What does HEPA mean and where did it come from? HEPA, which stands for High Efficiency Particulate Air, is a designation used to describe filters that are able to trap 99.97 percent of particles that are 0.3 microns. Though the HEPA standard and certification process wasn鈥檛 established until 1983, development of HEPA filters dates back to World War II, when American scientists with the Manhattan Project created the first HEPA filter to capture radioactive particles released during the creation of the atomic bomb. Why 0.3 microns? That micron size (0.3) is referred to by scientists as the MPPS, or the most penetrating particle size. Scientists have found that particles of that size evade air filters more than larger or smaller particles. We鈥檒l get into why in a little bit. What are HEPA filters made from and how do they work? Most modern HEPA filters consist of interlaced glass fibers that are twisted and turned in myriad directions to create a fibrous maze. China Air Filters manufacturers website:http://www.bagerpurifier.com/air-filters/