HEPA stands for High Efficiency Particulate Air (filter). HEPA is a technology developed in the 1940's by the U.S. Atomic Energy Commission to full-fill a top-secret need for an efficient way to filter radioactive particulate contaminants. HEPA is a type of highly efficient filtration media that removes microscopic particles from air passing through the filter. There are different ranges of efficiencies depending on the particle size. The most efficient form of HEPA removes 99.97% of particles with a size of 0.3 microns that pass through the filter. A HEPA air purifier will not capture chemical gasses.