The 16% figure you mention comes from the idea that, during normal respiration, the oxygen percentage in exhaled air is roughly that.
I always wondered what the origin of the 16% limit was. Thanks.
The saturation diving community has to be careful with this; maybe some saturation rules crept into your friends' safety consciousness.
@Akimbo probably has some good stories about this.
I don't know about how "good" the stories are but some are very illustrative. The first I learned of the virtually instant blackout that occurs when exposed to anoxic and near-anoxic environments was on the US Navy's Mark II Deep Dive System that was built to support SeaLab III. Washington engineers, in their infinite wisdom and general lack of diving expertise, routed 440 VAC inside the PTC (diving bell). Obviously this is very dangerous on many levels so they figured their butts were covered by protecting all electrical systems with a very low pressure helium purge... which leaked many times faster than expected. In practice, people would forget to turn the pure Helium supply off after dives and testing, especially when it was in the shipyards for overhauls.
The bell would fill with pure helium, displacing the air down the bottom hatch. The stories were that more than one "yardbird" (shipyard worker) climbed into the bell and fell back out to the deck plates before they could get more than halfway into the hatch. I came onboard months after it left the yards but these stories were part of my classroom sat training. The diving medical officers didn't have an explanation why they blacked out so fast. You would think there would be enough Oxygen in the bloodstream and tissues to keep you conscious for 30 seconds to a minute.
This phenomenon was clearly demonstrated to me during pre-dive checks that included testing every BIBS masks -- the oral-nasal masks hanging in every lock for emergency and treatment gasses. The plumbing systems in this particular chamber still contained deep mix; 1% in this case for a 0.3 PPO
2 at 850'. I was going through the checklist, pressed the first BIBS mask against my face, and that is the last I remember until I woke up flat on the deckplate with a knot on the back of my head. Fortunately, the training included "never strap the mask on your face until you analyze the gas". I would be dead if the mask didn't fall off on my way down. I was too embarrassed to report it since I knew better.
I couldn't remember any warning or even exhaling before going black. The closest explanation I have heard is the progression of symptoms relating to shallow water (hypoxic) blackout in a PFI freediving class. Basically, less critical functions are progressively shut-down as the Oxygen level diminishes. It makes sense but you have to wonder what environmental conditions would have selected these traits in human evolution. Apparently the reflex to breathe from CO
2 buildup continues to function long after blackout occurs.
I have written to various state's Attorney Generals over the years suggesting pure Nitrogen as an available, humane, and reliable means of execution. My first choice is to be shot by a jealous husband at 90, but anoxia is my distant second choice... not that I actually get to choose.
