The Federal Aviation Regulations - 49 CFR 121.331 - require that airline aircraft operating at altitudes above 14,000 feet must provide emergency oxygen for at least 10% of the passengers. In the event of cabil pressurization failure, the aircraft must descend to an altitude where oxygen is not necessary within less than four minutes. More complex for Jet aircraft which operate at higher altitudes, but still must provide adequate oxygen to allow descent to a safe altitude before injury occurs.