[–] KeinPolizist 0 points 2 points 2 points (+2|-0) ago (edited ago)
Yes, the first 33' feet underwater are the most dangerous when breathing compressed air, as far as expansion goes. At the surface you are under one atmosphere of pressure (which if I remember correctly is 14.7 PSI). For every 33' you descend, you are adding another atmosphere of pressure. This pressure compresses the air you are breathing. At 33', a breath of air will take up 50% the volume it would at the surface, meaning if you were to take a breath at that depth and hold it and swim to the surface the air in your lungs would double in size, and burst your lungs.
At 15' the air is compressed to 75% of what it would be at the surface, meaning when it expands the air will take up 1.5 times the volume it did at the original depth. That's still more than enough to cause major injury.
The farther down you go the gas becomes more compressed and changing elevations has a less dramatic effect on the gases volume. The closer you are to the surface, the more pronounced the effect is, and that's when its most dangerous.
This is why one of the first things you learn in scuba is to never hold your breath.