HEPA is short for High Efficiency Particulate Air. ULPA is Ultra Low Particulate Air. Each are designed to trap small airborne particles and contaminants. HEPA have minimum efficiency of 99.97% at 0.3 microns. ULPA are 99.999% at 0.12 microns or higher. However, if you take air flow and other variables into account, HEPA are cheaper, have a lower resistance to air flow, and usually have a longer service life than ULPA filters.