HEPA filters use a fine mesh of fibers to trap particles from the air passing through them. The air cannot flow in a straight path through the filter; instead, it navigates through a complex web that captures pollutants 1. The four main mechanisms through which HEPA filters capture particles are:

•  Direct Impaction: Larger contaminants collide directly with the fibers.
•  Sieving: Particles too big to fit through the gaps between fibers get trapped.
•  Interception: Smaller particles adhere to fibers when they come within one particle radius.
•  Diffusion: Smallest particles are bounced around by gas molecules, increasing the likelihood of being caught.