High-efficiency particulate air or HEPA is a type of air filter. Filters meeting the HEPA standard have many applications, including use in medical facilities, automobiles, aircraft and homes. The filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy (DOE). To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of 0.3 µm particles. This size constitutes the Most Penetrating Particle Size (MPPS), which is the most difficult size of particle to filter. Smaller and larger particles are filtered at even greater efficiency.