Abstract: A double filter insert for an air cleaning apparatus having an ionizer, consisting of a sheet-like pre-filter of polyester for retaining coarse dust particles and of an also sheet-like electrostatic filter of nonwoven fabric for removing also finest-grain floating particles from the air circulating through the air cleaning apparatus. According to the invention, the double filter insert (1) is characterized by at least one inspection opening (1.3) which, for making visible the degree of contamination of the electrostatic filter (1.2) is provided in the fabric of the overlying pre-filter (1.1), the electrostatic filter (1.2) in the new condition being of a distinctly brighter color than the pre-filter (1.1).