HEPA stands for High-Efficiency Particulate Air filter and refers to a material rated to trap 99.97% of airborne particles 0.3 microns and larger.

HEPA air filters are made from very tiny glass fibers that are made into a tightly woven paper. This creates a filter consisting of a multitude of very small sieves that can capture extremely small particles, including some biological agents. Once trapped, contaminates and particles are not able to flow back into circulation, due to the highly absorbent pores of the HEPA air filter.