HEPA (High Efficient Particulate Air) filters, by definition, remove at least 99.97% of airborne particles 0.3
micrometers (µm) in diameter.
HEPA filters are composed of materials made of randomly arranged fibers.
The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micron.
Key metrics affecting function are fiber diameter, filter thickness, and face velocity.
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 false. Unlike membrane filters, where particles as wide as the largest opening
or distance between fibers cannot pass in between them at all, HEPA filters are designed to target much
smaller pollutants and particles.
These particles are trapped (they stick to a fiber) through a combination 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 effect increases with diminishing fiber
separation and higher air flow velocity.
Diffusion: An enhancing mechanism is a result of the 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.
Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate
above 0.4 μm. In between, near the 0.3 μm MPPS, diffusion and interception predominate.
The filter resistance to flow is known as its “pressure drop”.