Air is the most common and only natural breathing gas. Other artificial gases, either pure gases or mixtures of gases, are used in breathing equipment and enclosed habitats such as SCUBA equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft and anaesthetic machines.

Most breathing gases are a mixture of oxygen and one or more inert gases. All breathing gases are alternatives to air and have been developed to improve on the performance of air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving.

A safe breathing gas has three essential features:

• it must contain sufficient oxygen to support the life, consciousness and work rate of the breather.
• it must not contain harmful gases. Carbon monoxide and carbon dioxide are common poisons in breathing gases. There are many others.
• it must not become toxic when being breathed at high pressure such as when underwater. Oxygen and nitrogen are examples of gases that become toxic under pressure.

The techniques used to fill diving cylinders with gases other than air are called gas blending.