Part of the training for a new diver is learning that a major concern with holding your breath is an Arterial Gas Embolism (AGE). As others have posted, most of the time, the effects are fairly immediate (for example, a stroke). Other concerns, like tension pneumothorax, can take hours to develop, but *I THINK* these are less frequent in general. (Not a medic of any form, here.) The medical types will tell you it's hard to diagnose things definitively over the Internet.
The 10' closest to the surface can be the riskiest in general for lung overexpansion injuries if not handled properly (i.e. always keeping you airway open), because the pressure ratio changes the most, and buoyancy is hardest to control. IIRC the record for a medically published lung overexpansion injury was a couple of feet, from someone standing up from the bottom of a pool. My first OW instructor had a saying, "if you hold your breath, 1' might cause problems. 3' probably will."
If you have further questions, you may want to ask a Moderator to move this thread to the
Dive Medicine forum, where several diving doctors hang out, and would be more likely to see your posts.
BTW, if you do see a doctor, most doctors and ERs don't have much knowledge about diving accidents and hyperbaric medicine. However in an emergency or semi-emergency, I believe DAN is happy to talk to you whether you're a member or not. They also have a referral list of local doctors that have knowledge of dive medicine. I think DAN even has a program where OW students can get free temporary memberships, by having their certified instructor filling out some form on their web site.
