B_Schwaz writes...

1a. Will Superman and Superboy actually die if they are deprived of oxygen for long enough? Or just be left unconscious?

1b. How long does it take without oxygen for them to die/be left unconscious?

2a. You've said that M'gann and J'onn need to breathe oxygen, but they come from Mars where there is little oxygen in the atmosphere. Is this because they have taken mostly human forms on Earth, even internally?

2b. How do M'gann and B'arzz breathe when they are in their true Martian forms?

2c. Do Martians breathe on Mars, and if so, what do they breathe?