Carbon dioxide tolerance can be measured by a carbon dioxide discard rate test.

“Then you take a fifth inhale as deep as you can through your nose. Fill your lungs as much as you can, and if you can try and expand make your stomach go out while you do that, that means that your diaphragm has really engaged. So you're inhaling as much as you possibly can. Then hit the timer and your goal is to release that air as slowly as possible through your mouth. So it looks like you have a tiny, tiny little straw in your mouth and you're letting it go. As slowly as you possibly can. Measure what we call the carbon dioxide blow off time or discard rate.”