One of the top retailers of HEPA filters is Airtech. We provide extremely high-quality items that satisfy the DOE's (Department of Energy) requirements. A mat of fibers laid at random makes up HEPA filters. Usually made of fiberglass, the fibers have sizes ranging from 0.5 to 2.0 micrometers. It is untrue to assume that a HEPA filter functions like a sieve through which particles smaller than the biggest aperture can pass. In contrast to membrane filters, which are made to block particles as large as the aperture or distance between fibers, HEPA filters are made to target far smaller contaminants and particles. BSL4.