Abstract: An imperforate duct has fluid-inlet and fluid-outlet openings at opposite ends thereof, between which is located a rotor that performs both a ventilating function and a filtering function by way of imperforate, rough surfaces which are adapted to retain solid and liquid particles contained in air or other gas that is being filtered, the rotor being followed by a gas-treating filter through which the fluid passes before emerging from the hood. An aerodynamic block against backflow is located between the inlet opening and the rotor, but not in contact with the rotor.