Originally Posted by TQA
Do not be tempted to extend a snorkel as you can not clear your exhaled CO2 from the tube.
Not a problem. I sell an Oxygen analyzer to scuba shops as part of some of my compressor stuff. One of my dive shop customers did a check to see how much oxygen was left in an exhaled breath. Normal air has approximately 21% O2. Our exhaled breath came in at about 19% which makes a lot of sense. After all, we blow our exhaled breath into the lungs of others when doing mouth to mouth!
This whole thing has nothing to do with the resistance of air flow in a long snorkel or breathing your exhaled breath. It is all about the difference in pressure on the lungs. You just can't breathe in when water pressure is squeezing your body.