If you suffer from allergies or asthma, then a vacuum with sealed HEPA filtration is the way to go. HEPA filters can remove 99.97% of airborne particles .3 micrometers in diameters. Particles of that size are the most difficult to filter, and thus are considered to be the most penetrating particle size. Particles that are larger or smaller are filtered with even higher efficiency.HEPA filters are composed of a mat of randomly arranged fibers. Key metrics affection function are fiber density and diameter as well as the thickness of the filter. HEPA filters are designed to target and capture by interception, impaction, and diffusion. The United States Department of Energy regulates the rating system for HEPA filters.For a HEPA filter to be effective, the vacuum cleaner must be designed as a sealed HEPA or True HEPA vacuum cleaner. Be wary of vacuum cleaners that advertise HEPA-like filters, as they do not meet the HEPA requirements.