HEPA filters work through a combination of mechanisms: interception, impaction, and diffusion. Interception occurs when particles follow a streamline that comes within one radius of a filter fiber and adhere to it. Impaction happens when larger particles are unable to avoid fibers by following the airstream and thus collide with and become embedded in them. Diffusion is effective for the smallest particles, which move erratically and are more likely to be trapped by filter fibers.
