Abstract: A filter window adapter unit adapted for use with a main split-sash window of school buses is provided. The adapter unit includes a mounting frame adapted to be installed over the main split-sash window and having a top frame member, opposed side frame members, and a bottom frame member all formed integrally together to define an adapter window opening. An outer track portion is formed in the opposed side frame members and extends between the top and bottom frame members. An inner track portion is formed in the opposed side frame members adjacent to the outer track portion and extends between the top and bottom frame members. An inner sash is disposed fixedly in a lower position in the inner track portion and carries a pane of glass. An outer sash is disposed movably in the outer track portion and carries an air filter. The outer sash is raised upwardly so as to cover a main window opening when the main split-sash is opened.