The ANS

Haha wah....I didn expect so many ppl to post an ans...haha good good...blog not dead yet...

Ok...I am also very interested, and I want to "suan" my physics teacher (just kidding) , so I went to do a research on this topic.....and....Dian Yang and I got it right!

As I explained earlier, there must be a balance between the pressure in the plane, and the pressure outside the plane. Or else, the plane will just blow up due to pressure differences (high pressure inside, low pressure outside). Thus, the air cabin pressure is lowered to that of the pressure at 10 000 feet, which is the best balance. The plane can be made with fairly light material (duralunium) and not explode in mid air. So that saves costs.
Now comes the interesting part....
Jets fly high in the atmosphere... typically between 26,000 and 41,000 feet... where the air is very thin. In order for passengers to breathe, extra oxygen has to be pumped into the cabin. If that extra oxygen wasn't added, we'd pass out in seconds. When the extra oxygen is pumped in, it's joined by other atmospheric gases. That gives in interior of the plane the "pressure"... or air density... you'd normally experience at 10,000 feet. Thus, the pressure is much higher inside the plane than it is outside.

In other words, the percentage of oxygen in the air cabin is increased by pumping in extra oxygen. This is necessary so as to allow normal breathing. The excess oxygen is of course, mixed with normal air from outside, thus we are able to breathe normally.
When does the oxygen mask come in then?
It is only deployed when the cabin faces a leak in air: ie; there is a drop in oxygen. As such, the captain deploys the last tank of oxygen, the oxygen mask, which contains normal air to passengers to breathe normally. He has to find a landing place fast as the oxygen will not last forever.
As to CTX's view...oxygen tanks DO NOT HAVE 100% OXYGEN!
We will die if we breathe in 100% oxygen. They just contain the right mixture for us to breathe normally. Even when you are on life support, or tht they are curing you from Carbon Monoxide posioning, they only feed u with 40% pure oxygen so as to use the vast amt to force ligand exchange between CO and O2 ligand in the haemoglobin. You will learn this under transition metals. Haha, so CTX, no 100% pure oxygen....

Interesting right?
For more info, visit this website:
http://www.beloblog.com/KGW_Blogs/travel/archives/2006/02/alaska_airlines.html

Have a nice week!