You've heard the spiel so many times you could probably recite it by heart.
"In the unlikely event of a sudden loss of cabin pressure, oxygen masks will drop down from the panel above your head... Secure your own mask before helping others."
But actually gaining sight of these mysterious devices is exceedingly rare. So rare, in fact, that when United Airlines crew deployed them on a flight to Denver in 2015 after several passengers said they felt unwell, the biggest concern for those on board appeared to be documenting the event with a selfie.
So how do the masks actually work, under what circumstances are they unleashed, and how much oxygen do they contain?
Is it about safety, or is it just an elaborate way to intoxicate passengers, keeping them pliant while their stricken plane plummets to Earth? That, in case you're scratching your head, is the verdict of Brad Pitt's character Tyler Durden in the film Fight Club:
"You know why they put oxygen masks on planes? Oxygen gets you high. In a catastrophic emergency, you're taking giant panicked breaths. Suddenly you become euphoric, docile. You accept your fate. It's all right here [points at an emergency instruction manual on a plane]. Emergency water landing - 600 miles an hour. Blank faces, calm as Hindu cows."
Conspiracy theorists, we're sorry. Oxygen masks really are there for your safety. More specifically, as the flight attendant's briefing explains, it's all about cabin pressure.
The air at high altitudes is thin, and oxygen levels very low, so aircraft have clever systems in place to keep cabin pressure at a level equivalent to an altitude of around 5000-8000 feet. Sitting on a plane, in a way, is like visiting Mexico City or Bogotá.
A loss of pressure, therefore, is a significant issue - and potentially dangerous. If it happens suddenly - because of a bomb, for example - the resultant forces may damage or destroy the plane. Luckily, the vast majority of decompressions are not explosive - and are easy for the crew to handle.
"Crashes or fatalities from pressure problems are extremely uncommon, even with a fairly rapid decompression brought on by a hole or puncture," explains Patrick Smith, a pilot and author of Cockpit Confidential. "If cabin pressure falls below a certain threshold, the masks will deploy from the ceiling, exposing everybody to the so-called 'rubber jungle'. Should you be confronted by this spectacle, strap your mask on and try to relax. The plane will be at a safe altitude shortly, and there are several minutes of backup oxygen for everybody."
Several minutes? That might be less than you'd expect. So presumably the descent to a safe altitude is done pretty hastily.
"Up front, the pilots will don their own masks and commence a rapid descent to an altitude no higher than 10,000 feet," he continues. "If the emergency descent feels perilously fast, this isn't because the plane is crashing: it's because the crew is doing what's it's supposed to do."
Smith speaks from experience. Recalling a flight from South America to the US, he says:
"All was quiet high over the Caribbean, when suddenly there was a loud whooshing sound that seemed to come from nowhere and everywhere at once. I could feel my ears popping and sure enough, a glance at the instruments showed we were quickly losing pressurisation.
"The captain and I put our masks on, took out the book, and began to troubleshoot. Part of that troubleshooting involved one of those steep descents. Commencing such a drop is a multistep process: set 10,000 in the altitude window; select 'flight level change' from the autoflight panel; increase the speed command to a point slightly below maximum; deploy the speedbrakes; retard the thrust levels to idle... To the passengers, I'm sure it felt like a rollercoaster, but everything was carefully coordinated and the autopilot was engaged the whole time."
But what if you're flying over the Himalayas and can't descent to 10,000 feet?
"Pilots will follow predetermined depressurisation routes that allow for a more gradual descent, in stages," says Smith. "Even over mountains, there is always the opportunity to reach a safe altitude before supplemental oxygen runs out."
In fact, Smith's estimate of "several minutes" may be too conservative. Other sources - and evidence from the, ahem, fatal crash of Helios Airways Flight 522 - suggest there is actually 12-15 minutes of oxygen at hand.
So, really, try to relax. And don't forget that selfie.
What if you decline to use the mask? If the cabin really has lost pressure, hypoxia (basically a lack of oxygen) will quickly take effect on the brain, with symptoms including nausea and headaches - but also euphoria.
Tracey Bryan outlined her experience of decompression, the 'rubber jungle', and a rapid descent, on the website Quora. "I was extremely calm," she said. "I even remember giggling, teasing other passengers for putting on the oxygen masks, and insisting that they weren't required. In fact, due to the delay in putting on the mask I'd become hypoxic and wasn't thinking clearly. Eventually, my friendly practically forced the mask onto my face and as the hypoxia faded I began to feel nervous."
According to Airbus, if a plane loses pressure at 40,000 feet, those on board have as little as 18 seconds of "useful consciousness" without supplemental oxygen. Once the euphoria is over, hypoxia renders one unconscious and can cause brain damage or death. So wear the mask.
And what about Tyler Durden's claim that oxygen makes you high? Alas, it doesn't. Pilots and astronauts will readily debunk that myth.
Finally, we'll offer a little detail on how the masks actually work. Firstly, what you're supplied isn't exactly oxygen - nor is it not compressed air in the scuba diving sense. Oxygen tanks are heavy and bulky so aircraft use a more complicated system. The panel above each seat actually contains a cocktail of chemicals that, when burned, release oxygen. They might include barium peroxide, a fine white powder used in fireworks, sodium chlorate, more commonly used as a weedkiller, and potassium chlorate, a staple of school science lab experiments (it reacts violently with sugar).
Tug the mask, like you're told in the demonstration, and the chemical process starts. Once it starts, it cannot be stopped until everything's burned up (around 12-15 minutes, as stated above).
Do not expect the bag to inflate. Passengers have reportedly suffered hypoxia after believing their mask was broken because the bag wasn't inflating, prompting them to remove it. Hence the warning given during every safety briefing.
"Oxygen is supplied in a constant flow," explained a BA spokesman. "The bag does not inflate like a respirator bag used in a medical theatre. How full it gets depends on an individual's rate of breathing. If the rate of breathing is very quick, air is inhaled at a faster rate and so the bag will inflate less. If all the air isn't inhaled, some will remain in the bag, partially inflating it."
The oxygen generator can also get extremely hot - so don't touch it - and passengers may even notice a burning smell (so don't be alarmed).
Are all these burning chemicals dangerous? Sort of. The crash of ValuJet Flight 592 in 1996, in which all 110 on board perished, was caused by an oxygen generator fire. However, the devices to blame were in the cargo hold, out of date, and missing their safety caps. A jolt lit one, and the rest followed. Understandably, masks are not deployed if there is an on-board fire as the production of oxygen may make matters worse.
And are they safe to inhale? "You might get some small bits of chemical dust," Arch Carson, an occupational health expert, told The Huffington Post last year. "But it's way better than the alternative, which is passing out due to lack of oxygen." Agreed.