1. Student Notes

2. Oxygen: The Good and the Bad
Student Notes
Oxygen: The Good and the Bad
Is necessary to sustain life
Too high an oxygen level can be just as harmful as too low
Oxygen is our life-support gas
The primary waste product is carbon dioxide
Oxygen and Metabolism
In your beginning scuba course, your instructor probably said: “At extreme depths, even the oxygen in the air you breathe can become toxic, but this only happens at depths far greater than recreational limits, so don’t worry about it.” End of subject. Now you are learning to dive with oxygen-enriched air, and oxygen toxicity and oxygen safety are very real concerns.
Oxygen is vital to our being. It is our essential life-support element(20). Inhaled oxygen is metabolized in our tissues, producing the energy necessary to sustain life. If we are deprived of oxygen, our survival time is measured in just minutes. Still, our bodies operate well only within a rather narrow range of oxygen partial pressures. Too high an oxygen level can be just as harmful as too low.
Moreover, because oxygen is so highly reactive, supporting combustion and combining aggressively with many substances, special care and precautions are required when handling pure oxygen or gas mixtures that are high in oxygen concentration. A later chapter of this book is devoted to oxygen handling and equipment considerations.
Oxygen and Metabolism
Oxygen is our life-support gas, the gas we extract from air in respiration and use in metabolism to generate heat and energy. It is absorbed through our lungs, combined with the hemoglobin in our red blood cells, and delivered to the tissues via the arterial system. There it is metabolized to produce our life-maintaining energy. The primary waster product is carbon dioxide, which- dissolved in the serum of our blood- is delivered back to the lungs via the venous side of the circulatory system and exhaled.

3. To avoid Oxygen Toxicity we have Partial Pressure Limits
Student Notes
To avoid Oxygen Toxicity we have Partial Pressure Limits
The generally accepted limit for nitrox diving is:
1.4 ata PO2
1.6 ata PO2 as a contingency
1.4 ata PO2 is more than adequate for 99.9% of the dives you may want to accomplish and is the standard at MLML
Oxygen Partial Pressure Limits
Susceptibility to CNS oxygen toxicity varies greatly between individuals and is affected by other factors and conditions as well. One’s oxygen tolerance has also been shown to vary from day to day. It is impossible to predictably relate CNS oxygen toxicity appearance to any definite PO2 and time exposure. Nevertheless, it is certain that the greater the oxygen partial pressure and the longer the time of exposure, the more likely it is that symptoms of CNS toxicity will develop. In setting oxygen exposure limits, it is best to err on the side of safety.
Beginning with its first presentation of oxygen-enriched air for scuba diving in the second edition of the NOAA Diving Manual (1979), NOAA has recommended a maximum oxygen partial pressure of 1.6 ata. In the new fourth edition (2001), they add the cautionary comment: “A slightly lower level provides less oxygen exposure risk.” Diving to a PO2 higher than 1.6 ata has been likened to knocking on the door of a casino. Once you go in, you could win, or you could lose a little, or you could lose a lot, but in the long run the house always wins. For recreational nitrox diving, the generally accepted PO2 exposure limit is 1.4 atmospheres absolute, with 1.6 ata reserved for contingencies. With appropriate selection of your enriched air nitrox mixture, 1.4 ata PO2 is more than adequate for 99.9% of the dives you may want to accomplish.
MLML limit is 1.4

