U.S. Army helicopter pilots in Afghanistan often operate at high (over 3,200 meters/10,000 feet) enough altitudes that they need oxygen to maintain alertness. Currently, crew members get the oxygen via tubes from hundred pound tanks. This restricts mobility inside the helicopter. So the army has developed a portable (2.3 kilograms/5 pound) device that you can wear on your chest. A sensor gives you additional oxygen when it detects a need. The PHODS (Portable Helicopter Oxygen Delivery System) provides enough oxygen for 2-3 hours.