HEPA Filters: How They Work to Filter Air

Many appliances from vacuum cleaners to air purifiers have HEPA filters fitted to the air stream to clean and remove pollutants much more effectively than ever.

If you are interested in knowing how HEPA filters work to finely filter the air passing through them as well as why we need them so much, this article is just what you were looking for!

As you are probably already very well aware, air pollution is a major problem in most areas of the world. hepa filter air purifierThis is an even greater problem for those living in cities and big towns where air quality is low and doesn't seem to be getting any better.

Air Pollution

Most of the time we are aware of pollution in the outdoor air, thanks to noticing it ourselves as well as being constantly reminded of it by mainstream news stories, word of mouth and the less known about but very real and often angry broadcasts of ecological activists via online media.

There are a number of poisonous vapors that are emitted from the exhaust pipes of the crushingly large numbers of internal combustion engines that power cars, trucks and other forms of transportation. Add that to the less well reported activities of certain forms of heavy industry and the lunacy of prolific agricultural crop spraying and the severe impact on air quality in many areas is widely experienced by most of the population.

However, it is not just the air outside that is severely impacted by all these toxic emissions.

Inside our homes, the air we breathe is generally more polluted than the outside atmosphere. There are a number of reasons for this. I will get into that in this article as well as covering the measures you can adopt that can help to reduce the level of air pollution inside your home that impact the health of you and your family.

Polluted Indoor Air

Airborne pollutants are allowed to increase in density as they become trapped within the closed walls, floor and ceilings of a modern house. This is in contrast to how the many particulates are able to easily disperse outside due to constant air movement.

You may be perfectly aware of the dust that floats around in the indoor air as it reflects in the shafts of sunlight coming in at the windows.

By daily cleaning and dusting, we do our best to remove as much of what we can see. However, the next day, most of dust we thought we'd cleaned up has returned to settle on things. That's because when we disturb settled dust, it disperses in the air in almost invisible clouds so that while surfaces look clean, the dust and other particulates are just floating around the room waiting to slowly settle once more over time.

The modern day vacuum cleaner does a better job at removing some of that dust, but unless it is fitted with a HEPA filter (we are getting to that…), much of the dust sucked up by the hose escapes back into the atmosphere!

Aside from the more obvious dust particles, many of the pollutants present in the household atmosphere are not so easy to spot. In most cases, a home's inhabitants are completely unaware of their presence.

A number of these pollutants can actually be detrimental to a person's health. This is more especially in those suffering with certain respiratory conditions including asthma, bronchitis, emphysema, COPD and related conditions when inhaled regularly over long periods of time.

Chemicals and Noxious Vapors

Most modern cleaning products that are in general use in homes add to the airborne pollution as they give off lingering fumes comprised of unhealthy and in some cases dangerous toxins. These vapors linger in the air where ineffective ventilation allows their density to increase over time.

A number of cleaning products contain chlorine bleach and/or ammonia. These are noticeable by their smell, which is distinctive and rather unpleasant. These chemicals can also irritate the nasal passages and respiratory channels down to the lungs.

Vacuum Cleaners

While vacuum cleaners do a very good job of trapping a lot of of household dust and dirt in the bag or dust container of bag-less vacuums, most of the finer particles still manage to escape back into the room. As those particulates are released into the air, they disperse into the atmosphere and gradually settle out over time.

This applies to most modern vacuum cleaners that are fitted with a filter that's meant to trap many of the smaller particles and prevent them re-entering the atmosphere. However, the very small particles still manage to pass through the filters.

It might seem obvious for the manufacturers to simply make the filters finer, or stack filters in series to trap all of the finest particles.

The problem with this approach in practice is finer filers would greatly reduce the air flow through the vacuum cleaner's system causing the motor to work very hard. This would use much more energy, making the appliance energy-inefficient and likely burn out he motor much faster, reducing its lifespan and requiring replacement much faster.

Another problem is that very dense filters would rapidly become clogged, causing the appliance to stop working frequently. Fortunately, a better solution exists!

The High Efficiency Particulate Air (HEPA) Filter

A number of higher quality vacuum cleaners are fitted with a combination of less dense filters to deal with larger particles plus a HEPA filter to deal with the finer particles. Let's look at how a HEPA filter works.

There are two separate mechanisms used to clean the airstream and this is how they are different from normal filters.

  1. Level 1: One or more outer filters capture larger particles of dirt and dust.
  2. Level 2: The smaller dust particles that get through the outer filter are trapped by a dense glass fiber mat. This inner filter as three different mechanisms designed to catch the much smaller dust particles as they pass through it in the airstream:
    • Impact: While travelling at higher air speeds, small particles directly impact with the fibers and are captured and trapped.
    • Interception: Small particles are trapped on the fibers as they brush past.
    • Diffusion: When travelling at lower air speeds, particles tend to pass through the filter in a random pattern called "Brownian motion" and are trapped on the fibers as they do so.

