The main purpose of the study was to evaluate tbe effect of different particle size distributions and flow rates on the pressure drop performance of HEPA filters with/without prefilters at a constant dust loading. Tests were conducted at constant velocity and dust loading for the new HEPA filters up to a loading which resulted in a pressure drop of 3" of H2O. The effect of the prefilter was then characterized by testing a new HEPA filter with prefilter installed. The cheaper prefilter was then replaced at a total pressure drop of 3" of H2O, after which additional testing was conducted on the HEPA with a new prefilter. Based on the results of the performance with and without prefilters, it was then possible to evaluate the potential benefits of using prefilters upstream of the HEPA filters, the reduced costs associated with the use of the prefilters, and the potential reduction In waste disposal volume.