The primary danger of decompression is hypoxia. Quick,  proper utilization of oxygen equipment is necessary to avoid  unconsciousness. Another potential danger that pilots, crew,  and passengers face during high altitude decompressions is  evolved gas decompression sickness. This occurs when the  pressure on the body drops sufficiently, nitrogen comes out  of solution, and forms bubbles inside the person that can have  adverse effects on some body tissues.  being stored in an unheated area of the aircraft rather than  an actual depletion of the oxygen supply. High pressure  oxygen containers should be marked with the psi tolerance  (i.e., 1,800 psi) before filling the container to that pressure.  The containers should be supplied with oxygen that meets  or exceeds SAE AS8010 (as revised), Aviator’s Breathing  Oxygen Purity Standard. To assure safety, periodic inspection  and servicing of the oxygen system should be performed.  Decompression caused by structural damage to the aircraft  presents another type of danger to pilots, crew, and  passengers––being tossed or blown out of the aircraft if  they are located near openings. Individuals near openings  should wear safety harnesses or seatbelts at all times when  the aircraft is pressurized and they are seated. Structural  damage also has the potential to expose them to wind blasts  and extremely cold temperatures.  An oxygen system consists of a mask or cannula and a  regulator that supplies a flow of oxygen dependent upon  cabin altitude. Most regulators approved for use up to 40,000  feet are designed to provide zero percent cylinder oxygen  and 100 percent cabin air at cabin altitudes of 8,000 feet or  less, with the ratio changing to 100 percent oxygen and zero  percent cabin air at approximately 34,000 feet cabin altitude.  [Figure 7-43]  Most regulators approved up to 45,000 feet  are designed to provide 40 percent cylinder oxygen and 60  percent cabin air at lower altitudes, with the ratio changing  to 100 percent at the higher altitude.  Rapid descent from altitude is necessary in order to minimize  these problems. Automatic visual and aural warning systems  are included in the equipment of all pressurized aircraft.  Crew and passengers use oxygen systems, in conjunction  with pressurization systems, to prevent hypoxia. Regulations  require, at a minimum, flight crews have and use supplemental  oxygen after 30 minutes exposure to cabin pressure altitudes  between 12,500 and 14,000 feet. Use of supplemental  oxygen is required immediately upon exposure to cabin  pressure altitudes above 14,000 feet. Every aircraft occupant,  above 15,000 feet cabin pressure altitude, must have  supplemental oxygen. However, based on a person’s physical  characteristics and condition, a person may feel the effects  of oxygen deprivation at much lower altitudes. Some people  flying above 10,000 feet during the day may experience  disorientation due to the lack of adequate oxygen. At night,  especially when fatigued, these effects may occur as low  as 5,000 feet. Therefore, for optimum protection, pilots are  encouraged to use supplemental oxygen above 10,000 feet  cabin altitude during the day and above 5,000 feet at night.  Pilots should be aware of the danger of fire when using  oxygen. Materials that are nearly fireproof in ordinary air may  be susceptible to combustion in oxygen. Oils and greases may  ignite if exposed to oxygen and cannot be used for sealing  the valves and fittings of oxygen equipment. Smoking during  any kind of oxygen equipment use is prohibited. Before  each flight, the pilot should thoroughly inspect and test all  oxygen equipment. The inspection should include a thorough  examination of the aircraft oxygen equipment, including  available supply, an operational check of the system, and  assurance that the supplemental oxygen is readily accessible.  The inspection should be accomplished with clean hands and  should include a visual inspection of the mask and tubing  for tears, cracks, or deterioration; the regulator for valve  and lever condition and positions; oxygen quantity; and the  location and functioning of oxygen pressure gauges, flow  indicators, and connections. The mask should be donned and  the system should be tested. After any oxygen use, verify that  all components and valves are shut off.  Most high altitude aircraft come equipped with some type  of fixed oxygen installation. If the aircraft does not have  a fixed installation, portable oxygen equipment must be  readily accessible during flight. The portable equipment  usually consists of a container, regulator, mask outlet,  and pressure gauge. Aircraft oxygen is usually stored in  high pressure system containers of 1,800–2,200 psi. When  the ambient temperature surrounding an oxygen cylinder  decreases, pressure within that cylinder decreases because  pressure varies directly with temperature if the volume of a  gas remains constant. A drop in the indicated pressure of a  supplemental oxygen cylinder may be due to the container  Figure 7-43.  Oxygen system regulator.  7-37  Oxygen Systems 