4. Physical Effects of High Oxygen Levels
Student Notes
Physical Effects of High Oxygen Levels
Central Nervous System Toxicity
has a wide range of signs and symptoms, the most dramatic being epilepsy-like convulsions
CNS toxicity can result from relatively short exposures to high partial pressures of oxygen
Pulmonary Toxicity or Whole Body Toxicity
results from prolonged exposure to elevated partial pressures of oxygen (above about 0.5 atmosphere)
not a concern for recreational nitrox diver
Physiological Effects of High Oxygen Levels
Oxygen is essential to us, and it also plays an important role in the treatment of diving maladies such as decompression sickness. Divers planning stage-decompression dives will breathe high concentrations of oxygen during decompression stops in order to offgas nitrogen more quickly, but safely. As nitrox divers, we use oxygen-enriched air to safely prolong our dive time or to increase our nitrogen safety margins. However, because we are utilizing an oxygen-enriched mixture, we must control and monitor the inspired partial pressure of oxygen we are breathing as well as pay attention to the other parts of dive planning. If we did not do so, we could easily get into trouble by diving to depths that allow the oxygen partial pressure in our breathing gas to become dangerously high.
With oxygen and nitrox, it is entirely possible to get “too much of a good thing,” There are two types of oxygen toxicity.
Central nervous system toxicity has a wide range of signs and symptoms, the most dramatic being epilepsy-like convulsions. CNS toxicity can result from relatively short exposures to high partial pressures of oxygen.
The other type is called pulmonary toxicity or whole body toxicity. Pulmonary toxicity results from prolonged exposure to elevated partial pressures of oxygen (above about 0.5 atmosphere). As the name implies, its most pronounced effects are on the lungs, producing signs and symptoms such as chest tightness, breathing discomfort and pain, shortness of breath, and coughing. Development of pulmonary toxicity requires long-term exposures such as encountered in saturation diving, certain military and commercial diving, or recompression chamber treatment.
Pulmonary toxicity is not a concern of the recreational nitrox diver; CNS toxicity is a concern.

5. Central Nervous System Toxicity
Student Notes
Central Nervous System Toxicity
Factors that can increase your susceptibility to CNS oxygen toxicity
heavy exercise, increased carbon dioxide build-up, chilling or hypothermia, and water immersion
One cannot predict oxygen toxicity
Central Nervous System Toxicity
Susceptibility to CNS oxygen toxicity varies greatly between individuals and is affected by other factors and conditions as well. One’s oxygen tolerance has also been shown to vary from day to day. It is impossible to predictably relate CNS oxygen toxicity appearance to any definite PO2 and time exposure. Nevertheless, it is certain that the greater the oxygen partial pressure and the longer the time of exposure, the more likely it is that symptoms of CNS toxicity will develop. In setting oxygen exposure limits, it is best to err on the side of safety.
Beginning with its first presentation of oxygen-enriched air for scuba diving in the second edition of the NOAA Diving Manual (1979), NOAA has recommended a maximum oxygen partial pressure of 1.6 ata. In the new fourth edition (2001), they add the cautionary comment: “A slightly lower level provides less oxygen exposure risk.” Diving to a PO2 higher than 1.6 ata has been likened to knocking on the door of a casino. Once you go in, you could win, or you could lose a little, or you could lose a lot, but in the long run the house always wins. For recreational nitrox diving, the generally accepted PO2 exposure limit is 1.4 atmospheres absolute, with 1.6 ata reserved for contingencies. With appropriate selection of your enriched air nitrox mixture, 1.4 ata PO2 is more than adequate for 99.9% of the dives you may want to accomplish.
Among the many factors that can increase your susceptibility to CNS oxygen toxicity (an “Ox-Tox Hit”) are heavy exercise, increased carbon dioxide build-up from whatever cause, chilling or hypothermia, and water immersion (as opposed to “chamber diving”). One cannot predict oxygen toxicity.

6. Central Nervous System Toxicity continued
Student Notes
Central Nervous System Toxicity continued
Central Nervous System Toxicity
The mnemonic acronym “ConVENTID” is useful for remembering the most obvious of them:
Convulsions
Visual disturbances
Ears
Nausea
Twitching or Tingling
Irritability
Dizziness or Dyspnea
It is also impossible to predict a reliable sequence of toxicity signs and symptoms. The first noticeable sign may be the epilepsy-like convulsions. This may not be serious in itself, but it is most certainly a problem if it occurs at a depth of 30 meters/100 feet while breathing out of a scuba regulator. Drowning is a very likely result. Many preliminary manifestations of CNS oxygen toxicity have been reported, either singly or in combination.
The mnemonic acronym “ConVENTID” is useful for remembering the most obvious of them.
ConVENTID stands for: Convulsions, Visual disturbances, Ears, Nausea, Twitching or Tingling, Irritability, and Dizziness or Dyspnea.

