HEPA filters really obtain a lot more effective the longer they are used (to a point certainly.) How is this so? It is due to the method these filters are developed. They are made from arbitrarily arranged fibers developed into a floor covering. Generally these fibers are made from fiberglass. The fibers have a size of.5 to 2 microns. The area between the fibers is a lot greater than 0.3 microns. Yet, despite these huge openings just 3 out of 10,000 fragments can make it through.