Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes. Compressed air is used for power tools such as air hammers, drills, wrenches and others. Compressed air is used to atomize paint, to operate air cylinders for automation, and can also be used to propel vehicles. Brakes applied by compressed air made large railway trains safer and more efficient to operate. Compressed air brakes are also found on large highway vehicles.
Compressed air is used as a breathing gas by underwater divers. It may be carried by the diver in a high pressure diving cylinder, or supplied from the surface at lower pressure through an air line or diver's umbilical. Similar arrangements are used in breathing apparatus used by firefighters, mine rescue workers and industrial workers in hazardous atmospheres.
Industrial use of piped compressed air for power transmission was developed in the mid 19th century; unlike steam, compressed air could be piped for long distances without losing pressure due to condensation. An early major application of compressed air was in the drilling of the Mont Cenis Tunnel in Switzerland in 1861, where a 600 kPa (87 psi) compressed air plant provided power to pneumatic drills, increasing productivity greatly over previous manual drilling methods. Compressed air drills were applied at mines in the United States in the 1870s. George Westinghouse invented air brakes for trains starting in 1869; these brakes considerably improved the safety of rail operations. In the 19th century, Paris had a system of pipes installed for municipal distribution of compressed air to power machines and to operate generators for lighting. Early air compressors were steam-driven, but in certain locations a trompe could directly obtain compressed air from the force of falling water.
Air for breathing may be stored at high pressure and gradually released when needed, as in scuba diving. Air for breathing must be free of oil and other contaminants; carbon monoxide, for example, in trace amounts that might not be dangerous at normal atmospheric pressure may have deadly effects when breathing pressurized air. Air compressors and supply systems intended for breathing air are not generally also used for pneumatic tools or other purposes.
Workers constructing the foundations of bridges or other structures may be working in a pressurized enclosure called a caisson, where water is prevented from entering the open bottom of the enclosure by filling it with air under pressure. It was known as early as the 17th century that workers in diving bells experienced shortness of breath and risked asphyxia, relieved by the release of fresh air into the bell. Such workers also experienced pain and other symptoms when returning to the surface, as the pressure was relieved. Denis Papin suggested in 1691 that the working time in a diving bell could be extended if fresh air from the surface was continually forced under pressure into the bell. By the 19th century, caissons were regularly used in civil construction, but workers experienced serious, sometimes fatal, symptoms on returning to the surface, a syndrome called caisson disease or decompression sickness. Many workers were killed by the disease on projects such as the Brooklyn Bridge and the Eads Bridge and it was not until the 1890s that it was understood that workers had to decompress slowly, to prevent the formation of dangerous bubbles in tissues.
Air under moderately high pressure, such as is used when diving below about 20 metres (70 ft), has an increasing narcotic effect on the nervous system. Nitrogen narcosis is a hazard when diving. For diving much beyond 30 metres (100 ft), it is less safe to use air alone and special breathing mixes containing helium are often used.
In industry, compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water. However, compressed air is more expensive than the other three utilities when evaluated on a per unit energy delivered basis.
Compressed air is used for many purposes, including:
When air at atmospheric pressure is compressed, it contains much more water vapor than the high-pressure air can hold. Relative humidity is governed by the properties of water and is not affected by air pressure. Management of the excessive moisture is a requirement of a compressed air distribution system. System designers must ensure that piping maintains a slope, to prevent accumulation of moisture in low parts of the piping system. Drain valves may be installed at multiple points of a large system to allow trapped water to be blown out. Taps from piping headers may be arranged at the tops of pipes, so that moisture is not carried over into piping branches feeding equipment.  Piping sizes are selected to avoid excessive energy loss in the piping system due to excess velocity in straight pipes at times of peak demand, or due to turbulence at pipe fittings.