I just finished my scuba certification, and they told us the number one rule of scuba diving is to *never* to hold your breath. Even when you’ve lost your regulator, you’re supposed to breath out a small stream of bubbles.

Is this only because of the overcompression injuries you can get if you hold your breath and rise (which allows the air to expand/decompress, potentially rupturing a lung, etc.) or are there other reasons?
Okay people, enough with the grouchy "you should know this".

First, if it’s just a matter of not holding your breath when you *rise* I totally get the whole "expanding air" / "ruptured lung" thing (reread my original question… I state it right there).

My questions is why you should not hold your breath when you’re stationary (not rising or sinking), such as during the exercises where do you regulator recovery (sweep and reach methods). It feels more natural to hold my breath during that, but the course instructions say never hold your breath; hence my question.

It’s just a bad habit to get into that you might do on ascent is all and it’s the primary reason why they tell you never to do it. If you’re conditioned to never hold your breath, chances are, you won’t be doing it when it’s really important, like an emergency swimming ascent. Don’t get me wrong, tons of divers do do it, it’s called skip breathing. It’s a way to wrangle a little more TBT out of your tank by reducing your gas consumption a bit. PADI and the rest don’t want you skipping because that could lead you to believe it was ok to do it. It’s not really and for the reason I mentioned above. Mental conditioning.