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This means that after any dive, it takes six hours for the 60-minute compartment to return to its starting amount of nitrogen, and 12 hours for the 120-minute half- time compartment to return to starting pressure—equilibra- tion with ambient pressure on land. From another perspective, oxygen can significantly enhance decompression. Decompression requirements are dictated by the on-gassing of inert gases. By breathing 100 percent oxygen, the inert gas gradient is significantly increased, thus increasing inert gas elimination from the body. For example, pure oxygen can be used to shorten decompression on the 20 and 10 fsw stops. In addition, high oxygen content mixtures can also be used to shorten decompression from the 30 fsw stop and deeper. Mixes rich in oxygen have proven to substantially improve decompression outcome when used as a supplemental decompression gas from both air and nitrox dives. 3.3.4.2 Decompression Sickness Decompression sickness (DCS, also known as “the bends”) is the result of inadequate decompression follow- ing exposure to increased pressure. During a dive, the diver’s body tissues absorb nitrogen from the breathing gas in proportion to the surrounding pressure. If the diver ascends too quickly, the nitrogen may come out of solu- tion and form bubbles in the body’s fluids and tissues. WARNING ALTHOUGH DECOMPRESSION SICKNESS MAY OCCUR AS A RESULT OF VIOLATING ACCEPTED SAFE DIVING PRACTICES, IT CAN ALSO OCCUR EVEN WHEN THE ACCEPTED GUIDELINES ARE BEING FOLLOWED PRECISELY. Bubbles form after many dives, often with no symp- toms; these are called silent bubbles. It’s probably not true that asymptomatic bubbles form after every dive; however, they are not uncommon. Bubbles cause dam- age in several ways: they can block blood and lymph cir- culation, depriving vital areas of oxygen and waste removal; extravascular bubbles can compress and stretch blood vessels and nerves creating pain, damage, and dis- ability; as foreign invaders to the body, bubbles can pro- voke a cascade of defenses including blood coagulation, release of vasoactive substances from cells lining the blood vessels, and the body’s immune system reacts by coating the bubbles with lipoproteins, which then dena- ture and release circulating fat emboli. Bubbles do not pass from body tissues into veins, unless the veins are already torn. Bubbles, even though tiny, are too big to

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