"Air is an intoxicating cocktail. Many people will tell you they enjoy the buzz of going deep on air. This feeling is partly a response to their subconscious awareness that they are no longer in complete control of their surroundings and that, if something were to go wrong, they may not be thinking clearly enough to handle it: something like the thrill of riding a roller-coaster. However the feeling is also due to anesthetic potential, a dangerous property of all gases. In the right quantity, any gas can knock you out. Some gases, like nitrous oxide (laughing gas) have long been known as excellent anesthetics. The major constituent gas in air, nitrogen, has anesthetic potential and, as you dive deeper and as the partial pressure of the gas increases, the depressant effect on your Central Nervous System becomes greater.
"Every year, a lot of people die diving deep on air. Many of the victims are expert divers, instructors, and instructor trainers; even respected authors of scuba-diving books! Yet the practice continues amid almost universal ignorance of a simple truth; survival is not a question of your courage or your competence as a diver. The risks you take in diving deep on air are all bound up with human physiology itself and none of us can beat that."
Adapted from Scuba Confidential by Simon Pridmore © used with permission.