HEPA filters are made up of a set of interwoven layers of glass or PTFE fibres, very close together, forming a very fine mesh. The captured particles remain trapped in these fibres and are therefore removed from the indoor air.
The HEPA filter (as well as the ULPA) stops the particles in different ways, depending on their size and behaviour:
Sieving: This is a firts level filtration, which mechanically blocks the largest particles (>1 micron) on the surface of the filter.Interception: This mechanism occurs when a particle (>0.5 micron) follows the airflow of purifier and cannot change direction. It then gets caught in one of the HEPA filter fibres and remains trapped.Inertial impaction: Thos occurs for "heavy" particles which, because of their inertia, move in a straight line until they collide with the filter fibres.Brownian diffusion: This concerns the finest particles (<0.3 micron), which move rapidly, not in a linear fashion but rather in a zigzag pattern. When these submicron particles pass close to a fibre, they are attracted and then retained by electrostatic forces.
Brownian diffusion: This concerns the finest particles (<0.3µm), which move rapidly, not in a linear fashion but rather in a zigzag pattern. When these submicron particles pass close to a fibre, they are attracted and then retained by electrostatic forces.
Focus on the fibres of a HEPA filter*
How a HEPA filter works
*Source: Physical and chemical characterization of Kuwaiti atmospheric dust and synthetic dusts: effects on the pressure drop and fractional efficiency of HEPA filters