Tony had told the truth when he recorded the message for Pepper. He wasn't afraid of the end. Not of death.
But he was scared of the dying.
Hypoxia was not the worst way to go, a confused, quiet slide into a dream he wouldn't wake up from.
But there were too many other ways to 'run out of oxygen' and too many had featured in his nightmares for it to be otherwise.
Suffocating, a bag over his head, rough hands on his shoulders, choking on sand and blood.
Water in his throat, his nose, his eyes. The awful frantic tension of actively trying not to breathe.
There was also a difference, he knew, between lack of air, lack of air pressure, and the wrong gas make-up of that air.
All their supplies were short, not only for creating oxygen but for removing the exhaled carbon dioxide. And he was far more scared of that.
He wished he didn't know as much about it as he did. Wished he didn't know that the accumulation of CO2 in the blood was the trigger for breathing to a far far greater extent than the depletion of oxygen. Wished he didn't know the symptoms. Couldn't all too easily imagine the panic-inducing hyperventilation. The gasping, dizzying, shaking, desperate breaths that would do you no good at all even if there was oxygen still in them.
He was breathing faster just thinking about it and he caught himself at it and stopped.
Too much knowledge was not always a good thing.
