Scene Context: Les wakes up in the medical bay of the Event Horizon and breathes through a mask because the ship's atmosphere consists of 100% nitrogen at a pressure of about half an atmosphere.
Essence of the Phenomenon: Nitrogen itself is not a breathing gas, but it functions well as an inert environmental filler. Such an atmosphere drastically reduces the fire risk in the event of a hull breach or a spark; however, without a dedicated oxygen supply, a human will quickly lose consciousness from hypoxia. NASA explicitly treats spacecraft atmospheric composition as a compromise between human respiration, fire safety, and operating pressure.
Current Limitations: A completely nitrogen-based spacecraft with personal oxygen life support for all crew members is not a modern spacecraft standard, but a rigid future engineering solution.