While the US HEPA standard usage of 0.3 micrometer particles to describe efficiency may seem arbitrary, particles of this size are actually the most difficult to filter. This renders them a ‘worst-case scenario’ reference particle.

Filter fibers trap contaminants in three main ways:

Interception takes place when a contaminant particle passes within the distance equal to one particle’s radius of a filter fiber, resulting in it touching the fiber and being removed from the airflow. Particles further than one particle radius from a fiber will not be trapped.

Inertial impaction occurs when a large particle, unable to adjust to the change in air direction near a filter fiber, becomes trapped on the fiber. The particle’s inertia ensures that it continues along its original path instead of circumventing the fiber, resulting in its capture.

Diffusion relies on the Brownian motion of gas particles. Small particles (typically 0.1 μm or less) tend to travel on a streamline in an erratic fashion, making random motions as they interact with gas molecules. This erratic motion causes the contaminant particles to become stuck to filter fibers.

Filtration Mechanisms

These three methods demonstrate why particles around 0.3 micrometers are the most difficult to filter. Particles less than 0.1 micrometers are easily trapped due to diffusion, while particles larger than 0.4 micrometers are trapped by inertial impaction. Particles between 0.1 and 0.4 μm are therefore too large for effective diffusion and too small for inertial impaction and efficient interception, so that the filter’s efficiency drops within this range. By specifying a HEPA filter’s efficiency at 0.3 μm, standards bodies are really describing a variant of the filter’s minimum efficiency.