of acute oxygen toxicity is acceptable. Oxygen supply during in-water decompression is via rebreather, open circuit diving regulator, full-face mask or diving helmet which has been prepared for . Built-in breathing system Oxygen supply to divers in decompression chambers is preferably through a built-in breathing system, which uses an oxygen mask plumbed into supply and exhaust hoses which supply oxygen from outside the chamber, and discharge the exhaled oxygen-rich gas outside the chamber, using a system equivalent to two demand valves, one upstream of the diver, to supply oxygen on demand, and the other downstream, to exhaust exhaled gas on demand, so that the oxygen partial pressure in the chamber is limited to relatively safe levels. If oxygen masks are used that discharge into the chamber, the chamber air must be replaced frequently to keep the oxygen level within safe operating limits. Anesthesia oxygen masks Anesthesia masks are face masks that are designed to administer anesthetic gases to a patient through inhalation. Anesthesia masks are either made of anti-static silicone or rubber, as a static electricity spark may ignite some anesthetic gases. They are either black rubber or clear silicone. Anesthesia masks fit over the mouth and nose and have a double hose system. One hose carries inhaled anesthetic gas to the mask and the other brings exhaled anesthetic gas back to the machine. Anesthesia masks have 4 point head strap harnesses to securely fit on the head to hold the mask in place as the anaesthesia doctor controls the gases and oxygen inhaled. Masks for high-altitude climbers Oxygen masks are used by climbers of high peaks such as Mount Everest. Because of the severe cold and harsh conditions oxygen masks for use at extreme altitude must be robust and effective. The oxygen storage tanks used with the masks (called oxygen bottles) are made of lightweight, high-strength metals and are covered in high-strength fiber such as kevlar. These special oxygen bottles are filled with oxygen at a very high pressure which provides a longer time duration of oxygen for breathing than standard pressure oxygen bottles. These systems are generally only used above . In recent years oxygen mask systems for high-altitude climbing which pump oxygen constantly have been increasingly replaced by systems supplying oxygen on demand via nasal cannulas. Oxygen helmets Oxygen helmets are used in hyperbaric oxygen chambers for oxygen administration. They are transparent, lightweight plastic helmets with a