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CHAPTER 3 — Underwater Physiology and Diving Disorders 3-43 exposure time, the more likely CNS symptoms will occur. This gives rise to partial pressure of oxygen ­ exposure time limits for various types of diving. 3‑9.2.2.1 Factors Affecting the Risk of CNS Oxygen Toxicity. A number of factors are known to influence the risk of CNS oxygen toxicity: Individual Susceptibility . Susceptibility to CNS oxygen toxicity varies markedly from person to person. Individual susceptibility also varies markedly from time to time and for this reason divers may experience CNS oxygen toxicity at exposure times and pressures previously tolerated. Individual variability makes it difficult to set oxygen exposure limits that are both safe and practical. C O 2 Retention . Hypercapnia greatly increases the risk of CNS toxicity probably through its effect on increasing brain blood flow and consequently brain oxygen levels. Hypercapnia may result from an accumulation of CO 2 in the inspired gas or from inadequate ventilation of the lungs. The latter is usually due to increased breathing resistance or a suppression of respiratory drive by high inspired ppO 2 . Hypercapnia is most likely to occur on deep dives and in divers using closed and semi ­ closed circuit rebreathers. Exercise . Exercise greatly increases the risk of CNS toxicity, probably by increasing the degree of CO 2 retention. Exposure limits must be much more conservative for exercising divers than for resting divers. Immersion in Water . Immersion in water greatly increases the risk of CNS toxicity. The precise mechanism for the big increase in risk over comparable dry chamber exposures is unknown, but may involve a greater tendency for diver CO 2 retention during immersion. Exposure limits must be much more conservative for immersed divers than for dry divers. Depth . Increasing depth is associated with an increased risk of CNS toxicity even though ppO 2 may remain unchanged. This is the situation with UBAs that control the oxygen partial pressure at a constant value, like the MK 16. The precise mech­ anism for this effect is unknown, but is probably more than just the increase in gas density and concomitant CO 2 retention. There is some evidence that the inert gas component of the gas mixture accelerates the formation of damaging oxygen free radicals. Exposure limits for mixed gas diving must be more conservative than for pure oxygen diving. Intermittent Exposure . Periodic interruption of high ppO 2 exposure with a 5 ­ 15 min exposure to low ppO 2 will reduce the risk of CNS toxicity and extend the total allowable exposure time to high ppO 2 . This technique is most often employed in hyperbaric treatments and surface decompression. Because of these modifying influences, allowable oxygen exposure times vary from situation to situation and from diving system to diving system. In general, closed and semi ­ closed circuit rebreathing systems require the lowest partial pres­ sure limits, whereas surface ­ supplied open ­ circuit systems permit slightly higher

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