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3-22 NOAA Diving Manual chest pain or discomfort, coughing, inability to take a deep breath without pain or coughing, a development of fluid in the lungs, and a reduction in vital capacity. Non- pulmonary symptoms of “whole-body” oxygen toxicity include skin numbness and itching, headache, dizziness, nausea, effects on the eyes, and a dramatic reduction of aerobic capacity during exercise. 3.3.3.3.3 Variations in Tolerance There is wide variation in susceptibility to oxygen tox- icity among individuals, and a significant variation in a single individual at different times. Part of this latter varia- tion is due to unknown causes, but a large part can be attributed to known environmental and physiological cir- cumstances. Susceptibility to CNS toxicity is increased by certain factors, particularly those that cause an increase in internal PCO 2 , such as exercise, breathing dense gas, or breathing against a resistance. Immersion, dramatic changes of temperature, and physical exertion also increases ones susceptibility to CNS oxygen toxicity. These differences make it difficult to predict the occur- rence of CNS oxygen toxicity. 3.3.3.3.4 Benefits of Intermittent Exposure Oxygen poisoning can be reduced or postponed by interrupting the exposure time (U.S.Navy Diving Manual 1999). If “breaks” in periods of low oxygen are taken dur- ing oxygen breathing, tolerance is greatly improved. In the U.S.N. tables for treatment of decompression sickness using oxygen, breaks of five minutes of air breathing are taken every 20 or 30 minutes of oxygen breathing at high PO 2 levels. This avoids oxygen convulsions in all but very rare cases and also postpones pulmonary toxicity. In situa- tions where supplemental oxygen or high oxygen content mixtures are used for decompression, it is strongly recom- mended that a five minute “air” break be taken every 20 minutes to minimize the risk of oxygen poisoning. 3.3.3.3.5 Concepts of Oxygen Exposure Management The traditional method used for prevention of CNS oxy- gen toxicity is to stay within exposure durations that are based on the oxygen level, or PO 2 , to which the diver is exposed (U.S.Navy Diving Manual 1999). These limits allow a cer- tain time at each PO 2 range. Such an approach has been practiced by the U.S. Navy and by NOAA for many years in their procedures for mixed gas and oxygen diving. As with decompression, a limit appears to be imple- mented as if it were a solid line dividing “no problems” from “guaranteed problems.” Actually, a limit is a solid line drawn through a wide gray area of gradually increas- ing risk. The limits given here and in other limit-based algorithms (such as a decompression table) are recom- mended guidelines for use under normal conditions. They have been proven in practice. They work for most people most of the time, but they are not guaranteed to work for all people all of the time under all circum- stances. They may need to be more conservative when conditions are more stressful. Diving with procedures described in this chapter imposes a relatively low risk of oxygen toxicity. The expo- sures are short and outside the limits that are expected to cause problems. 3.3.3.3.6 Prevention of CNS Poisoning With the help of experts, NOAA developed estimated oxygen exposure limits that were published in the 1991 version of the NOAA Diving Manual . These limits are shown in Table 3.4. They are intended for a diver doing dives for research, sampling, inspection, observation, and light to moderate work at the higher PO 2 levels.