air respirators, where a hose supplies air from a stationary source; and combination respirators that integrate both types. According to the NIOSH Respirator Selection Logic, atmosphere-supplying are recommended for concentrations of hazardous particulates or gases that are greater than the immediately dangerous to life or health level; where the required assigned protection factor exceeds those of air-purifying respirators; during firefighting (self-contained breathing apparatuses only); in oxygen-deficient atmosphere; and in an unknown atmosphere. Self-contained breathing apparatus A self-contained breathing apparatus (SCBA) typically has three main components: a high-pressure air cylinder (e.g., 2200 psi to 4500 psi), a pressure gauge and regulator, and an inhalation connection (mouthpiece, mouth mask or full face mask), connected together and mounted to a carrying frame or a harness with adjustable shoulder straps and belt so it can be worn on the back. There are two kinds of SCBA: open circuit and closed circuit. Most modern SCBAs are open-circuit. Open-circuit industrial breathing sets are filled with filtered, compressed air. The compressed air passes through a regulator, is inhaled and exhaled out of the circuit, quickly depleting the supply of air. Air cylinders are made of aluminum, steel, or of a composite construction like fiberglass-wrapped aluminum. The "positive pressure" type is common, which supplies a steady stream of air to stop fumes or smoke from leaking into the mask. Other SCBA's are of the "demand" type, which only supply air when the regulator senses the user inhaling. All fire departments and those working in toxic environments use the positive pressure SCBA for safety reasons. The closed-circuit type SCBA filters, supplements, and recirculates exhaled gas like a rebreather. It is used when a longer-duration supply of breathing gas is needed, such as in mine rescue and in long tunnels, and going through passages too narrow for a large open-circuit air cylinder. Supplied air respirator Supplied air respirators make use of a hose to deliver air from a stationary source. It provides clean air for long periods of time and are light weight for the user, although it limits user mobility. They are normally used when there are extended work periods required in atmospheres that are not immediately dangerous to life and health (IDLH). Disadvantages Extended use of respirators can result in higher levels of carbon dioxide than recommended in a work environment, and can lead to headaches, dermatitis and acne. Regulation The choice and use of respirators in developed