HEPA

High Efficiency Particulate Air filter — a filter class defined by a minimum particle-removal efficiency, commonly at least 99.95% or 99.97% depending on the classification standard used.

HEPA is a performance class, not a single fixed test method — EN 1822 (H13/H14) and other national standards define HEPA slightly differently. A filter's actual classification depends on which standard and test method was used to rate it.