Oxygen Toxicity Limits & Symptoms
Oxygen toxicity limits can be very confusing, especially for PPO2 (Partial Pressure of Oxygen) levels above 1.6 ATA used in chamber-based hyperbaric treatment (recompression) and decompression tables. For example, here is a chart of one of the most common DCS (Decompression Sickness) treatment tables. Note that the PPO2 of pure oxygen at 60'/18.3M is 2.82 ATA — or more than twice the normal limits recreational divers observe.U.S. Navy Diving Manual, Revision 7A, 30 April 2018.
Figure 17-4. Treatment Table 5, Page 17-43 (Page 899 in Acrobat file)
Some confusion comes from recreational diving courses that only teach the minimum subset necessary for that specialty. Hopefully this, plus contributions from other ScubaBoard members, will provide a more complete understanding.
“ Why should anyone use high oxygen levels and risk oxygen toxicity? ”
The "simple answer" for divers is twofold: Rapid removal of diluent gas (nitrogen and/or helium) from the body and reducing diluent gas absorption. Hyper-oxygenation can be the objective for non-diving HBOT (HyperBaric Oxygen Treatments) for CO poisoning, gangrene, burns, etc.
The following is an excerpt from the US Navy Diving Manual, with the following modifications:
I accentuated selected text from the manual with Bold to emphasize especially important points.
Akimbo:
U.S. Navy Diving Manual, Revision 7A, Volume 1, 30 April 2018, starting on Page 3-42 or Acrobat Page 200
3-9.2 Oxygen Toxicity. Exposure to a partial pressure of oxygen above that encountered in normal daily living may be toxic to the body. The extent of the toxicity is dependent upon both the oxygen partial pressure and the exposure time. The higher the partial pressure and the longer the exposure, the more severe the toxicity. The two types of oxygen toxicity experienced by divers are pulmonary oxygen toxicity and central nervous system (CNS) oxygen toxicity.
3‑9.2.1 Pulmonary Oxygen Toxicity. Pulmonary oxygen toxicity, sometimes called low pressure oxygen poisoning, can occur whenever the oxygen partial pressure exceeds 0.5 ata. A 12 hour exposure to a partial pressure of 1 ata will produce mild symptoms and measurable decreases in lung function. The same effect will occur with a 4 hour exposure at a partial pressure of 2 ata.
Long exposures to higher levels of oxygen, such as administered during Recompression Treatment Tables 4, 7, and 8, may produce pulmonary oxygen toxicity. The symptoms of pulmonary oxygen toxicity may begin with a burning sensation on inspiration and progress to pain on inspiration. During recompression treatments, pulmonary oxygen toxicity may have to be tolerated in patients with severe neurological symptoms to effect adequate treatment. In conscious patients, the pain and coughing experienced with inspiration eventually limit further exposure to oxygen. Unconscious patients who receive oxygen treatments do not feel pain and it is possible to subject them to exposures resulting in permanent lung damage or pneumonia. For this reason, care must be taken when administering 100 percent oxygen to unconscious patients even at surface pressure.
Return to normal pulmonary function gradually occurs after the exposure is terminated. There is no specific treatment for pulmonary oxygen toxicity.
The only way to avoid pulmonary oxygen toxicity completely is to avoid the long exposures to moderately elevated oxygen partial pressures that produce it. However, there is a way of extending tolerance. If the oxygen exposure is periodically interrupted by a short period of time at low oxygen partial pressure, the total exposure time needed to produce a given level of toxicity can be increased significantly.
Akimbo:
3‑9.2.2 Central Nervous System (CNS) Oxygen Toxicity. Central nervous system (CNS) oxygen toxicity, sometimes called high pressure oxygen poisoning, can occur whenever the oxygen partial pressure exceeds 1.3 ata in a wet diver or 2.4 ata in a dry diver. The reason for the marked increase in susceptibility in a wet diver is not completely understood. At partial pressures above the respective 1.3 ata wet and 2.4 ata dry thresholds, the risk of CNS toxicity is dependent on the oxygen partial pressure and the exposure time. The higher the partial pressure and the longer the 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.
Akimbo:
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.
CO2 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 CO2 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 ppO2. 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 CO2 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 CO2 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 ppO2 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 mechanism for this effect is unknown, but is probably more than just the increase in gas density and concomitant CO2 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.
Akimbo:
Intermittent Exposure. Periodic interruption of high ppO2 exposure with a 5-15 min exposure to low ppO2 will reduce the risk of CNS toxicity and extend the total allowable exposure time to high ppO2. 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 pres3- sure limits, whereas surface-supplied open-circuit systems permit slightly higher limits. Allowable oxygen exposure limits for each system are discussed in later chapters.
3‑9.2.2.2 Symptoms of CNS Oxygen Toxicity. The most serious direct consequence of oxygen toxicity is convulsions. Sometimes recognition of early symptoms may provide sufficient warning to permit reduction in oxygen partial pressure and prevent the onset of more serious symptoms. The warning symptoms most often encountered also may be remembered by the mnemonic VENTIDC:
V: Visual symptoms. Tunnel vision, a decrease in diver’s peripheral vision, and other symptoms, such as blurred vision, may occur.
E: Ear symptoms. Tinnitus, any sound perceived by the ears but not resulting from an external stimulus, may resemble bells ringing, roaring, or a machinery-like pulsing sound.
N: Nausea or spasmodic vomiting. These symptoms may be intermittent.
T: Twitching and tingling symptoms. Any of the small facial muscles, lips, or muscles of the extremities may be affected. These are the most frequent and clearest symptoms.
I: Irritability. Any change in the diver’s mental status including confusion, agitation, and anxiety.
D: Dizziness. Symptoms include clumsiness, incoordination, and unusual fatigue.
C: Convulsions. The first sign of CNS oxygen toxicity may be convulsions that occur with little or no warning.
Akimbo:
Edit 15 November 2021: Updated links for Version 7A of the US Navy Diving Manual and changed the use of colors for compatibility with different ScubaBoard Styles.
