What Is a HEPA Filter?

The EPA says HEPA is a type of pleated mechanical air filter common in portable air cleaners, an acronym for high efficiency particulate air filter as officially defined by the U.S. Department of Energy, and that it can theoretically remove at least 99.97 percent of dust, pollen, mold, bacteria and other airborne particles 0.3 microns in size. It says the 0.3 micron specification corresponds to the worst case, the most penetrating particle size, and that larger or smaller particles are trapped with even higher efficiency.

In vacuums, EPA rules at 40 CFR 745.83 define a HEPA vacuum as one designed with a HEPA filter as its last filtration stage, capable of capturing particulates of 0.3 microns with 99.97 percent efficiency, and built so that all air drawn in is expelled through the filter with none leaking past it. The EPA suggests considering a vacuum with a HEPA filter to reduce dust build-up, because it can help keep some vacuumed dust from escaping back into the air.

HEPA filters trap particles, not gases. The EPA says mechanical air cleaners and filters such as HEPA and other pleated filters effectively trap particles from smoke and do not produce ozone, but that no air cleaner or filter will eliminate all of the air pollutants in a home, and that most filters are designed for either particles or gases, so many air cleaners contain one filter for particles and another for gases.