Absolute Rating vs. Nominal Rating for Filters"Filters are rated on their ability to remove particles of a specific size from a fluid, but the problem is that a variety of very different methods are applied to specify performance in this way. Pore size ratings refer to the size of a specific particle or organism retained by the filter media to a specific degree of efficiency. A filter that is marked '10 micron ' has some capability to capture particle as small as 10 micrometers. However you do not know exactly what this means unless you also have a description of the test methods and standards used to determine the filter rating. The two most used reported media ratings are nominal and absolute micron rating."