While you are skin diving, your lungs do not usually experience any harmful effects from decreasing pressure changes when you are holding your breath.
As you take a breath and descend, the increasing water pressure compresses the air in your lungs, which re-expands during your ascent so that when you reach the surface your lungs return to approximately the original volume.
However, when you are scuba diving, the situation is very different, mainly because self-contained underwater breathing apparatus and its related equipment delivers air that is equal to the surrounding water pressure - ambient pressure.
Because your lungs will be at their normal volume at depth, the air will expand as you ascend.
This is why the most important rule in scuba diving is to 'breathe continuously and never hold your breath, especially when ascending.
Some lung overexpansion symptoms - as a result of some forms of lung overexpansion injuries.
Breathing normally through scuba keeps the airway to your lungs open which allows any expanding air to escape so your lungs keep their normal volume.
If you were to hold your breath and block your airway during an ascent, your lungs would over-expand and possibly cause a lung rupture, similar to an over filled balloon.
Lung over-expansion injuries are among the most serious of scuba diving injuries because air can enter the bloodstream and chest cavity leading to paralysis and death in some cases.
The tendency for learners to hold their breath should be avoided because even slight effects from decreasing pressure changes that happen in just a few meters of water can cause injuries. This is possibly the most dangerous example of all scuba bad habits.
Student divers can easily avoid life-threatening lung expansion injuries by continually breathing and not holding your breath.
Scuba Instructors reaffirm proper breathing techniques for novices taking their first scuba lessons by having them blow a tiny stream of bubbles whenever the student removes the regulator from their mouth.
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