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        Boyleís Law. Boyleís Law states that the the Pressure of a gas is inversely proportional to volume at constant temperature.  In English, that means if you take a flexible container (a balloon, perhaps) full of a gas and put it under pressure (squeeze it), it gets smaller. In fact, if you double the pressure on the flexible container, it will shrink to half the size.  On the flip side of that, if you release the pressure on a gas-filled flexible container, it gets bigger.  Cut the pressure in half and the flexible container will expand to twice the size.  The change in size of the flexible container is not due to a change in the amount of gas (number of molecules) it contains.  The gas inside becomes more or less dense in response to pressure changes.  If you happen to be a physics brainiac and donít understand the plain English, let me help: P1 X V1 = P2 X V2.
         The nature of gasses described by Boyleís Law is the most dangerous aspect of scuba diving. No matter what agency you certify through, the number one rule will be the same. Never hold your breath.  Breathe continuously.  Iíll explain why. As you go deeper, you subject your body to more and more pressure. For every 33 feet of sea water or 34 feet of freshwater, you apply an additional atmosphere of pressure (14.7 psi) to your body.  So, at 33 feet of sea water (fsw), your body is under twice as much pressure as it is at sea level. At 66 fsw, you are under 3X as much pressure, and at 99 fsw, you are under 4x as much pressure.  So what?  Why is that a problem?  Well, at this point, it really isnít. Thatís all about to change. 
         Letís take the balloon from the previous paragraph. We fill it on the surface then take it to 33 fsw.  Since the balloon is under twice as much pressure, it is 1/2 the size and the air inside it is twice as dense. We take it to 66 fsw. The pressure is 3X, the volume is volume is 1/3, and the air inside is 3X as dense.  At 99 fsw, pressure is 4X, volume is 1/4, and air density is 4X.  Again, whatís the problem?  Weíre almost there. Letís take that balloon down again, but this time, letís keep adding air in it to maintain the original volume.  At 33 fsw, pressure is 2X and the density of the air inside is 2X so we had to double the amount of air in the balloon to maintain a constant volume. At 66 fsw, pressure is 3X, density is 3X, and we added enough air to maintain constant volume. The balloon now contains 3X as much air as it did on the surface.  At 99 fsw, pressure is 4X, density is 4X, and the amount of air required to maintain constant volume is 4X. So, in order to inflate the balloon to the same volume requires 4X more air at 99 fsw than it does at sea level. Letís tie off the balloon and let it go to the surface.  The pressure on the surface is 1/4 what it is at 99 fsw.  The density of the air in the balloon expands to 1/4, so the volume of the balloon must expand by 4X.  If that balloon was fully inflated at 99 fsw and it expands by 4X by the time it reaches the surface, what do you think might happen to that balloon?  Right.  Itís bad news for the balloon. 
         Now, letís say that balloon is your lungs which you keep inflating with denser and denser air as you descend by breathing.  What happens if you hold your breath while you ascend?  That expanding air has no where to go so your lungs become over inflated and you get an overexpansion injury and air goes to places it isnít supposed to be.  Overexpansion injuries are extremely dangerous and are discussed in more detail in the Diving Injuries section. If you breathe continuously, that expanding air has a way to escape and doesnít cause the injury.  In short, DONíT HOLD YOUR BREATH!