To combine the forcing and exhausting systems, it is not necessary to provide two ducts, one forcing and one exhausting, throughout the length of the heading. An arrangement that serves the same purpose is the overlap system. In this system a main exhausting duct is used within a convenient distance of the face, often about 100 ft (30.5 m). Some of the intake air in the heading, before reaching the end of this duct, enters a short length of tubing and is blown onto the face. The advantages of both systems are thus obtained. Precautions must be taken against recirculation of air by the forcing unit, to prevent concentration of dust, and in collieries, combustible gases, at the face. The two ducts must overlap by a minimum distance which, in practice, is usually taken as 30 ft (9.1 m).