HEPA filters remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter. Particles of this size are the most difficult to filter and are thus considered the most penetrating particle size (MPPS). Particles that are larger or smaller are filtered with even higher efficiency.

HEPA filters are composed of a mat of randomly arranged fibers. Key metrics affecting 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 cannot 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 following three mechanisms:

Interception, where particles following a line of flow in the air stream come within one radius of a fiber and adhere to it.

Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly. This increases with diminishing fiber separation and higher air flow velocity.

Diffusion, an enhancing mechanism that is a result of a collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter. This behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above. It becomes dominant at lower air flow velocities.

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