spearing mostly...just thought it could be a safety measure to prevent swb and make you more comfortable while down...just thought it to be an interesting topic.
This is a good question. I wish I could answer it.
I have oxygen here on the farm that we use for shipping baby shrimp. It would be interesting to just lie on my couch and compare, breathing up with just air, and then trying with....say just 4 breaths of pure o2, and see it there is a big increase.
As mentioned, the difference is when you dive down. Even at 40 feet you've more than doubled the parrtial pressure of O2 in your lungs,( which is already 400% higher than normal from breathing pure O2) and this could cause physiological problems in O2 absorbtion.