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        If you ask the typical non diver what the hazards of scuba are, youíre likely to get answers that range from man-eating great white sharks the size of small cruise ships to murderous bands of rogue killer jellyfish.  This strange idea that every creature that lives in the sea is bent on eating every human silly enough to enter the water is a misconception. Generally speaking, if you donít harass the undersea creatures, theyíll leave you alone. The true hazards of scuba are overexpansion injuries, drowning, and decompression sickness.  Although these hazards are very dangerous, once you understand them, they are very easy to manage.

         Overexpansion injuries. Overexpansion injuries are caused by holding your breath during ascent.  If youíll recall Boyleís Law, volume varies inversely with pressure. That means if you take a breath while diving on scuba then ascend, the air in your lungs will expand as the pressure on your body from the water decreases. If you hold your breath, this expanding air has nowhere to go, so it ruptures the alveoli of the lungs and gets into places it isnít supposed to be.  Common symptoms of overexpansion injuries include numbness, loss of hearing, vision, or speech, chest pain, difficulty breathing, unconsciousness, and death.  Overexpansion injuries can manifest themselves in four ways.
         1.  Arterial Gas Embolism (AGE).  In this case, air gets into the bloodstream and works its way to the small capillaries of the brain where they block the blood flow.  The result is a stroke. This can result in paralysis, brain damage, and even death.
         2.  Mediastinal Emphysema.  This condition results when air collects in the cavity surrounding the heart. This puts pressure on the heart and can result in heart failure.
         3.  Pneumothorax. Pneumothorax occurs when air gets in the pleural cavity on the outside of the lungs. This condition causes chest pain and can result in collapsed lung(s).
         4.  Subcutaneous Emphysema. This one is the least dangerous of the four manifestations of overexpansion injuries.  In this case, air collects in the subcutaneous tissues usually around the shoulders and neck. The skin is itchy and spongy.
         Obviously, none of these conditions is desirable. Fortunately, the risk of overexpansion injury is very easy to manage.  Donít hold your breath. Breathe continuously.  Since most people have been breathing fairly regularly since the day they were born, this skill should require very little effort to master.  Just to make sure this is clear, Iíll say it again.  To avoid overexpansion injuries, donít hold your breath.
         Drowning.  In scuba, drowning occurs when a diver runs out of air at depth. Like overexpansion injuries, drowning is completely avoidable.  An important piece of gear that makes up the scuba unit is the submersible pressure gauge (SPG).  The SPG tells you how much air you have left. Look at it every now and then. There are even some computers that will tell you how long the air will last based on your current rate of air consumption. Of course, even those wonít help if you donít occasionally look at them.  In addition to the SPG, you either have a buddy or special training and equipment for solo diving. If you have a buddy, you can share air and ascend safely to the surface. If you have training and equipment for solo diving, you have a redundant air source you can switch to and ascend safely to the surface.  The risk of drowning is very easy to manage.  Make sure you have the proper equipment and training and look at your computer/SPG regularly.
         Decompression Sickness (DCS). DCS is a result of staying too long for the depth and omitting a required decompression stop or ascending too fast. Henryís Law is the culprit in this case.  At depth your body is under pressure.  According to Henryís Law, under pressure, gases become more soluble in liquids. That being the case, nitrogen absorbs into the tissues. When you ascend, you reduce the pressure allowing the nitrogen to dissolve in the tissues. The nitrogen wants to come back out. If you stay too deep too long and absorb too much nitrogen to be safely off-gassed during a normal ascent or if you ascend too fast, the nitrogen comes out in the form of bubbles. These bubbles collect in the tissues, joints, under the skin, even possibly against the spinal column. The symptoms of DCS include rash, difficulty breathing, chest pain, joint and/or muscle pain, numbness, paralysis, and unconsciousness.
         While DCS isnít as dangerous as overexpansion injuries or drowning, avoiding it isnít as cut and dry. You can do everything right and still get DCS.  An undeserved DCS hit is rare, but not impossible.  There are many factors that affect your susceptibility to DCS. Those factors include age, physical condition, dehydration, exhaustion, sickness, stress, and drug or alcohol use. In addition, the no decompression limits are based on a theoretical model, not the specific diver using them. To avoid DCS, monitor your depth and time and stay within the no decompression limits, ascend no faster than 30 feet per minute, make a safety stop at 15 feet for 3-5 minutes and donít push the limits.