HEPA filters are composed of a randomly arranged fiber mat. The key issues affecting their function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 µm.

The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fiber) by one of the three mechanisms.

HEPA filters are composed of a randomly arranged fiber mat. The key issues affecting their function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 µm.

The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fiber) by one of the three mechanisms.