Air Filtration Principles

Four different collection mechanisms govern particulate air filter performance: inertial impaction, interception, diffusion, and electrostatic attraction. The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size.

• Impaction occurs when a particle traveling in the air stream and passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber.
• Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through.
• Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber.
• Electrostatic attraction, the fourth mechanism, plays a very minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force.

Impaction and interception are the dominant collection mechanisms for particles larger than 0.2 m, and diffusion is dominant for particles smaller than 0.2 m. The combined effect of these three collection mechanisms results in the classic collection efficiency curve.

The information on this page is adapted from a National Institute for Occupational Safety and Health whitepapers entitled: "Filtration and Air-Cleaning Principles." and “”Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks.”