How Do HEPA Filters Work?

There are different types of HEPA filters, but True HEPAs filter by mechanical air filter method. Air is forced through a fine mesh, which can trap particles of 0.3 µm in size.

HEPA filters are made by randomly arranging the polypropylene or fiberglass fibers (between 0.5 and 2.0 µm). While sometimes the fibers are arranged in a tangle, the fibers can pass air through and trap some of the particles. The important part is that the fibers create narrow pathways for the air to pass through. Larger particles are trapped because they can’t keep up with the constant movement and maneuver through the fibers. But some smaller particles have very little inertia and can pass through with the air and along with the twists and turns of the fiber mesh.

Critical factors of the HEPA filter are the fiber diameter, filter thickness, and face velocity. The filters capture particles through diffusion, interception, and impaction.

Diffusion: Diffusion is the intermingling of substances by the natural movement of their particles. At lower airflow, particles below 0.3 µm are captured due to the collision of gas molecules with small particles (0.1 µm).

Interception: Interception occurs when a dust particle follows the air but still comes in contact with the fiber as it passes. This is dependent on the fiber and is enhanced when the size of the fiber is closest to the size of the particle. Most mid-sized particles are captured by interception. ‍

Impaction: Larger particles can’t avoid fibers while they curve through the air, so they must embed on a fiber directly.