What does HEPA stand for and how it works?

HEPA filters are recommended for combating smog, dust, mites, viruses, fungi, and many other airborne particles and microorganisms. Their exceptionally wide range of action makes them increasingly popular, becoming almost an indispensable element of modern vacuum cleaners, car air conditioning systems, and, of course, air purifiers. But how do HEPA filters work, and what is behind their tremendous success?

Pre-HEPA History

Air-purifying HEPA filters, although until recently used almost exclusively in places with very high air cleanliness requirements, such as the food, electronics, and pharmaceutical industries, as well as sterile laboratories and operating rooms, are now something common that ordinary people encounter almost daily. Initially a military invention designed to withstand intense bombardment in shelters, these filters enable more effective home vacuuming and provide a high level of comfort every day for people suffering from asthma or severe allergies.

The history of modern HEPA filters began in 1940 in the USA, with the onset of the Manhattan Project, the American scientific research program aimed at constructing the atomic bomb. Due to the appearance of tiny, irradiated particles released into the air during experiments with radioactive materials, there was a need to develop highly specialized filters that could effectively capture all harmful particles and microorganisms. The devices built for this purpose, ensuring the highest level of safety and capturing over 99% of contaminants, were christened with the acronym HEPA, developed as High Efficiency Particulate Air.

What is a HEPA Filter?

We already know where HEPA filters came from, but what exactly is hidden under the term High Efficiency Particulate Air Filter? The simplest translation would be simply "high-efficiency particulate air filter," meaning it is a device capable of capturing particles with an efficiency of over 99.95% that are 0.3 micrometers in size, particles smaller than the thickness of a human hair, which averages 50 micrometers in diameter. This means that HEPA filters can capture not only various types of dust but also microscopic contaminants, including health-threatening mold spores and other fungi, respiratory disease-causing atmospheric aerosols such as smog, as well as plant pollen, animal allergens, and droplet-transmitted bacteria and viruses.

Interestingly, although the efficiency of HEPA filters is assessed based on their ability to remove 0.3-micrometer particles from the air, they are built to reliably capture even smaller, but equally harmful, particles and microorganisms. This is due to the fact that particles around 0.3 microns in diameter are the hardest to capture and pose the most challenges during air filtration, while even smaller bacteria and viruses are easily trapped in the filter's fiber network. This is all thanks to the appropriate construction of thin sheets used to trap contaminants, allowing for the capture of health-threatening substances in three different ways.

How Does HEPA Filter the Air?

The main part of a HEPA filter is a large-format sheet folded in a zigzag pattern made of randomly arranged fibers of glass or synthetic material, which, due to its random and uneven structure, creates a complex maze of air channels where particles trying to pass through get trapped and captured. It’s not just about directly "sifting" large particles, such as PM10, a component of smog with a size of "only" 10 micrometers, but also about creating a situation where even particles small enough to fit between individual fibers are trapped inside the filter. To achieve such a good effect, HEPA filters are adapted to capture harmful particles in several ways:

1. Filtration by Impact

This is the simplest capture process, used to trap the largest airborne particles, including toxic substances produced by burning solid and liquid fuels. This method uses the dense arrangement of filter fibers to trap harmful particles analogously to a net, preventing them from passing through holes that are too narrow and blocking them between individual fibers. HEPA thus effectively deals with all kinds of pollen, making it a great solution for people with seasonal allergies.

2. Filtration by Interception

For particles smaller than 1 micron, standard sieving methods stop working because these particles can easily fit through openings that allow air to flow freely through the filter. Fortunately, despite their small size, they still have some weight and, carried along with the airflow, cannot keep up with the frequent changes in direction within the maze created by the irregularly arranged fibers, eventually colliding with one of the filter's threads and attaching to its surface. At this stage, HEPA captures primarily bacteria and dust mite spores present in dust, significantly impacting the health of people in the room.

3. Filtration by Diffusion

The final filtration method applies to particles smaller than 0.3 micrometers, such as various viruses (including coronavirus). Known as diffusion, it uses the phenomenon of Brownian motion, which is the chaotic movement of particles small enough to bounce off air molecules while floating, constantly changing their flight path. For this reason, although they would be able to pass through the filter with a steady airflow, they fly in a complex zigzag pattern, hitting the chaotically arranged fibers and getting trapped inside the filter.

How to Recognize an Effective HEPA Filter?

In fact, many HEPA filters have 100% efficiency against various particles, struggling only with the most troublesome ones, measuring 0.3 µm, whose filtration takes place on the border between interception and diffusion, allowing some to escape. However, even in their case, every HEPA filter must have at least 99.95% efficiency – which is sufficient for use in hospitals, laboratories, the pharmaceutical industry, and food production plants. Of course, highly specialized facilities in the nuclear, pyrotechnic, or energy industries that need to meet exceptionally stringent requirements should be equipped with filters related to HEPA, known as ULPA filters, capable of capturing up to 99.999995% of particles. However, in normal conditions, even EPA filters, with an efficiency of 85% to 99.5%, significantly improve air hygiene when installed in vacuum cleaners.

Regardless of the degree of efficiency, it is worth remembering that, over time, filters lose their effectiveness. And although models designed for regular cleaning are available, most air purifiers, vacuum cleaners, and air conditioners still use replaceable HEPA filters, which last from 12 to 24 months before wearing out and needing replacement. Given the current prices of air purifier inserts, this is a cost of only about 50 zł, which is really a small expense considering the enormous benefits that HEPA filters bring daily to allergy sufferers, people with respiratory diseases, and those living and working in smog-prone urban areas.

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