A Bremmer HEPA filter is a specific type of high quality air filter that meets the HEPA filter standard, which is to remove at least 99.97% of particles from the air down to at least 0.3 microns in size.
HEPA stands for High Efficiency Particulate Air. A HEPA air filter is made up of thousands of extremely fine fibres arranged into a matt to intercept both microscopic particles and larger ones. An air purifier will push air from the room through the filter to remove pollutant particles. By definition HEPA filters can trap close to 100% of the particles in the air that are 0.3 microns or smaller. (Bearing in mind that there are 10,000 microns in a centimetre, a particle 0.3 microns in diameter is really, really tiny.)