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.
That's all the information that you're given about oxygen masks. They don't inform you about why do you need it, how it works and most importantly, Brad Pitt from fight club was wrong, oxygen masks don't make you high. There is a significant amount of people that believe oxygen masks on planes are there only to keep you calm and put you on high and this misconception is solely down to the fact that there is a real lack of knowledge about oxygen masks on planes.
Why do you need it
The higher the altitude, the thinner the air and therefore, very low oxygen levels. Airplanes have systems in place that keeps cabin pressure at a level that's equal to an altitude of about 1500 - 2500 meters, airplanes can sometimes go over an altitude of 12000 meters. The air at such levels is extremely thin, so a loss of cabin pressure at this stage can be very dangerous, it is said that you would have about 18-20 seconds of 'useful consciousness' before going in to a state of euphoria, after the euphoria you can lose consciousness entirely, and after that there's a chance of brain damage or even death. So please, if you find yourself in such a situation, wear the mask! The reason that you're told to put your mask on first is because of the little time you have before going into a state of euphoria.
How it works
Since oxygen tanks are very heavy and bulky, the design for oxygen masks in airplanes is a little more complicated. The process starts with the tug on the mask that you're always instructed to do, that tug releases chemicals like barium peroxide, sodium chlorate and potassium chlorate. These chemicals make a tiny explosion that releases an amount of oxygen that lasts about 20 minutes. In that time the plane will descend to a safer altitude.
Another thing that you should be made aware of is that the bag doesn't inflate, and since it makes a tiny explosion there could be a feeling of heat and a burning smell. This doesn't mean that the masks don't work!
I hope that this post managed to shed some light on a topic that not a lot of people are knowledgeable about. I hope that you will never need to use oxygen masks but if you do, please follow the instructions carefully.
Some other instructions you might want to follow:
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References
http://www.telegraph.co.uk/travel/travel-truths/truth-about-oxygen-masks-on-planes/
https://curiosity.com/topics/airplane-oxygen-masks-make-their-own-oxygen-curiosity-2782371/