Agreat deal of debate surrouds the mechanics of oxygen toxicity and the symptoms that indicate an approaching attack. Much of the information presented here is designed to afford divers the ability to make educated decisions and responsible choices.
Nonetheless, it may also benefit a small number of very aggressive underwater explorers who are pioneering an expanded understanding of oxygen tolerance. Most divers will find that by following several very simple guidelines, they will limit themselves to responsible oxygen exposures that are perfectly sufficient for their operational needs.
The following guidelines are widely accepted as conservative limits as originally published by NOAA. Nonetheless, individuals should always remember that oxygen limits are imprecise. During testing, while many individuals managed amazing tolerances, others experienced problems very early in the tolerance time limits, and, to complicate matters, others managed very short exposures on one day, and lengthy exposures on another. Furthermore, elevated CNS percentages become more problematic when combined with contributing factors like work, water temperature, CO2 accumulation, immersion in water, drugs, and fatigue. It is good to remember that susceptibility to oxygen toxicity is a very personal affair.
Unfortunately a great deal of emphasis has focused on oxygen toxicity limits, leaving individuals with the sense that precise measures exist for tracking and measuring risk of oxygen toxicity. In truth these limits are guidelines, generally outlining zones of increasing risk. Very little risk appears in the lower range below 1.4 PO2 and most responsible divers maintain a careful approach to elevated oxygen mixtures above this range.
While diving, most individuals will have little trouble maintaining a low level of risk with respect to oxygen exposure. Divers need merely adjust the oxygen mixture downward with longer dives while always maintaining a good margin for error. For example, GUE standard diving mixes prescribe a maximum PO2 of 1.3 with variation for increased effort, stress, or environmental conditions.
A PO2 of 1.4 is the maximum recommended for a diver under stress, while a PO2 of 1.6 is the absolute maximum permissible, and usually reserved for a resting diver during decompression. Oxygen tolerance is highly variable and can be altered by many factors, including physical condition, cumulative exposure, immersion, exertion, and water temperature. Tracking the effect of one or multiple exposures involvescalculating the time spent at a given PO2 and relating it to the recommended limits. The time spent at this PO2 is described as a percentage of the total available time, also known as the oxygen clock.
The upper oxygen range beyond 1.4 PO2 is reserved for experienced divers conducting decompression dives and therefore able to "break" (stop breathing the high oxygen mix for periods of time) from the elevated oxygen to lower oxygen mixtures (technical diving). Use of oxygen mixtures approaching 1.6 PO2 should be done only where "breaks" are feasible and divers can control exertion level and stress while having access to lower oxygen mixtures to allow for alternating cycles of low oxygen breathing.
Understanding the potential variability in oxygen tolerance is important not only when considering oxygen toxicity but also when discussing the oxygen clock. This hypothetical clock works by relating the chosen maximum time at a given PO2 (i.e. 45min at 1.6) to a percentage of accumulated time at this
PO2. Of course, the success or failure of this clock depends on the accuracy of the time limit; a limit made ambiguous by susceptibility. The 100% limit (i.e. 45min at 1.6) suggests that to go beyond this limit markedly increases one's risk of oxygen toxicity. Nonetheless, GUE and WKPP exploration divers regularly exceed several thousand percent, highlighting the substantial variation that exists within oxygen limits. This variation can be explained either by the effective procedures and individual physiologies at work in these dives, or it may represent the failing of the conceptual model of oxygen used in this context. Most likely, it suggests something in between.
Oxygen toxicity measurements can best be seen as general rules of thumb. They are the diving community's "best guesses" regarding oxygen limits, limits based on highly variable and conflicting data. Objectively, these limits are probably overly conservative for the vast majority, good for some, and not good enough for a very few. However, given the risk posed by oxygen toxicity (i.e. seizure and likely drowning), it is best that most divers stay near the limits. Nonetheless, these "limits" should not be seen as absolute, but rather as a range that should be used as generally sensible guidelines.
Ardent belief that these "limits" are absolute has generated its own set of problems in technical diving, leading some to assume that they must shave an extra 5% or 10% off their clock by using odd gas mixtures. Using odd gas mixes designed to shave exposure from a somewhat arbitrary limit often creates more problems then it solves. Divers are likely best served by regular breaks from oxygen (for both decompression benefit and extended O2 tolerance) and the conservative use of PO2 (such as 1.4 or less for diving). Careless oxygen use can be fatal; therefore, for most divers, aggressive oxygen exposure is not worthwhile. Furthermore, it is important that divers carry out regular break cycles (going off oxygen), particularly on long dives where there is significant oxygen accumulation. While a number of different break cycles are successful, most divers breathe oxygen for 20 minutes, then follow it with a 5-minute break on a mixture with reduced oxygen content (historically air but more sensibly a breathable Trimix mixture). Divers with only 20 or 30 minutes of total decompression can insert the break in the middle of their decompression to derive maximum benefit from the break.