Like the traveler on an aircraft, you are vulnerable.  At any time, the cabin may lose pressurization and you would not have enough oxygen to breathe.   If there were to be a rapid decompression, vapor would fill the room, the pressure would drop, and your lungs would deflate.  Your lungs would be unable to provide you the required oxygen, because there is simply not enough pressure.  Of course, if everything worked as expected, the oxygen masks would drop providing you with enough pure oxygen to survive until the situation was under control.

(In the video:  The Masks Drop at 1:28)

But did you remember what the flight attendant said?  “Always put your own mask on first.”  Why?  Because if you tried to put your child’s mask on first you would pass out before you might help either her or you.  You must put your own mask on FIRST.  Once you do, you can help others.  So make, sure you are taken care of, then move on to helping those closest to you.

Notice that you don’t have to be safely on the ground or 100% secure to help others, but you do have to at least have the tools to function as a servant.