HEPA filter effectively trap a variety of different size particles through a combination of these three mechanisms.

HEPA History

Here is a little history on where the original design for HEPA filtration came from:

It was initially created and developed by none other than the nuclear industry. There was a great need for its high level of filtering capability for cleaning up any dangerous, radioactive particles present in the atmosphere.

While domestic situations don't really call for such a high level of filtration, their ability to remove an extremely high number of very small particles from the air made them perfect for air filtering and purification. As such, their application shifted to air purifiers.

It is their ability to effectively cleanse the air passing through them to such a high degree that made it possible for air purification appliances to remove a large number of airborne toxins and allergens effectively.

A true HEPA filter is one that is able to trap 99.97% of dust particles down to 0.3 microns in diameter. This is according to the "US National Institute for Occupational Safety and Health" (NIOSH).

If you didn't already know, one micron equals one millionth of a meter. The diameter of a typical human hair measures between approximately 50–150 microns.

For some perspective, diesel car exhausts emit particulates are described as either PM10 (smaller than 10 microns), or PM2.5 (smaller than 2.5 microns). As you can see, HEPA filters are able to work at a scale some 10-30 times smaller than this!

Types of HEPA Filters

A genuine HEPA filter when fitted to an air purifier can be described as being much more hygienic than an ordinary filter. It is capable of trapping mold spores as well as a number of airborne bacteria and viruses.

This is serious air cleaning right down to the microscopic scale.

A HEPA filter also traps a number of airborne gaseous pollutants. These include those found in some furnishings, curtains and blinds, paints, concrete and other building materials as well as those discharged from a wide variety of common household cleaning chemicals.

NIOSH recognizes nine different grades in respiratory equipment. These are based on three efficiency levels: (ie 95, 99 and 99.97%) combined with three levels of filter degradation resistance (ie N, R and P), defined as:

A filter may be labeled, for example N95 (not resistant to oil and 95% efficient,) or alternatively P100 (oil proof and 99.97% efficient).

An additional classification is added using the letters "A" through "E" and reflect the effectiveness of particle capture and airflow resistance.

Type "A" are the least effective. Type "E" are most effective and this is the classification for military grade filters. These are able to cope with biological, chemical and radiological particles.

The final classification is based on the filters being fire resistant (Type 1) or semi-combustible (Type 2).

HEPA Filters in General Use

If you suffer with an allergy certain respiratory conditions, you can take peace of mind from the above classifications, even if some of them are not necessarily essential in a mere domestic air purifier or vacuum cleaner!

However, most people find the key point is being certain a product is fitted with genuine HEPA filtration that can filter 99.97% of particles at 0.3 microns. This should be described on the label as "true HEPA" or "absolute HEPA."

Checking the product's labelling to see the manufacturer's stated particulate size is highly recommended. What you should avoid are any products with ambiguous or vague descriptions about he type of filtering provided. Words such as "HEPA-like" or "HEPA-type" do not quantify their status in any way and you should treat such products as NOT containing true, genuine HEPA.

It is worth noting that a product containing a true HEPA filter will always quote its classification figures on the label!

Additional Filtering

Some professional-grade vacuum cleaners and certainly with most high-end air purifiers are fitted with additional mechanisms designed to deal with even smaller particles than 0.3 microns. In the case of air purifiers, this is to increase their effectiveness at cleansing and even sterilizing the air.

Some may contain activated carbon granules. Others will add ionizers for eliminating gaseous pollutants that are not trapped by regular filters. Others contain internal ultraviolet light emitters to kill certain pathogens as they pass through the air purifier.

Do HEPA Filters Need to be Cleaned?

In short, the answer is that they do require periodic cleaning and less frequent replacement in order to maintain the high level of air cleansing and purification performed by the appliance.

Generally, removing and replacing filters is made easy by the manufacturer with quick access to the filter when the time comes to clean or replace it. You should read the manufacturer's instruction manual as this point will be covered.


There is no doubt that air purifiers that contain HEPA filtration are much more effective at cleaning and detoxifying the air than those without. This is the best option to consider when buying an air purifier or vacuum cleaner.

It should be noted that these appliances may consume more power than non-HEPA purifiers. This is mainly as the motor needs to work a little harder to force air through what is a much finer filter. However the benefits far outweigh the slightly higher power consumption.

For allergy sufferers or people with respiratory problems, breathing in cleaner, unpolluted air in the home is highly beneficial. Breathing cleaner air certainly helps to improve respiratory health, while also helping to reduce reliance on medications and steroid inhalers.

In short, air purifiers with HEPA filters contribute greatly to maintaining a better quality of life.

If you are in any doubt about how airborne pollution in the home affects your health while interacting with any medications you may take, always consult with your doctor. Likewise, follow your doctor's advice about medication dosages needed when using a HEPA filter fitted air purifier in your home.

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