Ultravation^® Progressive filters are constructed with increasing fiber density from the upstream to the downstream side, so they gather the largest airborne particles in the first part of the filter, and progressively smaller particles through the filter, then the tiniest particles at the end. This means the filters capture and hold more dirt than standard filters. Plus, Ultravation's 100% synthetic, continuous fibers are given a permanent electrostatic charge when they are manufactured, so they literally attract the smallest particles and hold them in the media.