ASTM F2101 (MIL 36954C), used to determine the bacterial aerosol filtration efficiency of various filtration materials. This test employs a ratio of the bacterial challenge counts to the number of bacteria that are trapped in the filter (do not make it through). The test procedure allows a reproducible bacterial challenge with an average droplet diameter of 3 microns (micrometers) to be delivered to test materials. This test method provides a standard procedure for comparison of filtration materials. Higher is better.