Closed-cycle protective breathing apparatus, commonly referred to as rebreathers, or CCBA provide trained aircrew members or ground personnel with eye and respiratory protection from toxic atmospheres. The purpose of this AIR is to describe the static and dynamic characteristics of closed circuit breathing apparatus. Considerable design consideration must be given to ensure conservation of, or removal of, vital respiratory gases in closed cycle breathing apparatus. Descriptions and illustrations are included that represent the basic flow paths of respiratory gases during operation. For more specific information on certain topics, references are also cited in the text, where applicable.