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PO2 Max - Why 1.4/1.6?

Discussion in 'Marine Science and Physiology' started by CuriousRambler, Jan 31, 2020.

  1. CuriousRambler

    CuriousRambler Solo Diver

    # of Dives: 25 - 49
    Location: California
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    I just wrapped up the online portion of my nitrox course a few days ago, and have been reading around here and elsewhere since, trying to gain a deeper understanding of things. One thing I haven't been able to find an explanation on: why was I taught to use a max PO2 of 1.4, or 1.6 for contingency planning? I understand the concept of what PO2 is, and I understand the risks/ramifications of exceeding these numbers, so I'm not asking "why can't I use 1.8, or 3.5?" I'm trying to get my head around why these are the numbers, from a theoretical perspective. I'll be amazed if this info isn't already here somewhere, but I'm not sure how to refine my search terms to get beyond conversations about which number to use, rather than why those numbers are the right ones to use.

    Operationally, I'm completely OK "just accepting" 1.2/1.4/1.6 are acceptably safe and well-validated numbers. I have no interest in debating the choice of these numbers, and I'm certainly not trying to suggest there's a better choice. Intellectually, I just want to know more. Were these numbers arrived at simply through mass trial and error, were they backed out of decompression algorithms, or are they based on existing physiological research independent of the rec/tech dive world? I can imagine plenty of scenarios where human oxygen toxicity limits might be of interest to other sectors like aerospace, medicine, commercial diving, etc.

    I've found plenty of good "books to read" lists here and elsewhere, and I've got a copy of Deco for Divers on order to start with. Unfortunately it looks like it'll arrive a couple days after my next stint out of town, so I'll have to read something else on the plane. Is there maybe a better book on this topic I should earmark for the next one on the list?
     
  2. aviator8

    aviator8 Professional Photographer

    # of Dives: 25 - 49
    Location: Georgia
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    I believe this came from Haldane's original models where a PPO2 was determined to be 2.0 from his models and observational studies. Over time it became clear that these were not absolute hard limits as individuals are different and contributing factors such as stress, hydration, age, temp, etc could cause variance. Agencies began to endorse 1.6 as a safer limit. When the original models were build they were based off studies which were military divers and presumably younger and fitter than most of the diving population. As time progressed an even more conservative number of 1.4 was endorsed to further reduce risks.
     
  3. DavidFL

    DavidFL Wide-eyed nube in the Pub ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Orlando, FL
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    Nitrox - 1.40 or 1.60 PO2?

    It's a good question; versions of the question come up frequently with new nitrox divers. My own view is that the PO2 'limit' is established by training agencies based on a combination of theory, some relevant research, and the experience of divers. Like choosing decompression algorithms and settings, it becomes a personal choice, but the training agencies need to have some kind of reasonable setpoint that balances their own view of risk vs. advantages of using a higher percentage of O2 in a mix.

    To the OP's point about searching, it turns out trying to get beyond conversations about which number to use may not be helpful, since those conversations held by other divers with the same question are what may lead you to your own understanding of where the numbers "come from" and ultimately your own choices.
     
    markmud likes this.
  4. Erik Forsberg

    Erik Forsberg Sea Corgi

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    Long story short; the PP where O2 becomes toxic varies between persons, and also day form (hydration, exertion, CO2 build up etc). This was not readily understood for quite a while and the US navy (iirc) did a looot of testing for where divers' PPO2 toxicity levels where. While this was a rather cruel experiement, it also presented quite some data for others to draw conclusions from, add some safety margin and introduce as standard in civilian diving training.

    1.4 / 1.6 was chosen as to veeeeeery few persons would develop O2 toxicity below those levels.

    There is a post here on scubaboard if you did around when a diver did get an O2 hit from well below threshold though, and he lived to tell the tale so it's definitely worth digging for it.
     
    jeff h likes this.
  5. Graeme Fraser

    Graeme Fraser Tech Instructor

    # of Dives: 1,000 - 2,499
    Location: Narnia
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    You're right that oxygen toxicity limits are still relevant in other applications, it's just that one of the possible symptoms (convulsions) is much more dangerous when diving than in, say, a decompression chamber, or even hard hat surface supply.

    Not because convulsions are necessarily harmful in themselves, just that it will likely result in drowning as the regulator falls from the mouth.
     
    RyanT likes this.
  6. Duke Dive Medicine

    Duke Dive Medicine ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    The 1.6 comes from the NOAA oxygen exposure limit table. It's the highest inspired PO2 recommended for normal exposure, and for a long time it was an accepted operational limit. It's drifted down over the years mostly due to the recognition that CNS O2 toxicity is highly dependent on the situation and the individual. Quoting from the DAN Technical Diving Conference Proceedings (Vann RD, Mitchell SJ, Denoble PJ, & Anthony TG, 2010): "Which oxygen partial pressures are safe? The question does not have an unequivocal answer.... At present, the available information seems sufficient to say that the risk of oxygen may be worrisome at or above 1.3 to 1.6 atm, but even this non-specific conclusion is tempered by the possibility that intra- and inter-individual variability, environmental effects, pharmaceutical influences and O2-CO2 interactions could reduce the threshold to less than 1.3 atm."

    Linking the whole document from Rubicon Foundation here. There's some great background information on O2 toxicity research.

    Best regards,
    DDM
     
  7. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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  8. CuriousRambler

    CuriousRambler Solo Diver

    # of Dives: 25 - 49
    Location: California
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    There's some good info in that thread for sure. I read through that, and have several of the linked references downloaded to my iPad for my upcoming travel browsing. Definitely helped form a better picture in my head, once you wade through the arguments about why 1.2 is better, but chamber tests are harmless >2.0, and 1.4 will kill you. Clearly plenty of debate on the topic, which is why I specifically stated I have no interest in questioning the numbers, I just want to understand them better haha. Thanks for pointing me that way though.

    I definitely understand that aspect, I just thought maybe other industries (perhaps with better funding) may have done more extensive (and presumably lower risk, for your cited reasons) research into the topic than recreational divers might be able to mount. There seems to be some argument about the value of applying chamber experiments to diving, due to the lack of physical exertion in most (all?) chamber tests, but I imagine any data are better than none, even when individual limits seem to vary to the extreme as well.

    Thank you! I saw some allusions to NOAA exposure limit tables, but wasn't sure how extensively they relate to the topic at large; if they're the source, or if their tables are just another published version/iteration of the same fundamental information everyone's using. And thanks for the doc link. I think I downloaded it off the above posted thread already, but the cover sheet looks different from what I remember. I'm going to have to compare them side by side, and see if I've got one or two things to read hah. I'll also have to do some digging on the NOAA tables to see what pops up if I pull on that thread a bit.

    And thank you for this! Short enough, I wish I could burn through it right now, but I'm hosting folks for dinner, so it's also been added to my "read later" folder.

    I appreciate everyone's input, this stuff is rather interesting to me for some reason. The more I read, the more curious I seem to be.
     
    ScubaWithTurk likes this.
  9. jeff h

    jeff h Angel Fish

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    Location: Michigan
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    I unfortunately know two people that passed away cave diving at 1.4 PPO2 on separate occasions. Not sure if it's just individual sensitivity to PPO2, or combination of physiological factors such as fatigue, dehydration, or environmental factors such cold, stress, etc that can vary from day to day. Chamber dives are non-working, low stress environments, so likely not representative of what divers are susceptible to at depth. Better to be conservative when it comes to PPO2.
     
  10. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    The evidence is clear: immersion O2 tolerance is less than dry (chamber) tolerance.
     
    ScubaWithTurk likes this.

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