3-46 U.S. Navy Diving Manual — Volume 1 may be more inclined to notice warning symptoms during subse quent exposures to oxygen. 3‑9.2.2.4 Prevention of CNS Oxygen Toxicity. The actual mechanism of CNS oxygen toxicity remains unknown in spite of many theories and much research. Preventing oxygen toxicity is important to divers. When use of high pressures of oxygen is advantageous or necessary, divers should take sensible precautions, such as being sure the breathing apparatus is in good order, observing depth ­ time limits, avoiding excessive exertion, and heeding abnormal symptoms that may appear. Interruption of oxygen breathing with peri odic “air” breaks can extend the exposure time to high oxygen partial pressures significantly. Air breaks are routinely incorporated into recompression treatment tables and some decompression tables. 3-9.3 Decompression Sickness (DCS). A diver’s blood and tissues absorb additional nitrogen (or helium) from the lungs when at depth. If a diver ascends too fast this excess gas will separate from solu tion and form bubbles. These bubbles produce mechanical and biochemical effects that lead to a condition known as decompres- sion sickness . 3‑9.3.1 Absorption and Elimination of Inert Gases. The average human body at sea level contains about 1 liter of nitrogen. All of the body tissues are saturated with nitro­ gen at a partial pressure equal to the partial pressure in the alveoli, about 0.79 ata. If the partial pressure of nitrogen changes because of a change in the pressure or composition of the breathing mixture, the pressure of the nitrogen dissolved in the body gradually attains a matching level. Additional quantities of nitrogen are absorbed or eliminated, depending on the partial pressure gradient, until the partial pressure of the gas in the lungs and in the tissues is equal. If a diver breathes he­ lium, a similar process occurs. As described by Henry’s Law, the amount of gas that dissolves in a liquid is almost directly proportional to the partial pressure of the gas. If one liter of inert gas is absorbed at a pressure of one atmosphere, then two liters are absorbed at two atmo­ spheres and three liters at three atmospheres, etc. The process of taking up more inert gas is called absorption or saturation. The pro­ cess of giving up inert gas is called elimination or desaturation. The chain of events is essentially the same in both processes even though the direction of exchange is opposite. Shading in diagram ( Figure 3 ­ 16 ) indicates saturation with nitrogen or helium un­ der increased pressure. Blood becomes saturated on passing through lungs, and tissues are saturated in turn via blood. Those with a large supply (as in A above) are saturated much more rapidly than those with poor blood supply (C) or an unusually large capacity for gas, as fatty tissues have for nitrogen. In very abrupt ascent from depth, bubbles may form in arterial blood or in “fast” tissue (A) even through the

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