Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year. Compressed air is 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 can be used in or for air-start systems in aircraft engines, pneumatic powered weapons, use of pressurized gases to do work, vehicular transportation using a compressed air vehicle, energy storage, for breathing and to inflate buoyancy devices.