The confusion about Bacteria retention capabilities stems from the fact that pore size is rated in liquid. The retention characteristics of membranes in air are totally different than those achieved when they are tested in liquids. Actually, the same membranes that are hydrophobic in water become hydrophilic in air.
Bacterial and viral retention is a complex topic. Manufacturers and users need to understand the properties of filter membranes, the implications of pore size, and the limits of each type of filter test. A thorough knowledge of filtration can help manufacturers get the best filter for their application. The best thing to do when designing filter media is to evaluate what is required of a product ahead of time, and then make a decision about what materials should be used. Membrane manufacturers can often provide the necessary expertise to find the best solution.