When learning to scuba dive, they caution you not to ever hold your breath - not when at a stable depth, not when descending, and most certainly not when ascend

When learning to scuba dive, they caution you not to ever hold your breath - not when at a stable depth, not when descending, and most certainly not when ascending - because the pressure differential can cause serious injuries. (This goes to the point of "if you run out of air at the bottom and need to make an emergency ascent, hum for the entire trip up to make sure you're breathing out slowly") Now, you have larger pressure differentials when scuba diving - every 10m of depth is an atmosphere of pressure - but the half atmosphere that you get from 10k vs 40k feet is probably still enough to cause some serious issues. (10k feet is ~0.69 atm, while 40k is ~0.19)