The best HEPA filters are certified to meet stringent standards, such as the European Norm 1822 (also referred to as EN 1822) for filtration performance, and are identified with a certification number. Certified HEPA filters are also known as True or Absolute HEPA filters. European companies often refer to them as S-Class filters.

Originally developed for rating filtration systems for maintaining extremely clean environments in pharmaceutical, computer manufacturing, medical and other clean-critical facilities, certification using this standard assures you that the filter will perform as a true HEPA filter.

The EN 1822 is a two-part test. The first part of the test identifies the particle size that most easily penetrates the HEPA filter, to establish the "Most Penetrating Particle Size" or MPPS.

Once the MPPS is identified, the second part of the test challenges the filter ONLY with particles of the most penetrating size. If a filter can maintain 99.97% filtration under the very worst-case scenario, then you have a filter that delivers true HEPA filtration.

And because efficiency is strongly correlated to the speed at which the air passes through the filter, this test is done using the same rate of airflow as when the filter is in place and the air purifier is in use.

This is extremely important because all mechanical filtration air purifiers operate based on air flowing from the intake, through the filter system and then out the exhaust.

If a HEPA filter is tested at less than the actual airflow it will be subjected to when the unit is being used, these small particles can simply blow right through the filter into the room air during normal operation. This is one way in which a filter that may actually be HEPA will not deliver HEPA filtration in the real world.

The next important question is whether or not all the air going into the air purifier actually passes through the filter system. If air leaks out of the unit before it is filtered, some percentage of small, lung damaging particles and allergens are simply recirculated into the room air where the extremely light particles can stay airborne for hours. 

So, not only is a HEPA filter certified using standard EN 1822 crucial, it is also important that the air purifier be extremely well sealed, so that air does not leak before the filter.

In order to demonstrate that they are delivering the highest levels of filtration in a sealed system there are now some air purifier manufacturers that not only use HEPA filters certified using the EN 1822 standard but they are also certifying the entire air purifier.

These manufacturers are offering proof positive that no air leaks before the filter and that all air that goes into the unit is HEPA filtered before it leaves.

Therefore, to protect your health and in situations where someone in your home is already suffering with a condition that can benefit from a true HEPA filtration air purifier, certified air purifiers (not just certified filters) are the only way to obtain total assurance of HEPA filtration.

This doesn't mean that companies that do not certify the entire air purifier leak air before the filter but you have to use your judgment. Look for heavy rubber seals where the housings come together, as well as other evidence of high quality construction, such as heavy-duty plastic that maintains its structural integrity.

Some high quality companies offer a wealth of testing on not just the filter but the entire system and may offer significant filtration beyond just HEPA standards. Given that perhaps 90% of particles in room air are smaller than 0.3 micrometers, these increased efficiencies remove far more small, lung damaging particles from the air than typical HEPA units.

When selecting air purifiers where the whole machine is not certified, be sure that the HEPA filtration is tested and certified using the EN 1822 standard. This is a solid indication that you are dealing with a manufacturer that is serious about delivering true HEPA filtration.

If you choose an air purifier based on anything less than described above, you may or may not be getting HEPA (or anything close to HEPA) filtration. The minimum standard should be that the manufacturer makes a HEPA claim and states the filtration efficiency and particle size retained, so that you can be sure that it is actually the HEPA standard of 99.97% efficiency capturing particles as small as 0.3 micrometers.


Beware Misleading Tactics

Unfortunately, "creative" use of the term HEPA is rampant in the air purifier industry and we have seen so-called "True HEPA" claims on air purifiers with no efficiency and/or particle sizes listed. And the rest of the claims they make in their advertising do not come close to true HEPA performance.

Beware of the misleading tactics of many air purifier manufacturers. Some manufacturers play fast and loose with the term HEPA and make claims such as "traps 100% of dust mites, ragweed and common grass pollens", with no mention of filtration efficiency or particle size. Manufacturers that make statements like this are counting on uneducated consumers choosing their product based on information that is misleading at best.

The reality is that dust mites do not cause allergic reactions; it is an allergen contained in their fecal material, which is very small. Dust mites themselves are generally about 125 micrometers long and most pollens are between 5 and 50 micrometers in size. These are huge particle sizes and nowhere near as small as the 0.3 micrometers required for true HEPA filtration.

Certified HEPA performance represents some design challenges to air purifier manufacturers. Creating a sealed system and dealing with the increased levels of airflow resistance of true HEPA filtration require companies to meet very high standards of engineering, design and manufacturing. This is one of the main reasons that truly effective, high performance air purifiers cost more than mass-market products.

When evaluating air purifiers, regardless whether they claim HEPA or not, look for specific efficiency and particle size ratings as well as a highly sealed housing so you can make an educated decision. If a manufacturer doesn't show these numbers you're better off somewhere else.

And, beware of companies that will state filtration efficiency without a particle size or a particle size without filtration efficiency. One without the other is meaningless.


Beyond Filtration Efficiency

But filtration efficiency is not the only issue. It is relatively easy for an air purifier to achieve HEPA filtration. The question is can it filter enough air in order to make a difference in the room. HEPA filtration without sufficient airflow will not make any meaningful improvement in room air because there will not be enough of the room air being filtered.

An excellent specification that combines both airflow through the air purifier and the level of filtration efficiency (the percentage of particles captured) and the particle size is known as Clean Air Delivery Rate (CADR).

In summary, filtration of fine particles can really improve indoor environmental conditions for people with allergies, asthma and other respiratory ailments as well as protect the health of everyone in your home.

The very best performance will be obtained by use of EN 1822 certified HEPA air purifiers (the entire air purifier), followed by high quality, well sealed air purifiers using EN 1822 certified HEPA filters. High filtration but less than HEPA air purifiers made by reputable manufacturers may also be good choices but be sure to look for both filtration and particle size specifications in an extremely high quality product.