Abstract: A duct cleaning apparatus for cleaning an inside surface of a duct including an elongated flexible element extending inside the duct having a first end supported in the duct spaced from an inside surface, a second opposed free end and having a length sufficient to engage the inside surface of the duct, preferably greater than the diameter of the duct. Fluid flow through the duct, particularly turbulent flow, causes the elongated flexible element to oscillate or flap, striking the inside surface and removing foreign material from a predetermined portion of the inside surface of the duct. The apparatus of this invention may be used to create a fire break, for example, by removing flammable particles or dust from an area of the duct, preventing a flame from propagating along the duct through the flammable material.