You might know it as a designation for vacuum cleaner or air purifiers. In a consumer context, "HEPA" has become overused and has lost its technical meaning. The one in your vacuum cleaner is probably just a decent filter. On the other hand, the filters used in nuclear, pharmaceutical, and medical facilities, as well as in asbestos, lead, and mold abatement must meet certain requirements. Those high-grade HEPA filters must be at least 99.97 percent efficient at trapping particles 0.3 microns (about 300 times smaller than the diameter of a human hair) in size-you can see how a vacuum cleaner with those standards wouldn't be of much use cleaning your carpet. The HEPA requirements are set by the Institute of Environmental Sciences and Technology (IEST), a not-for-profit that creates standards and recommended practices.

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