HEPA stands for "High Efficiency Particulate Air" and is one of the most widely used filter technologies in air purifiers. But how exactly does such a filter capture tiny particles, and why is it so effective?
The construction of a HEPA filter
A HEPA filter consists of a densely woven mat of randomly arranged, extremely thin fibers (often glass fiber or synthetic fibers), pleated in a zigzag pattern to fit as much filter surface area as possible into a compact space. Air is forced through this fiber network, trapping particles within the fiber structure.
Three ways particles are captured
Contrary to what you might expect, a HEPA filter does not simply act as a sieve that stops particles larger than its holes. Three physical mechanisms act simultaneously:
- Impaction: larger, heavier particles move in a straight line and collide directly with a fiber.
- Interception: medium-sized particles follow the airflow but strike a fiber when they pass too close to it.
- Diffusion: the smallest particles (such as many virus particles) move unpredictably (Brownian motion) and therefore collide with fibers more frequently, making them caught with extra effectiveness.
This combination ensures that a HEPA filter effectively captures both large and extremely small particles, with the most difficult particle size being around 0.3 micrometers (where none of the three mechanisms works optimally), the so-called "most penetrating particle size".
A common misunderstanding
Many people think that a HEPA filter works like a fine sieve, where everything larger than the opening gets stuck and everything smaller slips through without problems. That is not how it works: the smallest particles are actually trapped extra effectively thanks to diffusion, while medium-sized particles (around 0.3 micrometers) are relatively the hardest to capture. That is precisely the size at which the HEPA classification is tested.
Why is this relevant for air purifiers?
Because a HEPA filter is tested and certified at exactly this most difficult particle size, the classification (e.g., HEPA13) provides a reliable indication of effectiveness, regardless of whether it concerns particulate matter, pollen, bacteria, or virus particles. This makes HEPA one of the few filter claims that is actually standardized and verifiable.
What a HEPA filter does not do
A HEPA filter captures particles, but does not remove odors or gases; an activated carbon filter is required for that. Also, HEPA filters do not kill microorganisms; they physically trap them within the filter material.
Maintenance of a HEPA filter
A HEPA filter is not washable without damaging the fiber structure, unlike some pre-filters. As more particles are trapped, air resistance increases, which can slowly reduce airflow and therefore effectiveness. It is therefore recommended to replace a HEPA filter periodically, usually after 6 to 12 months of use, depending on the air quality in your home.
HEPA in practice
The Dynter AirPure Plus PA244 uses a HEPA13 filter within a 4-stage filtration system: first a pre-filter for coarse dust and hair, then the HEPA13 filter for fine particles, and an activated carbon filter for odors and volatile substances. Together, this system filters, according to specification, 99.97 percent of particulate matter, pollen, bacteria, viruses, and microplastics from the air.
In our experience, a HEPA filter is one of the few components of an air purifier where you know immediately what to expect in terms of filtration performance based on the classification on the box (e.g., HEPA13). Do you want to see how this comes together in a complete device? Check out the AirPure Plus PA244.