HEPA (High Efficiency Particulate Air) is a filtering efficiency specification for filters developed by the Atomic Energy Commission during World War II to effectively remove radioactive dust from plant exhausts without redistribution. A HEPA filter must be capable of capturing 99.97% of all particles as small as 0.3 (micrometers, formerly microns) in size from the air which flows through it. The phrase “as small as” means that if all particles were that small, it would still have that efficiency. This should not be confused with the phrase “down to” which may mean a mixture of particle sizes for the stated efficiency. Certified HEPA filters, like those used by Miele and Lindhous, are tested and marked with a certification number which gives the user the assurance that the HEPA specification efficiency is actually being achieved by the filter. The HEPA acronym is used primarily in the USA whereas Europeans often refer to the same filter as an S-Class filter. This indicates that the filter meets stringent standards like the British Standard 5415 and German norm: DIN 2418B Ti-E EU 13 for filtration performance.