Because the economics of having large oxygen tanks aboard airliners simply doesn't work
out (not to mention that the air quality inside the plane would rapidly become unpleasant
if fresh air wasn't constantly supplied, regardless of the oxygen levels), commercial airplanes
have a very clever system installed to solve the problem of ultra-low pressure atmosphere
at cruising altitudes.
In most modern airliners (the Boeing 787 Dreamliner not withstanding), outside air is "bled off"
from the compressor stage of the turbine engines and eventually piped into the passenger areas.
However, a bit of processing is needed first as the compressed air is extremely hot (on
the order of nearly 400 degrees Fahrenheit or 200 degrees Celsius) at this stage.
Thus, before it enters the passenger compartment, it is first allowed to expand and is run through
a heat exchanger and air cycle system to cool it off sufficiently.
This system also can work as a heater, with some of the hot air mixed in with the cooled
air to regulate cabin temperature.
Once cooled and filtered, the pressurized air, which now has sufficient oxygen density
to keep people happily conscious, is piped into the cabin area, usually at levels around
12 psi (about equivalent to atmospheric pressure at 7,000 feet).
Why 12 psi instead of something like sea-level pressures of about 14.7 psi?
12 psi is sufficient for the majority of passengers while simultaneously reducing the structural
strain on the aircraft itself over something like sea level atmospheric pressures.
As for the air already in the cabin, this is vented out through an outflow valve (or
multiple valves in larger aircraft), usually located near the rear of the plane.
(Fun Note: Before smoking was banned on commercial aircraft, the area around this outflow valve
was generally stained dark brown from tobacco smoke.)
This outflow valve opens and closes automatically to maintain a steady pressure inside the cabin,
while the entire system is ensuring that fresh air is continually being piped into and eventually
blown out of the aircraft.
In fact, while many complain of airplanes seeming "stuffy," this system ensures that
all the air in the aircraft is being completely replaced on average every 2-3 minutes.
Yes, that means that your car, house or office is likely significantly more "stuffy" than
a commercial airplane flying at 35,000 feet.
(Note: the Boeing 787 Dreamliner handles cabin pressurization a little differently, using
a modernized version of the electric compressor system seen on many older aircraft.)
Unfortunately, sometimes planes lose cabin pressure.
Whatever the cause, the loss of pressure (usually set at atmospheric pressures past 14,000 ft)
will result in oxygen masks deploying.
From here, useful consciousness may only last as little as 5-15 seconds, depending on remaining
cabin pressure, which is why it's critical to immediately put your mask on, rather than
helping someone else first.
You can help them much better when you're not unconscious or dead.
So how do these airline oxygen masks actually work?
It turns out, the economics of having a centralized oxygen tank to provide even emergency oxygen
for passengers likewise simply doesn't add up.
Similarly, having tiny individual pressurized oxygen tanks also isn't feasible.
In fact, these masks aren't hooked up to any tank or air line at all.
So how are you able to breathe oxygen through them?
Science!!!
While designs can vary slightly, in general, when you pull on the device to place it over
your face, the tug on the mask's lanyard releases a spring-loaded mechanism that sets off a
small explosive charge.
The resulting spark triggers a mixture of lead styphnate and tetracene to generate heat,
which will eventually cause a chemical reaction that produces oxygen for your mask.
(This is why they tell you to tug on the mask to get the oxygen flowing- you've got to set
off the explosive charge to get the whole thing going.)
That's right.
What you breathe through the mask didn't begin as pure oxygen.
Rather, the plane is equipped with numerous small chemical oxygen generators (also known
as "oxygen candles," about the size of a small package of tennis balls) which contain a mixture
of mostly sodium chlorate (NaClO3), less than 5% barium peroxide (BaO2) and less than 1%
potassium perchlorate (KClO4).
When these chemicals are heated by the lead styphnate and tetracene, each undergoes a
reaction that ultimately results in a fair bit of filtered, life sustaining oxygen running
through the tube to you.
Of course, you might also smell a faint burning odor, but this is nothing to be alarmed about;
it just assures you that the system is working.
In fact, if the plane is actually on fire, the masks usually won't deploy, so as not
to make the fire worse with the extra oxygen.
This brings us to the question of why the plastic bag on the breathing apparatus won't
necessarily inflate as you're using the device.
