A container designed as a refillable cylinder used to hold compressed medical air [a mixture of approximately 21% oxygen (O2) and 78% nitrogen (N2) and other inert gases] under safe conditions at high pressure (e.g., 50 - 200 Bar). It is typically filled with medical air when delivered from the gas supplier and includes a valve stem, an opening/closing valve, and will be graded according to size (capacity) and colour-coded to denote medical air content. The cylinder is typically made of steel and must be used together with a pressure regulator in order to release the air at the correct working pressure. Medical air is used for life support, therapeutic purposes and in anaesthesia.