I'm not sure if this is appropriate or not, but I started digging through my personnal notes that I have accumulated over the years, and thought I'd post what information I have regarding oxygen toxicity. Jarrod Jablonski of GUE and extreme caver of the WKPP wrote the following remarks as it pertains to this subject.
I certainly can't validate the accuracy of his remarks or mistakes/typos he may have made in writing them. These remarks were also made some years ago, and I can't be certain whether these are his current views (I haven't heard him talk differently). So, y'all will just have to take this at face value. IMO, JJ is one of a handfull of guys who have been there-done it and has a genuine level of expertise which is why I decided to post this.
Enjoy.
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Damage starts to occur as the lungs are exposed to higher PO2's and/or more time at elevated PO2 . The damage seems largely reversible unless it is allowed to go to far. Relatively small doses of increased PO2 seem to reverse rapidly while longer doses take more recovery time. The damage to the lungs is represented by swelling of the lung tissue, irritation, and coughing or breathing discomfort (usually in later stages). The damage to one's lungs is usually measured through Vital capacity (which is measured
with a Spiro meter and evaluates forced exhalation and lung volume). Vital capacity continues to decline in a generally consistent manner the longer you remain exposed. The decline can be reduced with the use of breaks off the high oxygen gas (i.e. "air" breaks- we usually use He based gas). OTU's (Oxygen Tolerance Units) are used to track oxygen exposure. They are essentially the same thing as the older UPTD measurement. One OTU
accumulates for every minute of pure oxygen at one ata. This was developed to allow a standard method to discuss oxygen dose accumulation. At a PO2 of .5 or less there does not seem to be any significant alteration in lung function ergo the .5 part of the equation. The OTU equation allows one to calculate the OTU's for any PO2. So you merely calculate the PO2 and solve for OTU. For example, (1.6-.5/.5).83 is 1.92 OTU/min. You can solve this for each time/PO2 step. Other equations such as the Harabin equation
attempt to create a linear representation of degrading lung elasticity (also known as vital capacity). This is done by fitting a linear equation to data of vital capacity reductions. For example, %VC drop=-.11(PO2-.5)t where t= time.
Dive safe,
JJ
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This question requires a far longer response than would be practical right now. I will see what I can put together in the near future but lets start with this general outline. Oxygen Exposure is problematic for several reasons including:
1) High degree of variability in oxygen tolerance
2) Inconsistent measurements and difficulty assessing oxygen tolerance
3) Poor understanding of oxygen toxicity mechanisms
In general the biggest problem with oxygen relates to the significant variability between individuals and within one individual over time. Plots of PO2 and time look more like a wide bore shotgun blast than sensible graph. Subjects would often manage huge tolerances (sometimes several hours) one day and then tox in a matter of minutes the next day.
Unfortunately tolerances would vary so dramatically that there was not any notable success in establishing a trend such as increased or decreased tolerance. Most oxygen tolerance testing was done with pure oxygen commonly at 30,60, and 90'. Many individuals managed amazing tolerances that make current limits seem ridiculously conservative. However, other individuals experienced problems very early in the tolerance time limits. Actually measuring tolerance proved to be a significant problem with exposure testing while early tests often used time limits based upon actual toxic events (ie seizures) and later studies limited time to what has become the commonly recognized "symptoms" known by the acronym VENTID. As a result some individuals that managed significant times when pushed to seizure were then limited by the occurrence of a symptom perhaps prematurely. The occurrence of symptoms did seem to at least loosely relate to toxicity incidence but unreliably and with many complications. This complication in the measuring process further skews what one might refer to as the "actual" toxicity time limits
Furthermore, our inability to get a handle on the mechanisms behind oxygen toxicity confuse limiting its impact and/or measuring the time limits. For example, consider immersion in water and its role in the development in oxygen toxicity. Merely being immersed (as opposed to being in a dry chamber) increases the risk of oxygen toxicity. Being immersed in hot or cold water (either pole is similar in impact) decreases tolerance. Numerous factors affect one's likelihood of succumbing to toxicity but our understanding of these mechanisms and our ability to manipulate them is a very inexact process and highly problematic
The preceding issues are important in considering oxygen toxicity manifestations and in discussing the "oxygen clock". This clock "works" by relating the chosen maximum time at a given PO2 (ie 45@1.6) to a percentage of accumulated time at this PO2. Of course, the success or failure of this clock relates to the accuracy of the time limit which is confused by susceptibility. The 100% (ie 45min at 1.6) limit suggests that as you exceed this parameter your risk of oxygen toxicity markedly increases.
Actually it is a bit more complicated than this but in general this is the idea. However, on our dives we regularly exceed several thousand percent and have pushed into the 10,000% range leaving these numbers to seem meaningless. While they are not meaningless and are in fact a good but very general rule of thumb it is important to see these "limits" for what they are- a forced best guess based on highly variable and conflicting data.
Realistically it seems that these limits are probably overly conservative for the vast majority, good for some, and not enough for a very few. However, given the risk (ie seizure and likely drowning) it is prudent for most people to stay near the limits. These "limits" are really more of a range that should be used as a generally sensible guideline.
The problem in technical diving is that an ardent belief in these numbers leads people to assume that they must save the extra 5 or 10% off their clock by using odd gas mixtures. It is very likely that divers are best served by regular breaks from oxygen (for deco benefit and extended O2 tolerance) and the conservative use of PO2 (such as 1.4 or less for diving). Oxygen tolerance limits are in many ways similar to Decompression limits in that they are likely too conservative for many and highly variable for most individuals. Furthermore, violating these "limits" may increase the risk of an unpleasant outcome or more likely will produce no noticeable impact. However, the risk of violation (which includes pain and death) is likely not worthwhile for the vast majority of divers. These ranges should be viewed with respect but also an understanding of their history and an appreciation for the variation that leads some divers to reevaluate their role in technical diving.
Safe diving,
JJ
