, or more in special cases. Navies, professional diving organizations, hospitals, and dedicated recompression facilities typically operate these. They range in size from semi-portable, one-patient units to room-sized units that can treat eight or more patients. They may be rated for lower pressures if not primarily intended for treatment of diving injuries. In the larger multiplace chambers, patients inside the chamber breathe from either "oxygen hoods" – flexible, transparent soft plastic hoods with a seal around the neck similar to a space suit helmet – or tightly fitting oxygen masks, which supply pure oxygen and may be designed to directly exhaust the exhaled gas from the chamber. During treatment patients breathe 100% oxygen most of the time to maximise the effectiveness of their treatment, but have periodic "air breaks" during which they breathe chamber air (21% oxygen) to reduce the risk of oxygen toxicity. The exhaled treatment gas must be removed from the chamber to prevent the buildup of oxygen, which could present a fire risk. Attendants may also breathe oxygen some of the time to reduce their risk of decompression sickness when they leave the chamber. The pressure inside the chamber is increased by opening valves allowing high-pressure air to enter from storage cylinders, which are filled by an air compressor. Chamber air oxygen content is kept between 19% and 23% to control fire risk (US Navy maximum 25%). If the chamber does not have a scrubber system to remove carbon dioxide from the chamber gas, the chamber must be isobarically ventilated to keep the CO2 within acceptable limits. Therapeutic recompression Hyperbaric oxygen therapy was developed as a treatment for diving disorders involving bubbles of gas in the tissues, such as decompression sickness and gas embolism, and it is still considered the definitive treatment for these conditions. The recompression treats decompression sickness and gas embolism by increasing pressure, which reduces the size of the gas bubbles and improves the transport of blood to downstream tissues. Elimination of the inert component of the breathing gas by breathing oxygen provides a stronger concentration gradient to eliminate dissolved inert gas still in the tissues, and further accelerates bubble reduction by dissolving the gas back into the blood. After elimination of bubbles, the pressure is gradually reduced back to atmospheric levels. The raised oxygen partial pressures in the blood may also help recovery of oxygen-starved tissues downstream of the blockages. Emergency treatment for