In Hulk (2003) when the fighter jet, takes Hulk to the "top of the world" (an altitude of over 95000 feet, and into the beginnings of space, what actually happens to Hulk? He is seen suffocating, his green sheen wears off and he finally collapses mid-way but what is actually happening to him?
In reference to the movie, the official novelisation would suggest that he passed out due to a lack of oxygen:
The strategy, developed on the fly, in every sense of the word, by Thunderbolt Ross, was devastatingly simple: Take the Hulk on a ride to the top of the world, and see what some thin air did for him.
The Hulk clung to the plane as it rose up through the clouds. Frost began to cover him, hanging from his hair, his eyebrows. The world started to fade around him. Close to losing consciousness, he stared into the eyes of the pilot through the cockpit’s windshield. The pilot flinched, concerned that the Hulk would abruptly recover and try to tear the wings off his plane. But he needn’t have worried, for the Hulk closed his eyes completely, and as the plane tilted back—having gone as high as it could safely go—the monster slid off.
Note that although the Hulk is able to hold his breath for a considerable amount of time, he doesn't seem to be able to cope with hypoxia...
But all the Angry Man did was glance up disdainfully at the helicopter, and then he took a deep breath and went under again.
Ross remembered the Hulk’s ability to survive on a single breath from the encounter with the gas. The situation wasn’t promising, and Ross was beginning to realize that this wasn’t simply a case of man against man. This was man against a force of nature . . . and unfortunately, in such conflicts, man always came up on the losing end.
Hulk needs to breath, although he's extraordinarily able to keep his breath during long periods of time.
On the event of suffocation, usually victims get unconscious before dying. When Hulk becomes unconscious (or loses enough conscience level) it's unable to maintain it's rage and so it reverts to be Bruce Banner.