HEPA filters actually obtain a lot more efficient the longer they are utilized (to a factor of course.) Just how is this so? It is because of the method these filters are made. They are made from arbitrarily arranged fibers developed into a mat. Normally these fibers are made from fiberglass. The fibers have a size of.5 to 2 microns. The area in between the fibers is much more than 0.3 microns. Yet, despite these big openings only 3 from 10,000 bits could make it through.