These are progressive density media, designed to trap dirt all through their thickness for longer filter life and no face loading. Glass fibers are loosely woven on the air-entering side, progressing to closely woven fibers on the air-exit side. A resinous bonding agent in the media increases rigidity and resistance to compression so the filter will not collapse in the airstream. Nominal 1" and 2" adhesive treated spun glass. For industrial and commercial air filtration with fairly heavy dirt loads. Tinted blue on the air-entering side. Used in conjunction with holding frames and as protective prefilters for high efficiency panels.