Apparatus for the elimination of noxious ammonia fumes associated with diazo copiers includes a plenum chamber disposed about the output aperture of the machine developer chamber. Vacuum means are provided to draw the ammonia laden air out of this chamber. Preferably this apparatus is used in conjunction with a novel filter connected between the chamber and the vacuum means. The filter comprises a filter body of a material such as corrugated paper having a plurality of longitudinal substantially parallel channels, the walls of which are impregnated with a material, such as phosphoric acid, which will react with ammonia to generate odorless end products. A granular layer impregnated with such a material may be disposed in contact with the output end of this filter body.