"So, this may surprise you, but you're still slowly suffocating even with an oxygen mask.

Because there's so little atmospheric pressure, the oxygen concentration in your blood diminishes even when breathing a higher mixture. They can't increase the mixture too much, or you start to get oxygen narcosis.

I read a couple books about it, and while the oxygen helps a lot, it's not a cure all. You're still on a timer to get to the top and back down. Even when sleeping, you're body is growing weaker."