Abstract: A down-flow chamber in which, from a filter in the ceiling of a working room, a germfree and dustfree flow of air is directed downwardly to a working face that is surrounded by walls with exhaust openings for the flow of air at the edges and in which, at the front, a transparent wall is mounted. Between the lower edge of the transparent wall and an exhaust grid in front of the working face there is a vertical opening of such a height that arms can be put through the opening to be able to carry out operations in the working room, wherein at the lower edge of the transparent wall a flexible lid preferably of smooth plastics material is mounted that is hanging down and is substantially extending to the exhaust grid. Further, a device is provided on the chamber that prevents the lid from moving outwardly from the vertical plane through the opening.