Certified HEPA (tested to MIL STD 282) can effectively trap 99.97% of dust at 0.3 micrometers. Not all HEPA filters can effectively stop dust; while vacuum cleaners with HEPA filters, water, or cyclones may filter more effectively than without, they may still exhaust millions of particles per cubic foot of air circulated. Central vacuum cleaners can be effective in removing dust, especially if they are exhausted directly to the outdoors.