It is an acronym for "high efficiency particulate air [filter]" (as officially defined by the U.S. Dept. of Energy). This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (m).Purify air , Absorption of bacteria, fungi, dustOften replace the filter to help make indoor air more freshFilter captures 99.97% of allergenshelps reduce household odorsUsed to filtration for the devices