7. Central Nervous System Toxicity continued
Student Notes
Central Nervous System Toxicity continued
Central Nervous System Toxicity
Convulsions are the most obvious and most serious signs.
Possible precursors to convulsions are:
Visual disturbances, tunnel vision, dazzle or seeing “fireflies.”
Ear ringing, tinnitus, or sounds like an approaching train in a tunnel.
Nausea, including vomiting.
Twitching, especially of the lips and small facial muscles or the hands, or tingling (paresthesia) especially in the fingers.
Irritability, restlessness, euphoria, dysphoria (uneasiness or feelings of impending doom), anxiety, or general confusion.
Dizziness and vertigo or dyspnea (difficult or labored breathing).
Convulsions are the most obvious and most serious signs. Possible precursors to convulsions are:
Visual disturbances, tunnel vision, dazzle or seeing “fireflies.”
Ear ringing, tinnitus, or sounds like an approaching train in a tunnel.
Nausea, including vomiting.
Twitching, especially of the lips and small facial muscles or the hands, or tingling (paresthesia) especially in the fingers.
Irritability, restlessness, euphoria, dysphoria (uneasiness or feelings of impending doom), anxiety, or general confusion.
Dizziness and vertigo or dyspnea (difficult or labored breathing).
Other signs can include facial pallor, slowed heart rate (bradycardia), pupil dilation, hiccups, and hallucinations.
Appearance of any sign or symptom of oxygen toxicity is reason to terminate the dive. But, because precursor symptoms are highly variable–as well as subjective–and they may not occur, it is doubly important that divers keep their PO2 exposure within an acceptable limit. In one study, convulsions were the first noticed manifestation in 40% of the subjects studied. In another study, nausea was the most common first manifestation, followed by muscular twitching and vertigo.
If a convulsion were to occur underwater, there is little that can be done until the active phase of the seizure is over and the muscles relax. Muscle contraction may cause the diver to lose the regulator, but the victim also ceases to breathe during the active phase as the vigorous, uncontrolled muscle contractions interrupt breathing and the tongue blocks the airway. No attempt should be made to surface victims of an “Ox-Tox Hit” at this time because they are effectively holding their breath. Because the convulsion was precipitated by breathing a high partial pressure of oxygen, and oxygen tensions in the body are therefore high, the person remains well oxygenated during the convulsion, and hypoxia is not a problem. Carbon dioxide levels will also become very high because the muscles are exercising heavily while the victim is not breathing. When the post-convulsive, resting phase begins, the muscles relax, and the victim remains unconscious. At this point, the victim can be taken to the surface and first aid care begun.

8. Managing Oxygen Exposure
Student Notes
Managing Oxygen Exposure
The best way to avoid oxygen toxicity problems is to stay within correct oxygen exposure limits.
Managing Oxygen Exposure
The best way to avoid oxygen toxicity problems is to stay within correct oxygen exposure limits. As stated above, the generally accepted limit for diving is 1.4 ata PO2, with 1.6 ata PO2 as a contingency. These limits are not lines drawn with a sword in the sand–see the casino comments above. Many divers have dived to 200 feet/61 meters and even considerably deeper on air, and most have returned none the worse for wear. But prudence should be part of all dive planning, and if you have reason to dive beyond recreational limits, you should definitely be preparing for and taking a NAUI trimix training course as well as one in decompression techniques. In deep diving, trimix reduces not only your oxygen exposure but also your nitrogen exposure. (If you want to safely dive deep and remember what you saw and did down there, try trimix.)
The oxygen exposure limits described in this book carry an extremely low risk of oxygen toxicity. They are well below any levels that might reasonably be expected to cause problems.
In addition to a general PO2 limit, NOAA, in the third edition of its Diving Manual, introduced oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata (see Table 3-1). The table shows allowable time for a single dive at any PO2 as well as the maximum accumulated exposure time over any 24-hour period. For example, for a PO2 of 1.0 ata (equivalent to 124 feet/38 meters on air), the maximum dive time for a single dive is 300 minutes (and is the same for any 24-hour period). For a PO2 of 1.4 ata (111 feet/33 meters on EAN32) the time limit for a single dive is 150 minutes (and 180 minutes in any 24-hour period).

