Why is this rule important ? For one, If you hold your breathe while ascending you could die. But why ? What happens ? If you hold your breath you create a "closed bag" if I may with your lungs. No place for air to go. As you ascend your lungs will increase in size by nature of the ascent. You could get a lung rupture as a result. This rupture forces bubbles into the bloodstream, blocking blood flow to the brain and other parts of your body, leading to paralysis, serious injury, or death. If you hold your breath for even 1 meter or a few feet, your lungs could over expand, just like the small bag.
If you feel discomfort in your ears while ascending, stop, go down a few feet and wait for it to clear. Afterwards, continue to ascend. Remember it is never a good idea to dive while having a cold, and on decongestants. They could wear off, trapping air in the ears and sinuses. If they do wear off you could get a "reverse Block."
Now the S.A.F.E. part. This stands for Slowly Ascend From Every Dive.
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This is really the PADI motto, so to speak. But how ? The rate of ascent should be no faster than 60 feet per minute, but slower is better. Your body needs time to adjust to the changing pressures, time to regulate your buoyancy, keep track of your buddy, and to watch for obstructions.
The best way to ascend is to use a dive line if diving from a boat, or follow the bottom contour, to make a slow trip to the surface. Make sure you start up with plenty of air.
One last note, it is always a good idea to stop your ascent at 15 feet for 3 minutes, again to offset more pressure gases. This is known as a safety stop..
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Think of the 60 feet per minute as a speed limit. It is fine to go slower, slower is better. Be a S.A.F.E. diver: Slowly Ascend From Every dive