As a filtering material, impregnated glass fiber can be used to increase absorption, dust storage capacity and reduce the pressure drop of the clean filter. Synthetic and fiberglass flat cassette filters are made of the highest quality German materials. From the air intake side, the fiber is rare and has a loose weave. Towards the air outlet, the diameter of the fibers narrows, to collect as many particles of polluted air as possible. The filters are reinforced with a mesh on both sides to ensure longer filter life and stability.