English: Diagram showing the main parts of a HEPA filter on the large diagram; its operation is shown on the 3 smaller diagrams:

• the thick brown/tan lines represent fibers in the filter medium.
• the blue lines represent air flow.
• The dark circle represents a particle (dust, bacteria, spore, etc.)
• the dotted red line represents the trajectory of the particle; consider that as soon as a particle comes within one radius of a fibre it adheres to it.
• in the case of diffusion, the particle (usually 0.1 µm or smaller) has such a trajectory because of the collision with gas molecules.

On the 100 nm to 1-micron scales that are the most challenging for filters, it is helpful to think of air as a viscous fluid like molasses. As the fluid flows through the filters, particles are carried around the fibers of the filter medium, for the most part. Inertial impaction: Larger particles (>1 micron) may be massive enough to continue to move in nearly a straight line (Newton's Law), as the flow is diverted around a fiber. Interception: Smaller particles are not much affected by inertia, but, if they are carried within one particle radius of the fiber, they will be intercepted by it. Diffusion: Still smaller particles (<100 nm) move randomly among streamlines as they are buffeted by collisions with molecules. A particle along a streamline that approaches a fiber very closely may be knocked against the fiber by such a collision.