HEPA stands for "high efficiency particulate air" and is a type of filter standardized by the US Department of Energy for filtering particulate matter of a specific size out of the air by using a maze of interwoven glass fibers to trap those particles. HEPA air purifier models use fans to suck air into the device in order to trap microscopic particles in the air, which can include dust, bacteria, viruses, pollen, smoke and mold, and recirculates the clean air back into the room via a vent. HEPA standards are different in the US and Europe, with the US standard requiring HEPA filters to filter 99.97% of particles 0.3 microns in size out of the air. Coway includes an excellent breakdown of this type of filtration on its website.