Can Filters Capture Particles Smaller Than Their Pores? Understanding Interception

As a particle follows the airflow around filter fibers or another porous structure, it will physically contact a fiber whenever the distance between the particle's center and the fiber surface is less than or equal to the particle's radius. Once contact occurs, van der Waals forces, surface tension, and other forces on the fiber surface hold the particle in place and prevent it from continuing with the airflow.

This mechanism has several key characteristics:

It does not depend on particle inertia or electrical charge. Capture is determined mainly by particle size, fiber diameter, and the path the airflow takes around the fibers.

It is particularly effective for particles about 0.1–1 μm in diameter, including some PM2.5 and fine dust. At lower airflow speeds, particle paths around the fibers are more stable, making the particles more likely to pass close enough to a fiber surface to be intercepted.

A particle can be smaller than the gap between fibers. For example, a 1 μm particle may be captured even when the fiber gap is 5 μm, provided that the distance from the particle's center to the fiber surface is no greater than the particle's radius. This makes interception fundamentally different from sieving, which depends on a particle being larger than an opening.

Put simply, interception is like a pedestrian walking so close to a sticky wall that they brush against it and become stuck. The particle is not blocked because it is too large for a pore; it is captured because its path brings it into contact with the filter material.