Okay, let me back up and deal with some misinformation here.
One -- exhaling alone does not decrease the CO2 in your blood. The blood is coming back to the lungs and CO2 is diffusing into the gas that's in the alveoli. As the CO2 does this, the CO2 concentration in the alveoli increases, and the rate of diffusion slows. When you exhale, all you do is reduce the volume in your bronchial tree -- you do nothing to change the concentration of the gases there. In the absence of fresh gas to inhale, the CO2 will continue to climb. However, there is a psychological relief to exhaling, and it is necessary because excess volume will cause trauma to the lungs, as the gas in them expands on ascent...
Nobody said that exhaling
alone decreased CO
2 blood levels. Exhaling as atmospheric pressure reduces also reduces the number of CO
2 molecules in the in the lungs compared to a static breath holding condition. Therefore the diffusion rate increases over what it would have been at higher concentrations.
... Second, you can black out from CO2, but it takes high concentrations to do it. I have seen patients with three times the normal amount of CO2 in their blood, who were still conscious. A normal person without lung disease would be in violent panic far before they got to those kinds of CO2 levels…
Do you think those patients might black out at say 99' with 12 times normal PPCO
2 -- 4 ATA x 3x normal CO
2? Blackout aside, panic induced drowning dooms far more recreational divers anyway, long before anoxia does them in.
... Rebreather divers have the risk of getting there, because no matter how much they breathe, if the scrubber isn't removing CO2, neither will they...
True, but they rarely do (blackout from CO
2). Generally speaking, they are well trained to recognize the symptoms and react accordingly -- going open circuit on their bailout. It is not like their symptoms are subtle! Hypoxia and anoxia nails a lot more of them since detection is entirely instrument dependent.
... Very deep divers combine an element of narcosis with CO2 retention, and can and do tolerate enough CO2 to lose consciousness. When you are talking about recreational scuba depths, that's just not going to happen...
That is a highly questionable hypothesis. Several diving disciplines develop a mental and physical tolerance to CO
2. Narcosis also numbs people's senses in many and varying ways. The phenomenon is most studied, at the moment, among world class apneists (deep freedivers), but was recognized in military and commercial divers in the 1960 before widespread use of helium essentially eliminated severe narcosis from diving profiles. CO
2 blackout was also the "presumed" cause of several deaths before the mid-1960s when regulator performance was marginal and dead spaces in full face masks and regulators was huge (especially double hoses before inhalation & exhalation valves were introduced). Sure the PPCO
2 concentrations have to be high for blackout, but it is not that difficult under heavy workloads and more than 6 atmospheres in this old gear to reach that point. I have experienced it myself on an instrumented test dive in a wet pot.
... Shallow water blackout from low O2 levels, however, IS a real phenomenon. It is unlikely to occur, however, during any length of time when the average, untrained person is able to go without inhaling....
You're right, shallow water blackout is not a factor here. It IS a very big deal for freedivers who are not well oxygenated when they leave the bottom and are skilled at suppressing CO
2 symptoms.
The real point here is every sane diver agrees that it is stupid to attempt an emergency free ascent when you are not skilled at it. This discussion is about developing the skill and indirect benefits that will, IMHO, make anyone a more competent diver.
There is nothing unsafe about practicing free ascents as long as you keep the airway open, don't get bent, and have your regulator ready to stuff back in your mouth. It is silly to discourage or deny this self-rescue tool to the diving community. It just is not as hard as people try to make it out to be, as anyone who actually has tried it can attest.
Start slow, gradually increase depth until you are satisfied or have found your personal limits.