compressed air, air whose volume has been decreased by the application of pressure. Air is compressed by various devices, including the simple hand pump and the reciprocating, rotary, centrifugal, and axial-flow compressors. Compressed air exerts an expansive force that can be controlled and used in various devices including tires, air brakes, caissons, and diving suits. As a source of power it is used to operate pneumatic tools, e.g., pneumatic hammers and drills and spraying equipment. It is widely employed for cleaning dust and dirt out of mechanical equipment. It is used also in mining, tunneling, and the manufacture of explosives, since it is not a fire hazard. Compressed air is in readily available supply and is easily stored and transported.