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---------- Forwarded message ---------- Date: Fri, 14 Jun 1996 11:56:09 -0700 (PDT) From: "Peter N.R. Heseltine" <heseltin@hs*.us*.ed*> To: David Doolette <ddoolett@me*.ad*.ed*.au*> Subject: Re: More on a *safe* pPO2 David, Many thanks for your reply and: On Fri, 14 Jun 1996, David Doolette wrote: > 2. The 1.6 bar regression line in figure three appears to be based on three > data points, aside from being a bit of a leap of faith, it underscores my > comments on the rarity of problems at these relatively low PO2s. I agree. No one can deny that they do happen. I've been e-mailed several anecdotal events by the techdivers@terra.net, including Tom Mount. My point of departure from Bret Gilliam, is that one shoudl not use a time factor to determine whether a pPO2 between about 1.4 and 1.6 is "safe". These are stochastic events and all this model does is confirm that there is not (at present) a way to integrate duration of exposure at these pPO2s into a risk assessment. > 3. Figure 4 is one of my favorites, illustrating that there was significant > difference between curves for exposure time and probablity of toxicity of > 2.2 bar PIO2 dependent on how and when the studies were conducted. I think this is because the definitions of an event changed between the "old" and the "new" data sets. > I believe that if you are going to exposure yourself to the risk of > drowning, you need to limit your PO2 to a level that is highly unlikely to > ever cause a problem (somewher below 1.8-1.4 bar, choose your own poison) > and at these levels I don't believe a safe exposure time can be predicted. I absolutely agree! > 4. One has to exercise some caution in extrapolating the results of these > studies to recreational diving. Most of these studies used rebreathing > apparatus, and, while the data is interesting, and certainly indicative, it > is not completely transferable to open circuit demand diving. All meta-analyses suffer from an inherent set of bias. I do think that this is *very* applicable to sport diving, as it has begun to change. In the US, in Florida some 30% of scuba rentals are now nitrox. Further the constant mass flow rebreather (such as the Uwatec/Drager Atlantis I and the CCR500) are gaining popularity rapidly. For the first time, large numbers of divers are bing exposed to high (> 1.0 ATA) pPO2s and some are using rebreathers. I would like some caution expressed, based on the available facts. IANTD and Tom Mount and others have written to me saying that they teach and recommend planning pPO2s on nitrox (or at depth on air) to 1.4 ATA. Bret Gilliam (the new CEO of Uwatec USA, who markets the Atlantis rebreather) published in a dive newsletter that there was essentially *no* risk below 1.6 ATA, if the CNS clock was not exceeded. Hence my search for facts. Many thanks, Peter Heseltine
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