Loss of Cabin Pressurization

Loss of pressurization is a potentially serious emergency in an aircraft flying at the normal cruising altitude for most jet passenger aircraft. 

Loss of cabin pressure, or depressurization, is normally classified as explosive, rapid, or gradual based on the time interval over which cabin pressure is lost.

Depressurization of the aircraft cabin as a result of structural failure, pressurization system malfunction, an inadvertent crew action or a deliberate crew intervention.

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Source: SKYbrary