Some of you may find this disturbing to watch, but this (older) video highlights the very real and potentially deadly effects of hypoxia on a high altitude jump. In this particular case, all of the skydivers on this plane had varying degrees of hypoxia by the time they climbed to 21,000 feet. Clearly there was miscommunication during the briefing, because the pilot wasn't aware there were two separate O2 valves; one for him and one for the jumpers. That said, the purpose of posting this video isn't to blame anyone, it's to learn about something that is very serious and can affect us without even knowing it. Remember, these jumpers were high as a kite and had no idea what was going on -- one of them even "woke up" mid-freefall without knowing how he got there. Scary stuff.