What happens when a plane loses cabin pressure? What actions should be taken?

Supplemental Oxygen Depressurisation of the aircraft cabin as a result of structural failure, pressurization system.
Signs of sudden decompression include “a loud bang, thump or clap” as the air inside and outside the plane meet, debris flying around the cabin .
Is loss of cabin pressure dangerous? Yes..
In 2005, as referenced above, a Helios Airways plane en route from Cyprus to Athens crashed into a mountain after a loss of cabin pressure, killing all 115 passengers and six crew on board.
The official investigation into the incident found that the pressurisation system had been left in manual gear, causing cabin pressure to drop and the pilots to fall unconscious as they suffered hypoxia. Oxygen supplies for passengers ran out after 15 minutes, and when the plane, running on autopilot, ran out of fuel, it crashed 33km from Athens airport.

In another incident, Payne Stewart, a prominent golf star, was killed along with five other people when a Learjet 35 plane crashed in October 1999, en route from Orlando in Florida to Dallas in Texas. The official report could not pinpoint the specific cause of the crash, but said it was likely the result of a loss of cabin pressure and the failure to get emergency oxygen, leading to the two pilots losing consciousness.

Non-fatal incidents have also been documented. Earlier this year, in July, more than 30 Ryanair passengers, some bleeding from their ears, received hospital treatment after their plane lost cabin pressure at 36,000 feet. The plane made a “controlled descent”, losing 26,000 feet in seven minutes.

Again in July, an Air China aircraft made an emergency descent of 25,000 feet in 10 minutes after what was believed to have been a loss of cabin pressure. However, the plane then climbed again and continued to its destination, even though oxygen masks had been deployed.

In 2016 a door left partially open on a Korean plane saw passengers display signs of mild hypoxia, including headaches, nausea and ear pain.

Oxygen donning TUC Times ile ilgili görsel sonucu

Time of useful consciousness (TUC), also effective performance time (EPT), is defined as the amount of time an individual is able to perform flying duties efficiently in an environment of inadequate oxygen supply. It is the period of time from the interruption of the oxygen supply or exposure.

That’s not to say sudden decompression isn’t a danger. The FAA (in its Advisory Circular 61-107A) provides a helpful chart showing just how long crewmembers are able to perform flight duties with an insufficient supply of oxygen. In an aircraft at 22,000 feet, passengers and crew would have 5 minutes of “useful consciousness” after rapid decompression. But at 43,000 feet, the time drops to a mere 5 seconds, hardly long enough to don an oxygen mask. 
 (The same circular notes “One pilot does not need to wear and use an oxygen mask if both pilots are at the controls and each pilot has a quick donning type of oxygen mask that can be placed on the face with one hand from the ready position and be properly secured, sealed, and operational within 5 seconds. If one pilot of a two-pilot crew is away from the controls, then the pilot that is at the controls must wear and use an oxygen mask that is secured and sealed.”)
In the event of a larger hole in the fuselage, there’s more risk of hypothermia than decompression. “[T]he cabin temperature would drop from 70˚ degrees F to as low as -60˚F,” 

“With temperatures that low, it is only a matter of seconds before hypothermia sets in and everyone begins to freeze to death.”

So it isn’t the pressure drop that gets you after all. It’s the cold.

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