9. Managing Oxygen Exposure continued
Student Notes
Managing Oxygen Exposure continued
NOAA Oxygen Exposure Limits
In addition to a general PO2 limit, NOAA has oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata
PO2=1.4 ata 111’ on EAN32
In addition to a general PO2 limit, NOAA, in the third edition of its Diving Manual, introduced oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata (see Table 3-1). The table shows allowable time for a single dive at any PO2 as well as the maximum accumulated exposure time over any 24-hour period. For example, for a PO2 of 1.0 ata (equivalent to 38 meters/124 feet on air), the maximum dive time for a single dive is 300 minutes (and is the same for any 24-hour period). For a PO2 of 1.4 ata (33 meters/111 feet on EAN32) the time limit for a single dive is 150 minutes (and 180 minutes in any 24-hour period).
In all cases, a recreational nitrox diver’s single dive time will be limited by the no-decompression limits as well as the diver’s gas supply. These will be less than the NOAA single dive oxygen exposure limit, and the controlling limit, if it arises at all, would be the 24-hour oxygen exposure limit. Even then, only the most dedicated and determined nitrox diver would possibly exceed the 24-hour limit. (A diver using EAN36 and diving to the maximum dive times allowed by the NAUI EAN36 Dive Tables would have to perform seven square-profile dives to 27 meters/90 feet with dive times of 50 minutes for the first dive and 24 minutes for each repetitive dive with a minimum surface interval of 2 hours 39 minutes between each dive.)

10. Student Notes
Avoiding CNS Toxicity
CNS toxicity is avoided by abiding by easily managed limits.
Remember that the recommended maximum PO2 for recreational nitrox diving is 1.4 atmospheres, with a PO2 of 1.6 atmospheres as a contingency amount.
Plan your dives and choose a nitrox mix that is appropriate to the dive.
Avoiding CNS Toxicity
CNS toxicity is avoided by abiding by easily managed limits. Avoid excessive oxygen partial pressures. Remember that the recommended maximum PO2 for recreational nitrox diving is 1.4 atmospheres, with a PO2 of 1.6 atmospheres as a contingency amount. Plan your dives and choose a nitrox mix that is appropriate to the dive. In the next chapter you will learn how to find the maximum operating depth for any given enriched air nitrox mixture as well as how to determine the optimal mix when you know the planned depth of the dive.

11. Tracking Your Nitrox Cylinder continued
Student Notes
Tracking Your Nitrox Cylinder continued
Filling Out The Logbook
Once you have analyzed your cylinder and labeled it, you will be asked to complete the permanent Fill Station Logbook and sign that you have received the cylinder.
Enter your name, date, your certification, cylinder’s serial number, pressure, oxygen mix, maximum operating depth, signature.
Logbook tracks all nitrox cylinders leaving facility.
Logbook verifies that you either analyzed the contents or knew the particulars of the fill when you received your cylinder.
Filling Out The Logbook
Once you have analyzed your cylinder and labeled it, you will be asked to complete a permanent Fill Station Log and sign-off that you have received the cylinder. This logbook is kept by the facility that filled your cylinder. In the logbook, you will enter your name, the date, your certification, the cylinder’s serial number, the cylinder pressure, the oxygen percentage of the mix, and the maximum operating depth. You will also sign the logbook entry.
The cylinder log is used to keep track of all nitrox cylinders that leave the facility. It also verifies, by your signature, that you either analyzed the contents or watched as a blending technician analyzed them for you, and that you knew the particulars of the fill and its limits when you received the cylinder.
And so, at last, you have it all together, and you are ready to enjoy the world of nitrox diving. Armed with your cylinder of enriched air nitrox–perhaps blended to your personal specifications, your knowledge of the risks, limits, and advantages of the gods’ ambrosia/devil’s gas that you are about to use, an appropriate dive plan, and a compatible dive partner, you head off to the dive site. There you and your partner assemble and don your equipment, rehearse and review your dive plan (including a review of your MOD), mutually check each other’s gear, and begin your dive. And, once you make your descent, it really is “just like breathing air.”