More than just cosmetic, the bags serve as something of a reservoir for oxygen.
If you aren't taking a breath at all (and have a good seal with the mask tight against
your face) the bag keeps the precious, continuously flowing oxygen from escaping into the thin
air around you, enabling more of the collected oxygen to be taken in when you do take a breath.
When this is happening, or you are breathing out with the valves on the mask releasing
much of the used air, the bag may begin to inflate as oxygen collects.
When you breathe in, it will deflate.
So why won't it always inflate at least a little to show its working?
To begin with, you may not have a great seal with the mask on your face, particularly if
you have facial hair.
This will allow any produced oxygen (and air you exhale) to more readily escape.
(As long as the mask is reasonably secure on your face, this should still provide you
with sufficient oxygen to get by on as long as the plane isn't flying above 40,000 feet
and the pilot does his or her job and gets the plane down below 10,000 feet as rapidly
as safely possible.
And if you’d like to see the procedures generally used by commercial pilots for this,
go check out Mentour Pilot’s recent video Emergency Descend!!!
Cockpit Video https://youtu.be/yHawjB2PzK0 )
Even if you have a good seal, however, the rate at which the oxygen is generated is often
not enough to fully inflate the masks' bag before you take deep, potentially panicky
breaths, deflating it.
This is simply because the oxygen generation isn't on-demand (for the passengers anyway),
but simply a continuous-flow production of oxygen.
Despite the potentially slow production, the chemical oxygen generators do provide oxygen
at a sufficient rate to sustain passengers, generally designed such that peak oxygen production
occurs right away (when the plane may be at very high altitude) with the oxygen production
rates tailing off over the course of approximately 12-20 minutes before the system burns itself
out.
This should be long enough for the pilots to get the plane low enough so that the air
pressure is high enough for (relatively) normal atmospheric breathing.
And if you've ever been lucky enough to be in this sort of situation, you know that those
pilots can get the plane from altitudes like 35,000+ feet to safer atmospheric levels in
a surprisingly small amount of time in an emergency.
Speaking
of good timing being critical
to so many aspects of life, Vincero!...
Bonus Facts: • As a result of the way the system works
for pressurizing the airplane cabin and keeping a steady supply of fresh air, the humidity
levels are ultra-low, making it so you dehydrate very quickly on flights.
Particularly for long flights, it's critical then that you drink plenty of fluids.
This ultra-low humidity level, combined with the low cabin pressure, also reduces your
sense of taste and smell by as much as 30%, which is why airline food generally tastes
so bland.
To try to compensate for this somewhat, many airlines make sure their food is much more
strongly flavored or spiced than you'd normally find appetizing.
Fun fact about breathing which you may or may not have ever noticed before- the vast
majority of humans breathe out of only one nostril at a time, with it switching periodically.
The first known instance of someone studying this "nasal cycle" was German nose specialist,
Richard Kayser in 1895.
How your nose accomplishes this switch is via erectile tissue in your nose, which is
very similar to the erectile tissue in a penis or clitoris.
Erectile tissue will swell up in one nostril, mostly blocking it, and at the same time erectile
tissue in the other nostril will shrink, opening it up for breathing.
Even more interesting is that depending on which nostril you are predominately breathing
out of at any given moment, it seems to greatly affect your body and brain.
For instance, a study in 1988 showed that breathing through your right nostril significantly
increases blood glucose levels, while breathing through your left nostril has the opposite
effect.
Another study in 1993 showed that when you are breathing through your right nostril,
you will use significantly more oxygen than when breathing through your left.
Most interestingly of all, yet another study published in 1994, showed that when you are
breathing through your left nostril, the right hemisphere of your brain will be more active
or dominant and vice-verse when you are breathing through your right nostril.
And if you’re now wondering, even though this switching happens naturally in a cyclical
fashion, you can affect it in more ways than just plugging one nostril or the other to
force breathing through the one you want.
If you lay down on one side or the other, after around 12-15 minutes, the erectile tissue
in the nostril on that side will begin to swell up and the other side will decrease
its swelling so that if you are lying on your left side, then your right nostril will open
up and your left nostril will close up.
It is speculated that when you sleep on your side, this nasal cycle could contribute to
people naturally switching sides throughout the night at very regular intervals, even
if they weren't in the slightest bit uncomfortable.