Frequent fliers have heard it dozens of times over – "Should the cabin experience sudden pressure loss… Oxygen masks will drop down from above your seat. Place the mask over your mouth and nose."
The pre-flight safety announcement-cum-video may give nervous fliers the jitters: not only are we told how to jump out of the plane in the event of a crash, but we are told we might need help breathing, too.
But when might you need to heed the advice to "pull the strap to tighten and continue to breathe normally"?
Cabin pressure loss, for the most part. Aircraft cabins are pressurised using cooled and filtered air bled from the engines, keeping the air pressure inside the cabin at the equivalent of an altitude of 8,000ft - even though commercial aircraft often fly at 40,000 ft.
The dry cabin air might cause passengers to become a little dehydrated, but happily they are able to breathe unassisted, and continue watching the inflight film, quaffing a tomato juice, or browsing the duty-free catalogue.
But this changes when there is a loss of cabin pressure – either slow or sudden. This can happen for a number of reasons. Technical problems with the pressurisation system are one cause, but cracks in windows or the fuselage, incorrectly sealed doors, and breaches in the aircraft due to an explosion are also all potential triggers, allowing cabin air to escape.
"In one cabin decompression event, a cabin crew member was saved from ejection out of the aircraft, because a passenger was holding on to the cabin crew member's ankle." - Airbus training memo on cabin pressure loss
Small air leaks would likely cause a slow loss of pressurisation, in which case the pilot would have time to make an emergency descent to a safe altitude, of between 8-10,000 ft. Bumpy, yes, but hopefully not fatal.
Should a plane's pressurisation systems be in order, a drop in the cabin pressure that makes the plane think it is above 14,000 ft will see the oxygen masks fall from panels above passengers' heads – in a scene known to cabin crew as the "rubber jungle".
But if, for example, an explosive device suddenly hit the fuselage and caused a rapid drop in cabin pressure, you might have just seconds to play with, especially if the breach in the fuselage is large.
In 2007, Airbus issued a "cabin decompression awareness" note, admitting that "a recently published accident report involving a case of slow decompression suggested that the overall aviation industry does not provide sufficient training to flight crew and cabin crew."
The note said that at 40,000 ft, people have as little as 18 seconds of "useful consciousness" time if they are starved of oxygen. It stressed the risks of hypoxia – oxygen starvation – are all the greater as people may not realise they are suffering until they can no longer breathe and fall unconscious.
Oxygen masks hang from the ceiling in the cabin interior of Asiana Airlines flight 214 following a crash at San Francisco airport (Getty) Photo: Getty
Signs of sudden decompression include "a loud bang, thump or clap" as air inside and outside the plane meet, debris flying around the cabin, and "Unsecured items in the immediate area" near the breach being "ejected" from the aircraft.
Safety procedures noted in the briefing document stress recognising the symptoms of hypoxia – nausea, headaches, and euphoria among them – and donning the nearest oxygen mask and holding on to the nearest fixed item.
Alarmingly, the note added; "In one cabin decompression event, a cabin crew member was saved from ejection out of the aircraft, because a passenger was holding on to the cabin crew member's ankle."
At 40,000 ft, people have as little as 18 seconds of 'useful consciousness' time if they are starved of oxygen
None of the airlines Telegraph Travel contacted would speak on the topic of cabin depressurisation, although The Civil Aviation Authority said that companies have "very clear" procedures that cabin crew will follow in the event of gradual or sudden depressurisation.
Incidences of cabin pressure loss are not common, although the Civil Aviation Authority said it could not give exact figures and in the past two decades there have been a number of times it has occurred with fatal consequences.
In 2005, a Helios Airways aeroplane 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. In 2008, 16 passengers on a Ryanair flight from Bristol to Barcelona-Girona had to be taken to hospital after a drop in cabin pressure caused an emergency descent and diversion.
And last month a door left partially open on a Korean plane meant that airline officials were forced to admit passengers started to display the signs of mild hypoxia, including headaches, nausea and ear pain.
So – there is a reason the pre-flight safety briefing is there – if the rubber jungle appears, there is a reason you need to don the oxygen mask. Tighten the mask and breathe normally, and hope that the pilot has done the same, and is able to descend to an altitude at which all on board can breathe easily.