HEPA Filter


You may have heard of this term a lot when you are looking into air purifiers or vacuum cleaners. So what exactly is an HEPA filter?

HEPA is an abbreviation of High-Efficiency Particulate Air also sometimes called High-Efficiency Particulate Arresting is a type of mechanical air filter. Filters meeting the HEPA standard have many applications, including use in medical facilities, automobiles, aircraft and indoors.


We tend to think of air pollution as something that happens only outdoors. However, even at home or in the office, there are mold spores, pollen, and pet dander, dust mites, and tobacco smoke in the air which can irritate your lungs and contribute to allergies and asthma.

HEPA-Filter Function


The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect and impractical. HEPA filters are composed of a mat of randomly arranged fibres. Whereas the typical simplest kind of filter is a sieve: something with holes that are big enough to trap some particles and small enough to let others through. Some purifiers do use filters like this to stop bigger particles of dust and dirt.

HEPA filter fibres are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Unlike typical filters at this pore size, where particles as wide as the largest opening or distance between fibres can not pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles. These particles are trapped (they stick to a fibre) through a combination of the following three mechanisms:


where particles following a line of flow in the air stream come within one radius of a fibre and adhere to it.


where larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher air flow velocity.


an enhancing mechanism that is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behaviour is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; this mechanism becomes dominant at lower air flow velocities.