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Likelihood of getting oxygen toxicity at 1.4 PPO

Discussion in 'Marine Science and Physiology' started by ystrout, Jun 4, 2018.

  1. ystrout

    ystrout Nassau Grouper

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    Hi Guys,

    I recently started using nitrox and had some questions regarding max PPO.

    I understand I shouldn't go over 1.4 PPO, and don't ever plan to do so. But I remember the course telling us that 1.6 PPO was the max depth where you're VERY likely to suffer from oxygen toxicity. So how likely are you to actually have symptoms at 1.4 PPO? And what about 1.3 PPO?

    Let's assume someone is in good health with no pre-existing conditions that increase odds of oxygen toxicity.

    And so no one takes this thread the wrong way, I'm not planning to dive past 1.3-1.4 PPO, I'm just wondering for curiosity's sake.
     
  2. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    if 1.6 was "very" likely, then there would be a mountain of bent tech divers every time we do deco....

    the odds of you having any sort of issue, at recreational depths, under normal work loads *i.e. not a combat swimmer* at 1.4 is basically 0.

    If you are doing extreme depths, extreme durations, etc etc. then you may want to lower it, but for recreational depths, it's a non issue.

    Nitrox is also best done with standard or relatively standard gases, so with EAN32 being the most common, to get to 1.4 you have to be at 110 ish ft. You aren't going to spend a lot of time down there without going into deco or running out of gas, so it's not really something to worry about. Something to be aware of, but not something to worry about

    Disclaimer, not a doctor
     
  3. kelemvor

    kelemvor Big Fleshy Monster ScubaBoard Supporter

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    Those of us who are doing accelerated decompression using 100% O2 generally go to 1.6. At least, I do and those I've talked to about doing these stops have been using. I'm not sure I've heard phrases like "very likely" regarding toxing at 1.6.

    If when you get into more advanced training, you'll learn that exposure time is also a key factor in oxygen toxicity. I generally run 1.4 as my maxpp02 during planning and I also set an audible alarm on my dive computer to alert me if I've exceeded 1.4 during a non-mandatory deco dive.

    My point is that personally I feel safe using 1.4 as my safe limit for regular diving.

    However.. I'm also diving with a full facemask. That means that a toxicity event is less likely to be lethal for me.
     
    Jay likes this.
  4. MaxBottomtime

    MaxBottomtime Divemaster

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    1.6 was the limit when I was certified. It worked just fine for many decades. These days agencies want to limit their liabilities and now recommend 1.4, although it doesn't mean the physics of oxygen have changed.
     
  5. Wookie

    Wookie Orange Man Bad Staff Member ScubaBoard Business Sponsor

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    I would never suggest anyone else dive a 1.6 PPO2. Wait, yes I would, and I think it was silly to change the rules to speak to conservatism without evidence. I dive 1.6. There is not a great reason not to.
     
    oldschoolto, AfterDark, Jay and 4 others like this.
  6. Dr Simon Mitchell

    Dr Simon Mitchell Medical Moderator Staff Member

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    Hello,

    In the lead up to the DAN technical diving conference in 2009 Dick Vann and I spent some time trying to find verifiable anecdotes describing oxygen seizures at PO2s around 1.3 - 1.4. Setting aside concerns about the accuracy of PO2 measurement in individual cases, we found several reasonably convincing cases occurring at 1.3 and 1.4. These are described on pages 56 - 58 of the DAN Technical Diving Conference Proceedings [1] which you can find on-line. You would find the entire chapter in which these anecdotes appear (by Dick Vann and Bill Hamilton) to be very interesting I suspect.

    As tbone points out, the apparently small number of seizures occurring during a vast number of decompressions using PO2s between 1.3 and 1.6 suggest that the risk is very low, but it is certainly not zero. It is also very context sensitive (risk variability between individuals, perhaps some drugs, and exercise). Exercise whilst breathing dense gas and consequent CO2 retention is probably the biggest risk factor, but CO2 retention is unlikely to be relevant when oxygen exposures are peaking during long resting decompressions. We found some evidence to support this statement in a field study at Bikini Atoll [2]. I would be happy to send you this study if you PM me.

    Simon M

    1. VANN RD, MITCHELL SJ, DENOBLE PJ, ANTHONY TG (eds). Technical Diving. Proceedings of the Divers Alert Network 2008 January 18-19 Conference. Durham NC, Divers Alert Network, 401pp (ISBN 978 1 930536 53 1), 2009

    2. MITCHELL SJ, MESLEY P, HANNAM JA. An observational field study of end tidal CO2 in recreational rebreather divers on surfacing after decompression dives. Aerospace Med Hum Perform 86, 41-45, 2015
     
    fozzy, Steve-H, Esprise Me and 7 others like this.
  7. gcarter

    gcarter Orca

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    I generally try to keep to 1.4 for conservatism, but did a dive last summer to 1.5 without any concern.
     
  8. uncfnp

    uncfnp Solo Diver

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    But were any of these case at recreation limits. In other words, is there any evidence of oxtox at 1.4 during typical recreational dive times?
     
  9. JVos

    JVos Rebreather Pilot

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    The key to 1.6 being safe for deco is that no work is being done. Personally I always use 1.2 at depth as my max PO2.
     
    stevensamler likes this.
  10. Lorenzoid

    Lorenzoid idling in neutral buoyancy

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    I think the simple answer to the question is that there is insufficient data for science to have reached any sort of conclusion on "likelihood." There is just no way at present to give a numerical answer of the likelihood or odds at any given PPO2. As I understood Dr. Mitchell's comment above, the best we have at present are rough statements, such as "very low." So what does that mean to you? You have to use your own judgment. If researchers managed to dig up a few instances out there in which someone was reported to have seized at 1.3-1.4, are you of such a conservative mindset that you will limit yourself to 1.2? Maybe some people are that conservative.
     
    Esprise Me and lv2dive like this.

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