of the rails 80 and 82 when the filter cassette is brought in for servicing permits easy access to and removal of the additional filtering elements the choice of additional filtering elements for a particular installation depends on the primary requirements of the air filtering system in which the cassette is installed for example the filtering requirements for a machine shop and a beauty parlor are quite different a beauty parlor installation would probably require a relatively large amount of odor removing material and a relatively small amount of particle removing material whereas a machine shop installation would require a relatively large amount of particle removing material and little or no odor removing material the filter cassette disclosed herein permits an unlimited range of filter material combinations in the embodiment shown in fig4 and 5 a combination of filter elements is shown which comes closest to being a 34 standard package 34 i e it is suitable for a wide range of installations starting at the front or upstream side there is a metal grille or screen 92 which acts as a retainer for the inserted upstream filter element or elements the pre filter element 94 is a particle intercept filter for large particles the pre filter 94 and screen 92 rest on the bottom wall or floor 18 of the framework 12 and are supported and held in place by the flange 90 which extends along the bottom and both sides of the cassette after the rails 80 and 82 have been.