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.