There are many considerations when it comes to safety although let’s start with the cardinal rule of diving. Never hold your breath when diving. Let us now explore this in a little detail so that you have a full appreciation for what it entails. The medical reason why we should never hold our breath is to avoid an Arterial Gas Embolism and is caused by the rapid expansion of gasses during ascent. For the purpose of this exercise, we won’t get overly technical.
To understand this rule we first must appreciate the atmospheric changes that occur at depth. On the surface, the air you breathe it 1 atm. Now as you descend the pressure increases, so at 10 meters, its 2atm, 20 meters 3atm and so on. What this means is that the air you are breathing at depth is denser or more compressed.
In your tank or cylinder, your will have compressed air. The first stage converts this air to an intermediate pressure and the second stage converts it to ambient pressure. This means that if you are at 20 meters depth you will be breathing air that is 3 times denser than on the surface.
So what would happen if we are at 20 meters and we take a big breath and fill our lungs? Then we swim up to 10 meters, the ambient pressure reduces so the air in our lungs will expand. If we continued to hold our breath and swim back down to 20 meters the air would compress again. The problem is if we hold our breath and ascend with the air expanding, it will want to escape our lungs, so it will push into our bloodstream and cause an embolism. This is extremely dangerous and often fatal.
When you conduct you open water training, your instructor will often reinforce this point by making you blow bubbles when you take the regulator out of your mouth when performing a skill.