What should I do if I run out of air?

If you run out of air, you will need to operate according to certain emergency procedures.  First, stay calm and think.  You will ascend normally and signal to your buddy that you are out of air.  Then you will use your buddy's alternate air source (AAS) while ascending.  If you cannot use the AAS, then you can share your buddy's regulator.  If you cannot locate your buddy and you are on your own, you will need to perform a Controlled Emergency Swimming Ascent (CESA).  You will take a deep breath and slowly ascend, making an ahhhhh sound as you do so that you will take a long time to exhale.  This will allow you to get to the surface.  If you are too deep for a CESA, then you may have to make a emergency buoyancy ascent and this should be a last resort.  This means that you take off your weights and float to the surface with arms and legs spread to slow the ascent and all the while making the ahhhh sound as you breathe out.